Table of contents

Since the early 1980s, the Quark Matter conferences have been the most important forum for presenting results in the field of high-energy heavy-ion collisions. The 22nd conference in this series took place in Annecy, France, on 22–29 May 2011, and it attracted a record attendance of almost 800 participants. More than 500 requests to give presentations were received and, based on the recommendations of the International Advisory Committee, almost 200 were selected. This special issue of Journal of Physics G: Nuclear and Particle Physics contains the written reports of those oral presentations.

Quark Matter 2011 was scheduled to take place six months after the start of the heavy ion program at the Large Hadron Collider (LHC). Hence, these proceedings mark a historical milestone: two decades after starting to prepare for the LHC, the present volume documents the first substantial harvest of LHC heavy-ion data. In addition, these proceedings feature a complete overview of recent theoretical and experimental developments over two orders of magnitude in the center-of-mass energy of heavy-ion collisions. In particular, they include prominently the latest results from the heavy-ion experiments at Brookhaven's Relativistic Heavy Ion Collider and a broad range of theoretical highlights.

Early in the organization of Quark Matter 2011, it was recognized that the novelty of the results expected at this conference argues for a very rapid publication of the proceedings. We would like to thank all who helped meet the ambitious production schedule. In particular, we would like to thank the paper committees of the LHC experiments ATLAS, ALICE and CMS, and the RHIC experiments PHENIX and STAR who ensured, in a coordinated action, that all experimental contributions were received within four weeks of the end of the conference. We would also like to thank the many individual contributors, as well as the anonymous referees appointed by Journal of Physics G: Nuclear and Particle Physics, who respected the tight deadline. Last but not least, we would like to thank the staff of the journal, and in particular Suzie Prescott and Rachel Lawless: they handled an enormous number of communications and requests flawlessly and swiftly.

Yves Schutz and Urs Achim Wiedemann Organizers of Quark Matter 2011 Guest Editors

Results from RHIC led to the discovery of the strongly coupled quark–gluon plasma (sQGP), a state of the QCD vacuum. In order to arrive at a coherent picture of this state, both the properties of the sQGP and the conditions under which the sQGP is measured have to be established. In this paper, we present recent experimental results of the PHENIX Collaboration, further constraining the conditions and the properties of the sQGP at RHIC: measurement of direct photon elliptic flow provides strong constraints for hydrodynamic models employed to derive the initial temperature; measurement of v₂ and v₃ disentangles effects from the initial state and η/s; flow and R_AA measurements at lower $\sqrt{s}$ extend the results to a regime at lower temperature and higher baryon density.

We report selected results from STAR collaboration at the RHIC, focusing on jet–hadron and jet-like correlations, quarkonium suppression and collectivity, di-electron spectrum in both p+p and Au+Au, and higher moments (standard deviation, skewness and kurtosis) of net-protons as well as azimuthal anisotropy from the RHIC Beam Energy Scan program.

After 20 years of preparation, the dedicated heavy-ion experiment ALICE, which stands for A Large Ion Collider Experiment, took first data at the CERN LHC accelerator with proton collisions at the end of 2009 and with lead beams at the end of 2010. This paper will give a brief overview of the main results presented at the Quark Matter 2011 conference.

Results are presented from the ATLAS collaboration from the 2010 LHC heavy-ion run, during which nearly ten inverse microbarns of luminosity were delivered. Soft physics results include charged particle multiplicities and collective flow. The charged particle multiplicity, which tracks initial state entropy production, increases by a factor of 2 relative to the top RHIC energy, with a centrality dependence very similar to that already measured at the RHIC. Measurements of elliptic flow out to large transverse momentum also show similar results to what was measured at the RHIC, but no significant pseudorapidity dependence. Extensions of these measurements to higher harmonics have also been made and can be used to explain structures in the two-particle correlation functions that had long been attributed to jet–medium interactions. New hard probe measurements include single muons, jets and high p_T hadrons. Single muons at high momentum are used to extract the yield of W^± bosons and are found to be consistent within statistical uncertainties with binary collision scaling. Conversely, jets are found to be suppressed in central events by a factor of 2 relative to peripheral events, with no significant dependence on the jet energy. Fragmentation functions are also found to be the same in central and peripheral events. Finally, charged hadrons have been measured out to 30 GeV, and their centrality dependence relative to peripheral events is similar to that found for jets.

The CMS experiment at the LHC is a general-purpose apparatus with a set of large acceptance and high granularity detectors for hadrons, electrons, photons and muons, providing unique capabilities for both proton–proton and ion–ion collisions. The data collected during the November 2010 Pb–Pb run at = 2.76 TeV were analyzed and multiple measurements of the properties of the hot and dense matter were obtained. Global event properties, detailed study of jet production and jet properties, isolated photons, quarkonia and weak bosons were measured and compared to pp data and Monte Carlo simulations.

I present a brief discussion of the different approaches to the study of initial state effects in heavy-ion collisions in view of the recent results from Pb+Pb and p+p collisions at the LHC.

Global variables, such as the charged particle multiplicity and the transverse energy, are important observables to characterize relativistic heavy-ion collisions and to constrain model calculations. The charged particle multiplicity dN_ch/dη and the transverse energy dE_T/dη are measured at TeV in Pb–Pb collisions as a function of centrality and in pp collisions. The fraction of an inelastic cross section seen by the ALICE detector is calculated either using a Glauber model or the data corrected by simulations of nuclear and electromagnetic processes, or data collected with a minimum bias interaction trigger. The centrality, defined by the number of nucleons participating in the collision, is obtained, via the Glauber model, by relating the multiplicity distributions of various detectors in the ALICE Central Barrel and their correlation with the spectator energy measured by the Zero-Degree Calorimeters. The results are compared to corresponding results obtained at the significantly lower energies of the BNL AGS, the CERN SPS and the BNL RHIC, and with models based on different mechanisms for particle production in nuclear collisions. Particular emphasis will be given to a discussion on systematic studies of the dependence of the centrality determination on the details of the Glauber model, and the validity of the Glauber model at unprecedented collision energies.

We report on the results of femtoscopic analysis of Pb–Pb collisions at TeV and pp collisions at , 2.76 and 7 TeV with identical pions and kaons. Detailed femtoscopy studies in heavy-ion collisions at SPS and RHIC have shown that emission region sizes ('HBT radii') decrease with increasing pair transverse momentum k_T, which is understood as a manifestation of the collective behavior of matter. The trend was predicted to persist at the LHC. The data from Pb–Pb collisions confirm the existence of a flowing medium and provide strict constraints on the dynamical models. Similar analysis is carried out for pp collisions for pions and kaons and qualitative similarities to heavy-ion data are seen, especially in collisions producing large number of particles. The observed trends give insight into the soft particle production mechanism in pp collisions. 3D radii were also found to universally scale with event multiplicity in heavy-ion collisions. We extend the range of multiplicities both upward with the Pb–Pb data and downward with the pp data to test the scaling in new areas. In particular, the high-multiplicity pp collisions reach particle densities comparable to the ones measured in peripheral Cu–Cu and Au–Au collisions at the RHIC. This allows for the first time to directly compare freeze-out sizes for systems with very different initial states.

I review recent developments in the field of relativistic hydrodynamics and its application to the bulk dynamics in heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC). In particular, I report on progress in going beyond second order relativistic viscous hydrodynamics for conformal fluids, including temperature-dependent shear viscosity to entropy density ratios, as well as coupling hydrodynamic calculations to microscopic hadronic rescattering models. I describe event-by-event hydrodynamic simulations and their ability to compute higher harmonic flow coefficients. Combined comparisons of all harmonics to recent experimental data from both the RHIC and the LHC will potentially allow us to determine the desired details of the initial state and the medium properties of the quark–gluon plasma produced in heavy-ion collisions.

Recent collective flow measurements including higher moment event anisotropy from the PHENIX experiment are presented, and the particle type, beam energy dependence and the relation with jet modification are discussed. The measured higher order event anisotropy with event plane defined at forward rapidities and the long-range correlation with large η gaps are both consistent with initial geometrical fluctuation of the participating nuclei. In 200 GeV Au+Au collisions, higher order event anisotropy, especially simultaneous description of v₂ and v₃, is found to give an additional constraining power on an initial geometrical condition and viscosity in the hydrodynamic calculations. v₂, v₃ and v₄ are almost unchanged down to the lower colliding energy at 39 GeV in Au+Au. The measured two-particle correlation with subtraction of the measured v_n parameters shows a significant effect on the shape and yield in the associate particle Δϕ distribution with respect to the azimuthal direction of trigger particles. However, some medium responses from jet suppression or jet modification seem to be observed. Direct photon v₂ has been measured in 200 GeV Au+Au collisions. The measured v₂ is found to be small at high p_T as expected from non-suppressed direct photon R_AA ≃ 1, which can be understood as being dominated by prompt photons from initial hard scattering. On the other hand, at lower p_T < 4 GeV/c it is found to be significantly larger than zero, which is comparable to other hadron v₂, where thermal photons are observed.

We report on the CMS measurements of charged hadron anisotropic azimuthal distributions from Pb–Pb collisions at = 2.76 TeV and their decomposition into a Fourier series up to the sixth coefficient. The results are presented as a function of transverse momentum (p_T), centrality and pseudorapidity (η) and cover a broad kinematic range. The scaling with the respective participant eccentricity is studied. The sensitivity of different harmonic coefficients to the dynamical models that describe the medium produced in the collisions is discussed.

We present a differential measurement of the azimuthal anisotropy of charged hadron production in Pb+Pb collisions at = 2.76 TeV. This azimuthal anisotropy is expanded into a Fourier series in azimuthal angle, where the coefficient for each term, v_n, characterizes the magnitude of the anisotropy at a particular angular scale. We extract v₂–v₆ via a discrete Fourier analysis of the two-particle Δϕ–Δη correlation with a large Δη gap (|Δη| > 2), and via an event-plane method based on the forward calorimeter. Significant v₂–v₆ values are observed over a broad range in , η and centrality, and they are found to be consistent between the two methods in the transverse momentum region GeV. This suggests that the measured v₂–v₆ obtained from two-particle correlations at low with a large Δη gap are consistent with the collective response of the system to the initial-state geometry fluctuations, and is not the result of jet fragmentation or resonance decay.

The ALICE detector at the Large Hadron Collider (LHC) recorded first Pb–Pb collisions at TeV in November and December of 2010. We report on the measurements of anisotropic flow for charged and identified particles. From the comparison with measurements at lower energies and with model predictions, we find that the system created at these collision energies is described well by hydrodynamical model calculations and behaves like an almost perfect fluid.

We present transverse momentum distributions of inclusive charged particles and identified hadrons in pp and Pb–Pb collisions at TeV, measured by ALICE at the LHC. The Pb–Pb data are presented in intervals of collision centrality and cover transverse momenta up to 50 GeV/c. Nuclear medium effects are studied in terms of the nuclear modification factor R_AA. The results indicate a strong suppression of high-p_T particles in Pb–Pb collisions, consistent with a large energy loss of hard-scattered partons in the hot, dense and long-lived medium created at the LHC. We compare the preliminary results for inclusive charged particles to previous results from RHIC and calculations from energy loss models. Furthermore, we compare the nuclear modification factors of inclusive charged particles to those of identified π⁰, π^±, K⁰_s and Λ.

We report the measurements of nuclear modification factors of the Z bosons, isolated photons and charged particles in = 2.76 TeV Pb–Pb collisions with the Compact Muon Solenoid detector. The nuclear modification factors are constructed by dividing the Pb–Pb p_T spectra, normalized to the number of binary collisions, by the pp references. No modifications are observed in the isolated photon and Z boson production with respect to the pp references, while a large suppression is observed in the charged particles.

The nuclear modification factor R_AA has, for more than one decade, been one of the workhorse variables in the field of ultra-relativistic heavy-ion collisions. It describes the deviations of the yield of a given probe, such as π⁰ yields, as compared to the yield that would have been obtained from a simple-minded superposition of independent proton–proton collisions. In addition to new π⁰ suppression results in Au+Au collisions at = 200 GeV, we present recent results from π⁰ yields from the currently ongoing RHIC low energy scan, measurements of the jet fragmentation function in Cu+Cu and the R_AA from fully reconstructed jets in Cu+Cu collisions.

The generic features of parton energy loss effects on the quenching of single hadron spectra in heavy-ion collisions are discussed, paying attention to the expected differences from RHIC to LHC. The need for precise baseline measurements in p–Pb collisions at the LHC is also emphasized. Finally, I briefly mention the production of prompt photons in heavy-ion collisions as well as some open questions.

A thorough understanding of jet quenching on the basis of multi-particle final states and jet observables requires new theoretical tools. This talk summarizes the status and prospects of the theoretical description of jet quenching in terms of Monte Carlo generators.

I present recent results from jets and jet-like correlation measurements from STAR. The pp data are compared to those from Au–Au collisions to attempt to infer information on the medium produced and how hard scattered partons interact with this matter. Results from d-Au events are utilized to investigate the magnitude of cold nuclear matter effects on hard scatterings. The evolution of the underlying event from pp to d-Au collisions is studied. In heavy-ion collisions, background fluctuations are the major source of systematic uncertainties in jet measurements. Detailed studies are therefore being made of these fluctuations and recent progress in our understanding is reported. Jet and jet–hadron correlation results are presented and give clear indications that partonic fragmentation at the RHIC is highly modified in the presence of a strongly coupled colored medium, resulting in a significant broadening and softening of the jet fragment correlation. Finally, di-hadron correlations utilizing identified particles as triggers indicate that the 'ridge' is stronger for p+ K than for π but that the near-side peak per-trigger yield remains unaltered from d-Au to Au–Au collisions.

The energy loss of fast partons traversing the strongly interacting matter produced in high-energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. The multipurpose compact muon solenoid (CMS) detector is well designed to measure these hard scattering processes with its high-resolution calorimeters and high precision silicon tracker. In central Pb–Pb collisions, a center-of-mass energy of 2.76 TeV parton energy loss is observed as a significant dijet transverse momentum imbalance, when comparing to a reference distribution corresponding to pp collisions at the same energy. To gain further understanding of the parton energy loss mechanism, the redistribution of the quenched jet energy was studied using the transverse momentum balance of charged tracks projected onto the direction of the leading jet. In contrast to pp collisions, a large fraction of the momentum balance for asymmetric dijets is found to be carried by low momentum particles at large angular distance to the axis of the recoiling jet. Further, the fragmentation functions for leading and subleading jets were reconstructed and were found to be unmodified compared to measurements in pp collisions. The results yield a detailed picture of parton propagation in the hot QCD medium.

The ATLAS detector with highly segmented calorimeters covering the pseudo-rapidity interval |η| < 5 provides excellent capabilities for reconstructing jets in Pb+Pb collisions. In the Fall 2010 LHC Pb+Pb run at $\mbox{$\sqrt{s_{{\rm NN}}}$}= 2.76$ TeV, ATLAS recorded about 10 μb⁻¹ of Pb+Pb collisions. Using approximately 7 μb⁻¹ from this data set, jets were reconstructed using the anti-k_t algorithm for two jet size parameters, R = 0.2 and R = 0.4, and using a per-event estimation and subtraction of the underlying event. New, preliminary ATLAS measurements obtained from this analysis of single jet spectra and jet central-to-peripheral ratios (R_CP), jet charged particle fragmentation distributions, and dijet asymmetry distributions for R = 0.2 jets are presented.

I summarize recent results from lattice QCD on a few selected topics which are of interest to the heavy-ion physics community. Special emphasis is placed upon observables related to fluctuations of conserved charges and their connection to event-by-event fluctuations in heavy-ion collision experiments.

We present the first results using the STAR detector from the beam energy scan program at the Relativistic Heavy-Ion Collider (RHIC). In this program, Au ion collisions at the center of mass energies ($\sqrt{s_{\mathrm{NN}}}$) of 7.7, 11.5 and 39 GeV allowed the RHIC to extend its study of the QCD phase diagram from baryonic chemical potential values of 20 to about 400 MeV. For the high net-baryon density matter at midrapidity, formed in these collisions, we report several interesting measurements. These include the observation of the difference between anti-particle and particle elliptic flow, disappearance of the difference in dynamical azimuthal correlations with respect to the event plane between same and opposite signed charged particles, change in the slope of the eccentricity at freeze-out and directed flow of protons as a function of $\sqrt{s_{\mathrm{NN}}}$ and the deviation of higher order fluctuations from hadron resonance gas and Poissonian expectations. Possible interpretations of these observations are also discussed.

This paper presents results and plans of the NA49 and NA61/SHINE experiments at the CERN Super Proton Synchrotron concerning the study of relativistic nucleus–nucleus interactions. First, the NA49 evidence for the energy threshold of creating quark–gluon plasma, the onset of deconfinement, in central lead–lead collisions around 30A GeV is reviewed. Then the status of the NA61/SHINE systematic study of properties of the onset of deconfinement is presented. Second, the search for the critical point of strongly interacting matter undertaken by both experiments is discussed. NA49 measured large fluctuations at the top SPS energy, 158A GeV, in collisions of light- and medium-size nuclei. They seem to indicate that the critical point exists and is located close to baryonic chemical potential of about 250 MeV. The NA61/SHINE beam energy and system size scan started in 2009 will provide evidence for the existence of the critical point or refute the interpretation of the NA49 fluctuation data in terms of the critical point.

The ALICE experiment has been taking data since 2009, with proton and lead beams. In this paper, the different particle identification techniques used by the experiment are briefly reviewed. The current results on identified particle spectra in pp collisions at and 7 TeV, and in Pb–Pb collisions at are summarized. In particular, the energy dependence of the spectral shapes and particle ratios in pp collisions is discussed and the results are compared to previous experiments and commonly used Monte Carlo models. The baryon/meson ratio Λ/K⁰_S is studied in Pb–Pb collisions as a function of transverse momentum and centrality, and it is compared to previous results. The evolution of the particle spectra in Pb–Pb with collision centrality is compared to measurements at lower energies and discussed in the context of thermal and hydrodynamical models.

This paper consists of a brief overview of current understanding of collective behavior in relativistic heavy-ion collisions. In particular, recent progress in understanding the implications of event-by-event fluctuations has solved important puzzles in existing data—the 'ridge' and 'shoulder' phenomena of long-range two-particle correlations—and has created an exciting opportunity to tightly constrain theoretical models with many new observables.

Measurements of charged dihadron angular correlations are presented in proton–proton (pp) and lead–lead (PbPb) collisions, over a broad range of pseudorapidity and azimuthal angle, using the CMS detector at the LHC. In very high multiplicity pp events at center-of-mass energy of 7 TeV, a striking 'ridge'-like structure emerges in the two-dimensional correlation function for particle pairs with intermediate p_T of 1–3 GeV/c, in the kinematic region 2.0 < |Δη| < 4.8 and small Δϕ, which is similar to observations in heavy-ion collisions. Studies of this new effect as a function of particle transverse momentum are discussed. The long-range and short-range dihadron correlations are also studied in PbPb collision at a nucleon–nucleon center-of-mass energy of 2.76 TeV, as a function of transverse momentum and collision centrality. A Fourier analysis of the long-range dihadron correlations is presented and discussed in the context of CMS measurements of higher order flow coefficients.

Untriggered di-hadron correlation studies are shown which provide a map of the bulk correlation structures in Pb–Pb collisions. Long-range correlations are further studied by triggered correlations which address the dependence on trigger and associated p_T. Measured correlation functions are decomposed with a multi-parameter fit and into Fourier coefficients. The jet-yield modification factor I_AA is presented.

We report STAR measurements relating to higher flow harmonics including the centrality dependence of two- and four-particle cumulants for harmonics 1–6. Two-particle correlation functions versus Δη and Δϕ are presented for p_T and number correlations. We find the power spectra (Fourier transforms of the correlation functions) for central collisions drop quickly for higher harmonics. The Δη dependence of v₃{2}² and p_T and the centrality dependence of v₂ and v₃ are studied. Trends are consistent with expectations from models including hot-spots in the initial energy density and an expansion phase. We also present v₃ and v₂{2}² − v₂{4}² versus .

Over the last decade a fruitful interplay has developed between analyses of strongly coupled non-Abelian plasmas via the gauge/string duality and the phenomenology of the quark–gluon plasma created in heavy-ion collisions. The reasons why the gauge/string duality is not a precision tool for QCD physics at present are reviewed, with emphasis on conceptual issues. It is then argued that, nevertheless, the duality can provide valuable insights at both the quantitative and the qualitative levels. This is illustrated with a few examples and concluded with a brief discussion of future prospects.

The discovery at the RHIC of large high-p_T suppression and flow of electrons from heavy quarks has altered our view of the hot and dense matter formed in central Au+Au collisions at 200 GeV. Meanwhile, the nuclear modification of the J/ψ yield has shown weak energy density dependence when comparing measurements at CERN-SPS and at RHIC energies and measurements in different rapidities. PHENIX has carried out a comprehensive study of heavy quarks including a baseline measurement of heavy flavour, J/ψ and ϒ in p+p collisions, and the fractional contribution to J/ψ from ψ' and χ_c decays. The initial state and other cold nuclear matter effects were studied using a large data set collected in d+Au collisions. A modest nuclear modification factor of electrons from heavy flavour decays at mid-rapidity was observed in d+Au and in Cu+Cu collisions. We found a significant suppression of J/ψ in d+Au for all rapidity ranges covered by PHENIX. This suppression is stronger at forward rapidity than at mid-rapidity and the dependence with the nuclear thickness is not linear. A similar suppression trend was also observed for ϒ(1S+2S+3S) but with large statistical uncertainties. A more precise measurement of the J/ψ yield in Au+Au collisions in the forward rapidity is consistent with our previous observation. The centrality dependence of the J/ψ suppression also shows similar behaviour when looking at the data collected in Au+Au collisions at = 62 GeV and GeV, supporting previous comparisons with CERN-SPS data.

We present the first results from the ALICE experiment on the nuclear modification factors for heavy-flavour hadron production in Pb–Pb collisions at TeV. Using proton–proton and lead–lead collision samples at TeV and TeV, respectively, the nuclear modification factors R_AA(p_t) were measured for D mesons at central rapidity (via displaced decay vertex reconstruction), and for electrons and muons, at central and forward rapidity, respectively.

Quarkonia have been studied in different collision systems and energy in order to understand the effects of the hot and dense medium created in heavy-ion collisions. CMS is well suited to measure quarkonia decays to muons given the muon identification and charged particle tracking capability. We report here prompt, non-prompt and ϒ production measured by the CMS experiment in pp collisions at TeV. In addition, the and ϒ production in Pb–Pb at TeV and pp collisions at the same per nucleon energy are measured and compared. Prompt and non-prompt contributions are separated for the first time in heavy-ion collisions, as is the ground from the excited states in the ϒ family. Suppression in Pb–Pb at TeV is quantified for prompt , and ϒ(1S), as well as the relative suppression of ϒ(2S+3S) compared to ϒ(1S).

In this new energy regime, quarkonium provides a unique probe to study the properties of the high-density, strongly interacting system formed in the early stages of high-energy heavy-ion collisions. In ALICE, quarkonium states are reconstructed down to p_T = 0 via their µ⁺µ⁻ decay channel in the muon spectrometer (2.5 ⩽ y ⩽ 4.0) and via their e⁺e⁻ channel in the central barrel (|y| ⩽ 0.9). Measurement of the transverse momentum and rapidity distributions of the inclusive J/ψ production cross section in proton–proton collisions at = 2.76 and 7 TeV will be presented. We will discuss the dependence on charged particle multiplicity of the inclusive J/ψ yield in proton–proton collisions at = 7 TeV. Finally, the analysis of the inclusive J/ψ production in Pb–Pb collisions at = 2.76 TeV will be described. Preliminary results on the nuclear modification factor (R_AA) and the central to peripheral nuclear modification factor (R_CP) will be discussed.

These proceedings summarize my plenary talk at Quark Matter 2011 with a focus on the future perspectives of the low-energy programs at RHIC, FAIR, NICA and CERN.

The LHC brings nuclear collisions to the TeV scale for the first time and the first data show the qualitative differences of this new regime. The corresponding phase space available encompasses completely uncharted regions of QCD in which high-density or high-temperature domains can be identified. Proton–nucleus runs are essential for a complete interpretation of the data and for the study of new regimes dominated by large occupation numbers in the hadronic wavefunction. I comment here on the physics opportunities for p+Pb runs at the LHC and d+Au runs at the RHIC and the corresponding needs in view of the new Pb+Pb data from the LHC.

We discuss open physics problems which pertain to the nuclear structure that could be addressed with the measurements at a dedicated electron–ion machine at high energies. Physics possibilities at the Electron Ion Collider and the Large Hadron electron Collider proposals are briefly presented.

An outlook on physics at the high energy frontier of nucleus–nucleus collisions is presented, on the basis of the new results presented at Quark Matter 2011 by the LHC and RHIC experiments.

Hydrodynamical description of the 'Little Bang' in heavy ion collisions is surprisingly successful: here we systematically study propagation of small perturbations treated hydrodynamically. Using analytic description of the expanding fireball known as the 'Gubser flow', we proceed to linearized equations for perturbations. As all variables are separated and all equations solved (semi)analytically, we can collect all the harmonics and reconstruct the complete Green function of the problem, even in the viscous case. Applying it to the power spectrum, we found acoustic minimum at m = 7 and maximum at m = 9, which remarkably have some evidence for both in the data. We estimate effective viscosity and the size of the perturbation from the fit to power spectrum. The shape of the two-point correlator is also reproduced remarkably well. At the end, we argue that independent perturbations are local, and thus harmonics phases are correlated.

The measurements of charged-particle multiplicity and transverse energy at mid-rapidity in Pb–Pb collisions at $\sqrt{s_{\rm NN}}= 2.76$ TeV are reported as a function of centrality. The fraction of the inelastic cross section recorded by the ALICE detector is estimated using a Glauber model. The results scaled by the number of participating nucleons are compared with pp collisions at the same collision energy, with similar results obtained at significantly lower energies, and with models based on different mechanisms for particle production in nuclear collisions.

Measurements of the charged hadron multiplicity and transverse energy are presented for minimum bias PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair. The number of charged hadrons was obtained by two different methods based on the silicon pixel system of the CMS detector at the LHC. The two methods are in excellent agreement, resulting a charged hadron density of 1612 ± 55 for the 5% most central collisions. For the transverse energy measurement, CMS has almost Hermetic calorimetry coverage with fine granularity and excellent resolution. In addition, for particles near central rapidity, momenta from the tracker can be combined with the calorimeter data to give a significant improvement of the system resolution. A transverse energy density of 2.1 TeV is observed for the most central 2.5% collisions. The measurements are compared with heavy-ion results from earlier experiments, where a smooth dependence on the collision energy and impact parameter is observed.

We present a study of charged particle pseudorapidity density distribution over a broad range of pseudorapidity and centrality in Pb+Pb collisions at TeV with the ATLAS detector at the LHC. The global shapes of multiplicity density distributions over four units of pseudorapidity from −2 to 2 are presented. Centrality and collision energy dependence of mid-rapidity density per participating nucleon pair is shown and compared to results of previous measurements.

The transverse momentum spectra, elliptic flow and interferometry radii for Pb–Pb collisions at the LHC are calculated in the relativistic viscous hydrodynamics. For Glauber model initial conditions, we find that the data can be described using a small value of shear viscosity η/s = 0.08. The viscosities and the equation of state are as used for RHIC data.

The relative rates of single- and double-diffractive processes were measured with the ALICE detector by studying properties of gaps in the pseudorapidity distribution of particles produced in proton–proton collisions at = 0.9, 2.76 and 7 TeV. ALICE triggering efficiencies are determined for various classes of events using a detector simulation validated with data on inclusive particle production. Cross-sections are determined using the van der Meer scans to measure beam properties and obtain a measurement of the luminosity.

With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise () extraction of the quark gluon plasma (QGP) shear viscosity (η/s)_QGP from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with <5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an uncertainty for (η/s)_QGP. It is shown that a simultaneous comparison of elliptic and triangular flow, v₂ and v₃, puts strong constraints on initial state models and can largely eliminate the present uncertainty in (η/s)_QGP. The variation of the differential elliptic flow v₂(p_T) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model.

During the past decade, azimuthal correlation measurements have played a pivotal role in our understanding of the properties of high density QCD matter through their sensitivity to the early stage evolution of relativistic heavy-ion collisions. The CMS experiment has measured the anisotropy parameter, v₂, using four different methods: event-plane, two- and four-particle cumulants, and Lee–Yang zeros. Consistent results are found for the different methods after considering their respective sensitivities to non-flow correlations and event-by-event fluctuations in the initial conditions. The anisotropy is studied as a function of transverse momentum, pseudorapidity and centrality in a broad kinematic range: 0.3 < p_T < 12 GeV/c, |η| < 2.4, and in 12 centrality classes in the range 0–80%.

We report on the first measurements of the elliptic and triangular flow for charged pions, kaons and anti-protons in lead–lead collisions at measured with the ALICE detector at the LHC. We compare the observed mass splitting of differential elliptic flow at LHC energies to RHIC measurements at lower energies and theory predictions. We test the quark coalescence picture with the quark number scaling of elliptic and triangular flow.

Flow coefficients v_n for n = 2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at GeV, are presented. They indicate the expected growth of viscous damping for sound propagation in the quark–gluon plasma (QGP) produced in these collisions. Hydrodynamical model comparisons which include the effects of initial state geometry fluctuations highlight the role of higher harmonics (v_{n, n > 2}) as a constraint for disentangling the effects of viscosity and initial conditions and suggest a small specific viscosity for the QGP.

We present v₂ measurements from Au+Au collisions at = 7.7, 11.5 and 39 GeV for identified hadrons (π, K, K⁰_s, p, ϕ, Λ, Ξ) and light nuclei ( and ³He). For the first time, a significant difference in v₂ between baryons and anti-baryons is observed. The difference increases with decreasing center-of-mass energy. A systematically lower ϕ-meson v₂, compared to the other particles, is observed in the frame of the number-of-constituent quark scaling.

We investigate the effects of a temperature-dependent shear viscosity over entropy density ratio η/s, with a minimum near the phase transition, on the elliptic flow of hadrons in ultrarelativistic heavy-ion collisions at the RHIC and the LHC. We find that the suppression of the elliptic flow in Au+Au collisions at the RHIC is dominated by the viscosity in hadronic matter and in the phase transition region, but insensitive to the viscosity of the quark–gluon plasma (QGP). However, at the highest LHC energy, the elliptic flow becomes sensitive to the shear viscosity of the QGP and insensitive to the hadronic viscosity.

Results on two-particle angular correlations are presented in proton–proton collisions at center of mass energies of 7 TeV, over a broad range of pseudorapidity and azimuthal angle. In very high-multiplicity events at 7 TeV, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate p_T of 1–3 GeV/c, in the kinematic region 2.0 < |Δη| < 4.8 and small Δϕ. This structure, which has not been observed in pp collisions before, is similar to what is known as the 'ridge' in heavy-ion collisions. It is not predicted by commonly used proton–proton Monte Carlo models and is not seen in lower multiplicity pp collisions. Updated studies of this new effect as a function of particle transverse momentum, rapidity and event characteristics are shown.

Measurements of anisotropic flow in heavy-ion collisions provide evidence for the creation of strongly interacting matter which appears to behave as an almost ideal fluid. Anisotropic flow signals the presence of multiple interactions and is very sensitive to the initial spatial anisotropy of the overlap region in non-central heavy-ion collisions. In this paper, we report measurements of elliptic v₂, triangular v₃, quadrangular v₄ and pentagonal v₅ flow. These measurements have been performed with 2- and multi-particle correlation techniques.

High-multiplicity proton–proton collisions at the LHC may exhibit collective phenomena such as elliptic flow. We study this issue using DIPSY, a brand-new Monte Carlo event generator which features almost-NLO BFKL dynamics and describes the transverse shape of the proton including all fluctuations. We predict the eccentricity of the collision as a function of the multiplicity and estimate the magnitude of elliptic flow. We suggest that flow can be signaled by a sign change in the four-particle azimuthal correlation.

Dynamical fluctuations in global conserved quantities such as baryon number, strangeness or charge may be observed near a QCD critical point. Results from new measurements of dynamical K/π, p/π and K/p ratio fluctuations are presented. The commencing of a QCD critical point search at the RHIC has extended the reach of possible measurements of dynamical K/π, p/π and K/p ratio fluctuations from Au+Au collisions to lower energies. The STAR experiment has performed a comprehensive study of the energy dependence of these dynamical fluctuations in Au+Au collisions at the energies = 7.7, 11.5, 39, 62.4 and 200 GeV. New results are compared to previous measurements and to theoretical predictions from several models. The measured dynamical K/π fluctuations are found to be independent of collision energy, while dynamical p/π and K/p fluctuations have a negative value that increases toward zero at top RHIC energy. Fluctuations of the higher moments of conserved quantities (net-proton and net-charge) distributions, which are predicted to be sensitive to the presence of a critical point, are also presented.

Event-by-event fluctuations of the initial transverse density profile result in a collective flow pattern which also fluctuates event by event. We propose a number of new correlation observables to characterize these fluctuations and discuss how they should be analyzed experimentally. We argue that most of these quantities can be measured at the RHIC and the LHC, and compare prediction with recent data.

We present the first measurement of pion source radii in Pb–Pb collisions at the Large Hadron Collider. The radii were obtained by analyzing the Bose–Einstein enhancement in two-pion correlation functions. Like at lower energies, the radii drop with increasing transverse momentum, indicating the presence of collective expansion. In absolute terms, all three radii (R_out, R_side, R_long) are larger than at RHIC, roughly consistent with a linear scaling with the cube root of the particle multiplicity.

Bose–Einstein correlations are measured in samples of charged particles produced in proton–proton collisions at 0.9 and 7 TeV center-of-mass energies, recorded by the CMS experiment at the LHC. The signal is observed in the form of an enhancement of number of pairs of the same-sign charged particles with small relative momentum. The dependence of this enhancement on kinematic and topological features of the event is studied.

Identical neutral kaon pair correlations are measured in 7 TeV pp collisions in the ALICE experiment. K⁰_sK⁰_s correlation functions are formed in 3 multiplicity × 4 k_T bins. The femtoscopic kaon source parameters R_inv and λ are extracted from these correlation functions by fitting a femtoscopy × PYTHIA model to them, PYTHIA accounting for the non-flat baseline found in pp collisions. Source parameters are obtained from a fit which includes quantum statistics and final-state interactions of the a₀/f₀ resonance. K⁰_sK⁰_s correlations show a systematic increase in R_inv for increasing multiplicity bin and decreasing R_inv for increasing k_T bin as seen in ππ correlations in the pp system, as well as seen in heavy-ion collisions. Also K⁰_sK⁰_s correlations are observed to smoothly extend this ππ R_inv behavior for the pp system up to about three times higher k_T than the k_T range measured in ππ correlations.

A study of the energy behavior of the interferometry radii is carried out for RHIC and LHC energies within the hydrokinetic model (HKM). The hydrokinetic predictions for the HBT radii at LHC energies are compared with the recent results of the ALICE Collaboration. The role of the non-equilibrium and non-hydrodynamic stage of the matter evolution in the formation of the femtoscopy scales at LHC energies is analyzed. For this aim, we develop the hybrid HKM.

We present the new results of the Wuppertal-Budapest lattice QCD collaboration on flavor diagonal and non-diagonal quark number susceptibilities with 2+1 staggered quark flavors in a temperature regime between 120 and 400 MeV. A Symanzik improved gauge and a stout-link improved staggered fermion action is utilized; the light and strange quark masses are set to their physical values. Lattices with N_t = 6, 8, 10, 12 are used. We perform a continuum extrapolation of those observables for which the scaling regime is reached, and discretization errors are under control.

The experimental results on heavy ion collisions at the RHIC and LHC indicate that QCD plasma behaves as a nearly perfect fluid described by relativistic hydrodynamics. Hydrodynamics is an effective low-energy theory of everything stating that the response of a system to external perturbations is dictated by conservation laws that are a consequence of the symmetries of the underlying theory. In the case of QCD fluid produced in heavy ion collisions, this theory possesses anomalies, so some of the apparent classical symmetries are broken by quantum effects. Even though the anomalies appear as a result of UV regularization and so look like a short-distance phenomenon, it has been realized recently that they also affect the large-distance macroscopic behavior in hydrodynamics. One of the manifestations of anomalies in relativistic hydrodynamics is the chiral magnetic effect (CME). At this conference, a number of pieces of evidence for the CME have been presented, including (i) the disappearance of charge asymmetry fluctuations in the low-energy RHIC data where the energy density is thought to be below the critical one for deconfinement and (ii) the observation of charge asymmetry fluctuations in Pb–Pb collisions at the LHC. Here is given a summary of some of the recent theoretical and experimental developments and of the future tests that may allow us to establish (or to refute) the CME as the origin of the observed charge asymmetry fluctuations.

We consider the interference pattern for the medium-induced gluon radiation produced by a color singlet quark–antiquark antenna embedded in a QCD medium with size L and 'jet-quenching' parameter . Within the BDMPS-Z regime, we demonstrate that, for a dipole opening angle , the interference between the medium-induced gluon emissions by the quark and the antiquark is suppressed with respect to the direct emissions. This is so since direct emissions are delocalized throughout the medium and thus yield contributions proportional to L while interference occurs only between emissions at early times, when both sources remain coherent. Thus, for , the medium-induced radiation is the sum of the two spectra individually produced by the quark and the antiquark, without coherence effects like angular ordering. For , the medium-induced radiation vanishes.

We report on recent studies of the phenomenon of color decoherence in jets in QCD media. The effect is most clearly observed in the radiation pattern of a quark–antiquark antenna, created in the same quantum state, traversing a dense color deconfined plasma. Multiple scattering with the medium color charges gradually destroys the coherence of the antenna. In the limit of opaque media, this ultimately leads to independent radiation off the antenna constituents. Accordingly, radiation off the total charge vanishes implying a memory loss effect induced by the medium.

We calculate running coupling corrections for the lowest-order gluon production cross section in high energy hadronic and nuclear scattering using the BLM scale-setting prescription. At leading order, there are three powers of fixed coupling; in our final answer, these three couplings are replaced by seven factors of running coupling: five in the numerator and two in the denominator, forming a 'septumvirate' of running couplings, analogous to the 'triumvirate' of running couplings found earlier for the small-x BFKL/BK/JIMWLK evolution equations. It is interesting to note that the two running couplings in the denominator of the 'septumvirate' run with complex-valued momentum scales, which are complex conjugates of each other, such that the production cross section is indeed real. We use our lowest-order result to conjecture how running coupling corrections may enter the full fixed-coupling k_T-factorization formula for gluon production which includes nonlinear small-x evolution.

We calculate the structure functions for DIS off a large nucleus at the NLO accuracy in α_s using the operator product expansion (OPE) at high energy: we calculate the leading order and next-to-leading order (NLO) coefficient function of the OPE (the photon impact factor) and the evolution equation to the NLO of the relative Wilson lines operators. We obtain for the first time an analytic expression in coordinate space of the NLO photon impact factor.

We present STAR measurements of relative charm and bottom contributions to non-photonic electrons (NPE) from p+p collisions at and 500 GeV energies. We report the total bottom quark production cross section from p+p collisions at GeV extracted from the NPE spectrum and B to D ratios. We also present the NPE-hadron azimuthal correlations from Au+Au collisions at GeV from the 2010 RHIC run where we have collected high statistics data set with low photonic conversion background.

Heavy quark production is measured in the single-muon decay channel with the ALICE muon spectrometer. We present the differential production cross section of muons from heavy flavour decay at forward rapidity (2.5 < y < 4) in pp collisions at TeV and comparisons to perturbative QCD predictions. The analysis of the inclusive muon production in Pb–Pb collisions at TeV is described as well, and special attention is given to preliminary results on the nuclear modification factor.

We discuss resonance recombination for quarks and show that it is compatible with quark and hadron distributions in local thermal equilibrium. We then calculate realistic heavy quark phase space distributions in heavy ion collisions using Langevin simulations with non-perturbative T-matrix interactions in hydrodynamic backgrounds. We hadronize the heavy quarks on the critical hypersurface given by hydrodynamics after constructing a criterion for the relative recombination and fragmentation contributions. We discuss the influence of recombination and flow on the resulting heavy meson and single electron R_AA and elliptic flow. We will also comment on the effect of diffusion of open heavy flavor mesons in the hadronic phase.

Electron spectra measured with ALICE at mid-rapidity are used to study the production of hadrons carrying a charm or a beauty quark. The production cross section of electrons from heavy-flavour hadron decays is measured in pp collisions at = 7 TeV. Electrons from the beauty decays are identified via the displacement from the interaction vertex. From the electron spectra measured in Pb–Pb collisions, we determine the nuclear modification factor, which is sensitive to the heavy-quark energy loss in a hot strongly interacting medium.

We analyze the low-frequency part of charmonium spectral functions on large lattices close to the continuum limit in the temperature region 1.5 ≲ T/T_c ≲ 3 as well as for T ≃ 0.75T_c. We present evidence for the existence of a transport peak above T_c and its absence below T_c. The heavy quark diffusion constant is then estimated using the Kubo formula. As part of the calculation we also determine the temperature dependence of the signature for the charmonium bound state in the spectral function and discuss the fate of charmonium states in the hot medium.

The LHC centre-of-mass energy allows copious ϒ production in PbPb collisions. Detailed measurements of bottomonium production will help characterize the dense matter produced in heavy-ion collisions. The full spectroscopy of quarkonium states has been suggested as a possible thermometer for the quark–gluon plasma. CMS is well suited to measure ϒ decays to muons given the muon identification and charged particle tracking capability. Using muons of transverse momentum above 4 GeV/c and pseudorapidity below 2.4, the double ratio of the ϒ(2S + 3S) excited states to the ϒ(1S) ground state in PbPb and pp collisions, [ϒ(2S + 3S)/ϒ(1S)]_PbPb/[ϒ(2S + 3S)/ϒ(1S)]_pp, is found to be 0.31^+0.19_{− 0.15}(stat.) ± 0.03(syst.). The probability of obtaining the measured value, or lower, if the true double ratio is unity, is calculated to be less than 1%. ϒ(1S) suppression in PbPb at $\sqrt{s_{NN}}=2.76$ TeV is also presented in this proceeding.

We present the observation of the nucleus, the heaviest antinucleus observed to date. In total, 18 counts were detected at the STAR experiment at the RHIC in 10⁹ recorded Au+Au collisions at beam energies of = 200 and 62 GeV. The background has been estimated, and the misidentification probability is found to be lower than 10⁻¹¹.

The particle identification capabilities of the ALICE experiment are unique among the four major LHC experiments. The working principles and excellent performance of the central barrel detectors in a high-multiplicity environment are presented as well as two physics examples: the extraction of transverse momentum spectra of charged pions, kaons, protons, and the observation of the nucleus.

Results of the measurement of the π, K, p transverse momentum (p_t) spectra at mid-rapidity in proton–proton collisions at TeV are presented. Particle identification was performed using the energy loss signal in the inner tracking system and the time projection chamber, while information from the time-of-flight detector was used to identify particles at higher transverse momentum. From the spectra at TeV, the mean transverse momentum (⟨p_t⟩) and particle ratios were extracted and compared to results obtained for collisions at TeV and lower energies.

We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS energies, employing the hybrid version of UrQMD which models hadronization from a hydrodynamic phase by the Cooper–Frye mechanism, and matches to a final hadron-resonance cascade. We fit the results both before and after the cascade stage using the statistical hadronization model to assess the effect of the cascade phase. We observe a strong effect on antibaryon yields except anti-Ω, resulting in a shift in T and μ_B. We discuss the implications for the freeze-out curve.

Invariant cross sections for neutral pions and η mesons in pp collisions at , 2.76 and 7 TeV were measured by the ALICE detector at the LHC. Next-to-leading order perturbative QCD calculations describe the π⁰ and η spectra at 0.9 TeV, but overestimate the measured cross sections at 2.76 and 7 TeV. The measured η/π⁰ ratio is consistent with m_T scaling at 2.76 TeV. At 7 TeV, indications for a violation of m_T scaling were found.

The study of resonance production in pp collisions helps to understand hadronization mechanisms and tuning of the QCD-inspired particle production models. In Pb–Pb collisions, resonances allow one to probe the temperature and time evolution of the fireball. Transverse momentum spectra have been analyzed for K*(892)⁰, φ(1020) and Ξ(1530)⁰ resonances using data from pp collisions at 7 TeV collected by the ALICE detector. A comparison with Monte Carlo event generators shows different levels of agreement for meson spectra, while Ξ(1530)⁰ is always underestimated.

We present the study of K⁰_S and Λ production performed with the ALICE experiment at the LHC in Pb–Pb collisions at TeV and pp collisions at and 7 TeV. The K⁰_S and Λ particles are reconstructed via their V0 decay topology allowing their identification up to high transverse momenta. The corresponding baryon/meson ratios as a function of transverse momentum are extracted for Pb–Pb collisions in centrality bins and in the transverse momentum range from 1 to 6 GeV/c. They are also compared with those measured in pp events at the LHC energies of 0.9 and 7 TeV as well as in Au–Au collisions at and 200 GeV from RHIC.

The production of charged multi-strange particles is studied with the ALICE experiment at the CERN LHC. Measurements of the central rapidity yields of Ξ⁻ and Ω⁻ baryons, as well as their antiparticles, are presented as a function of transverse momentum (p_t) for inelastic pp collisions at TeV and compared to existing measurements performed at the same and/or at lower energies. The results are also compared to predictions from two different tunes of the PYTHIA event generator. We find that data significantly exceed the production rates from those models. Finally, we present the status of the multi-strange particle production studies in Pb–Pb at TeV performed as a function of collision centrality.

The nuclear modification factors R_AA and R_CP of Λ and K⁰_s in Pb–Pb collisions at = 2.76 TeV measured by ALICE at the LHC are presented. In central collisions, a strong suppression at high p_T (p_T > 8 GeV/c) with respect to pp collisions is observed. The p_T region below is dominated by an enhancement of Λ over the suppressed K⁰_s. The results are compared to those of charged hadrons and to Λ from lower collision energies.

We investigate the production of hadrons in nuclear collisions within the framework of the thermal (or statistical hadronization) model. We discuss the light-quark hadrons as well as charmonium and provide predictions for the LHC energy. Even as its exact magnitude is dependent on the charm production cross section, not yet measured in Pb–Pb collisions, we can confidently predict that at the LHC the nuclear modification factor of charmonium as a function of centrality is larger than that observed at RHIC and compare the experimental results to these predictions.

The PHENIX experiment at the RHIC has measured various light mesons using multiple decay channels over a wide range of transverse momenta. In these proceedings, we present a review of the most recent results on the production of ω, ϕ, K*, K_s in p+p collisions and their nuclear modification factors in d+Au, Cu+Cu and Au+Au collisions at 200 GeV. Results of ϕ production at = 62.4 GeV are also discussed.

Vector mesons are key probes of the hot and dense state of strongly interacting matter produced in heavy-ion collisions. Their dileptonic decay channel is particularly suitable for these studies, since dileptons have negligible final state interactions in QCD matter. A preliminary measurement of the ϕ and ω differential cross sections was performed by the ALICE experiment in pp collisions at TeV, through their decay in muon pairs. The p_T and rapidity regions covered in this analysis are p_T > 1 GeV/c and 2.5 < y < 4.

Jets are an important tool to probe the hot, dense medium that is produced in ultra-relativistic heavy ion collisions. The large collision energy at the LHC provides copious production of dijets with energies that can be cleanly identified above the heavy ion background. The multipurpose CMS detector is well designed to measure these hard scattering processes with its high resolution calorimeters and high precision silicon tracker. Jet quenching is observed by a significant imbalance of dijet transverse momentum in PbPb collisions at TeV. The fraction of unbalanced dijets is found to increase strongly with increasing collision centrality. The dijet imbalance persists to the highest jet momenta studied. The redistribution of the quenched jet energy is studied using the transverse momentum balance of charged tracks projected onto the direction of the leading jet. In contrast to pp collisions, a large fraction of the momentum balance for asymmetric jets is found to be carried by low momentum particles at large angular distance from the jet axis.

The first results of single jet observables in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}= 2.76$ TeV measured with the ATLAS detector at the LHC are presented. Full jets are reconstructed with the anti-k_t algorithm with R = 0.2 and 0.4, using an event-by-event subtraction procedure to correct for the effects of the underlying event including elliptic flow. The geometrically-scaled ratio of jet yields in central and peripheral events, R_CP, indicates a clear suppression of jets with E_T > 100 GeV. The transverse and longitudinal distributions of jet fragments are also presented. We find little or no substantial change to the fragmentation properties and no significant change in the level of suppression when moving to the larger jet definition.

The strong modifications of dijet properties in heavy-ion collisions measured by ATLAS and CMS provide important constraints on the dynamical mechanisms underlying jet quenching. In this work, we show that the transport of soft gluons away from the jet cone—jet collimation—can account for the observed dijet asymmetry with values of that lie in the expected order of magnitude. Further, we show that the energy loss attained through this mechanism results in a very mild distortion of the azimuthal angle dijet distribution.

We present results and predictions at next-to-leading (NLO) order for the recent LHC lead–lead run at a center-of-mass energy of 2.76 TeV per nucleon–nucleon pair. Specifically, we focus on the suppression of the single and double inclusive jet cross sections and demonstrate how the di-jet asymmetry, recently measured by ATLAS and CMS, can be extracted from this general result. The case of jets tagged by an electroweak boson is exemplified by the Z⁰+jet channel. We predict a signature transition from enhancement to suppression of the tagged jet related to the medium-induced modification of the parton shower. Finally, we clarify the relation between the suppression of inclusive jets, tagged jets and di-jets and the quenching of inclusive particles on the example of the recent ALICE charged hadron attenuation data.

For a quantitative interpretation of reconstructed jet properties in heavy-ion collisions, it is paramount to characterize the contribution from the underlying event and the influence of background fluctuations on the jet signal. In addition to the pure number fluctuations, region-to-region correlated background within one event can enhance or deplete locally the level of background and modify the jet energy. We show the first detailed assessment of background effects using different probes embedded into heavy-ion data and quantify their influence on the reconstructed jet spectrum.

Despite a wealth of experimental data for high-P_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. A key reason is that the pQCD parton spectrum at RHIC is falling so steeply that distinguishing even a moderate shift in parton energy from complete parton absorption is essentially impossible. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and, as a result, at RHIC energies only the pathlength dependence of energy loss offers some discriminating power. At LHC, however, this is no longer the case. Due to the much flatter shape of the parton p_T spectra originating from 2.76 A GeV collisions, the available data probe much deeper into the model dynamics. A simultaneous fit of the nuclear suppression at both RHIC and LHC energies thus has great potential for discriminating between various models that yield equally good descriptions of RHIC data alone.

Fully reconstructed jets and direct photon-tagged jet fragments significantly reduce energy-loss bias, the bias toward mostly measuring particles from partons which suffer little energy loss. In d+Au collisions, one accesses the physics at large x, which yields important constraints for nuclear parton distribution functions. In both d+Au and A+A collisions, coherent multiple-scattering models of energy loss can be tested. In this contribution, we present the current results from the PHENIX experiment on fully reconstructed jets and direct γ-hadron correlations. Baseline measurements of jets in p+p collisions as well as their yield and correlation modifications in d+Au and Cu+Cu will be given. From γ-hadron correlations, we present the fragmentation function in p+p and Au+Au collisions and its modification in Au+Au to z_T lower than what has previously been studied. Implications of these data on our understanding of both cold and hot, dense nuclear matter created at the RHIC are discussed.

Angular correlations between trigger (t) and associated (a) hadrons are measured by the ALICE experiment for 0.5 < p^{t, a}_T < 15 GeV, where p^t_T ⩾ p_T^a. The azimuthal pair correlation shapes are examined in a variety of centrality categories for pairs in |η| < 1.0 where |η^t − η^a| > 0.8. A series of two-particle Fourier components V_nΔ ≡ ⟨cos (nΔϕ)⟩ are extracted from the long-range azimuthal correlation functions. The sum of n < 6 terms matches the data. For each n, a fit is applied over all p_T bins simultaneously to test the collectivity hypothesis V_nΔ ≃ v^t_n v^a_n. The factorization holds at p^{t, a}_T below approximately 4 GeV but breaks progressively at higher momenta. The divergence between the data and the global fit quantifies the onset of nonflow dominance in long-range correlations due to the away side jet. The v_n values from the global fit are in close agreement with results from more established methods. At higher p_T where jet correlations dominate, the modification of conditional yields in central Pb–Pb collisions is measured with respect to pp (I_AA) and with respect to peripheral events (I_CP). Significant suppression is observed on the side opposing the trigger, while a moderate enhancement is measured on the near side.

Measurements of charged dihadron Δη–Δϕ correlations from the CMS collaboration are presented for Pb–Pb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair over a broad range of pseudorapidity and the full range of azimuthal angle. A significant correlated yield is observed for pairs of particles with small Δϕ but large longitudinal separation Δη, commonly known as the 'ridge'. The ridge persists up to at least |Δη| = 4 and the dependence of the ridge region shape and yield on collision centrality and transverse momentum has been measured. A Fourier analysis of the long-range two-particle correlation is presented and discussed in the context of higher order flow coefficients.

We present measurements of untriggered di-hadron correlations as a function of centrality in Pb–Pb $\sqrt{s_{{\rm NN}}} = 2.76$ TeV collisions, for charged hadrons with p_T > 0.15 GeV/c. These measurements provide a map of the bulk correlation structures in heavy-ion collisions. Contributions to these structures may come from jets, initial density fluctuations, elliptic flow, resonances and/or momentum conservation. We decompose the measured correlation functions via a multi-parameter fit in order to extract the nearside Gaussian, the longer range Δη correlation often referred to as the soft ridge. The effect of including higher harmonics (v₃ and v₄) in this procedure will be discussed. We investigate how the nearside Gaussian scales with the number of binary collisions. Finally, we show the charge dependence of the nearside Gaussian.

Two-particle correlation triggered by high-p_T particles allows us to study hard scattering phenomena when full jet reconstruction is challenging. An analysis of the first ALICE pp data where charged and neutral particles isolated or not are used as trigger particles is presented. The two-particle correlation between the trigger (t) and the associate (a) particles is studied as a function of the imbalance parameter and interpreted in terms of the jet fragmentation function.

Non-statistical event-by-event fluctuations of the mean transverse momentum of charged particles in pp and Pb–Pb collisions are studied using the ALICE experiment at the LHC. Little collision energy dependence is observed in pp. The data indicate a common scaling behaviour with event multiplicity from pp to semi-central Pb–Pb collisions. In central Pb–Pb, the results deviate from this trend, exhibiting a significant reduction of the fluctuation strength. The results are compared with measurements in Au–Au collisions at lower energies and with Monte Carlo simulations.

Event-by-event fluctuations of produced particle multiplicities are believed to be sensitive to a deconfinement phase transition and the critical point of strongly interacting matter. The NA49 collaboration has conducted a systematic study of various fluctuation observables. In this contribution, recent results on hadron ratio fluctuations in central Pb+Pb collisions at –17.3 GeV are reported. These results are complemented by the centrality dependence of hadron ratio fluctuations at . A universal scaling was found, describing the energy and centrality dependence of the (K⁺ + K⁻)/(π⁺ + π⁻) and ratio fluctuations purely by a change in the observed average hadron multiplicities. This scaling is broken for the fluctuations of the and K⁺/p ratios, possibly hinting at a change in the baryon number–strangeness correlation.

Dipole, triangular and higher harmonic flows that have an origin in the initial density fluctuations have gained a lot of attention as they can provide additional important information about the dynamical properties (e.g. viscosity) of the system. The fluctuations in the initial geometry should also be reflected in the detailed shape and velocity field of the system at freeze-out. In this talk, I discuss the possibility of measuring such fluctuations by means of identical and non-identical particle interferometry.

We discuss universal properties of higher order cumulants of net baryon number fluctuations and point out their relevance for the analysis of freeze-out and critical conditions in heavy-ion collisions at the LHC and RHIC.

We present results on the chiral and deconfinement properties of the QCD transition at finite temperature. We performed calculations using asqtad and HISQ/tree actions using lattices with temporal extent N_τ = 6, 8 and 12 allowing us to control the approach to the continuum limit. We analyze the chiral transition in terms of universal O(N) scaling functions. From peaks in the scaling functions we perform a simultaneous continuum extrapolation for HISQ/tree and asqtad to derive the critical temperature, T_c = 157 ± 6 MeV.

The QCD transition is studied on lattices up to N_t = 16. The chiral condensate is presented as a function of the temperature, and the corresponding transition temperature is extracted. The equation of state is determined on lattices with N_t = 6, 8, 10 and at some temperature values with N_t = 12. The pressure and the trace anomaly are presented as functions of the temperature in the range 100–1000 MeV. Using the same configurations, we determine the continuum extrapolated phase diagram of QCD on the μ–T plane for small to moderate chemical potentials. Two transition lines are defined with two quantities, the chiral condensate and the strange quark number susceptibility.

We consider the Polyakov loop-extended two-flavor chiral quark–meson model and discuss critical phenomena related to the spontaneous breaking of the chiral symmetry. The model is explored beyond the mean-field approximation in the framework of the functional renormalization group. We discuss properties of the net-quark number density fluctuations as well as their higher cumulants. We show that with the increasing net-quark number density, the higher order cumulants exhibit a strong sensitivity to the chiral crossover transition. We discuss their role as probes of the chiral phase transition in heavy-ion collisions at RHIC and LHC.

We employ the lattice QCD data on Taylor expansion coefficients to extend our previous parametrization of the equation of state to finite baryon density. When we take into account lattice spacing and quark mass dependence of the hadron masses, the Taylor coefficients at low temperature are equal to those of hadron resonance gas. Thus, the equation of state is smoothly connected to the hadron resonance gas equation of state at low temperatures. We also show how the elliptic flow is affected by this equation of state at the maximum SPS energy.

Recent results of a next-to-next-to-leading order hard-thermal-loop perturbation theory (HTLpt) calculation of the QCD thermodynamics are revisited. The focus is on the trace anomaly scaled by T² for pure-glue and N_f = 3 QCD. The comparison to available lattice data suggests that for pure-glue QCD agreement between HTLpt and lattice data for the trace anomaly begins at temperatures above 8 T_c while when including quarks (N_f = 3) agreement begins already at temperatures above 2 T_c. The results in both cases indicate that at very high temperatures the T²-scaled trace anomaly increases with temperature in accordance with the predictions of HTLpt.

The compact muon solenoid (CMS) is very well equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus–nucleus collisions. Such probes are especially relevant for studying the quark–gluon plasma; since they are produced at early times, they are experiencing the full evolution of the medium. The CMS is able to distinguish non-prompt from prompt J/ψ in pp and Pb–Pb collisions. We report here the nuclear modification factor of prompt J/ψ in Pb–Pb as a function of transverse momentum, rapidity and the number of nucleons participating in the collision.

We present results from the ALICE experiment on the inclusive J/ψ production in pp collisions at = 2.76 and 7 TeV. The integrated and differential cross sections are evaluated down to p_T = 0 in two rapidity ranges, |y| < 0.9 and 2.5 < y < 4, in the dielectron and dimuon decay channel, respectively. The measurement at = 2.76 TeV, the same energy as Pb–Pb collisions, provides a crucial reference for the study of hot nuclear matter effects on J/ψ production. The J/ψ yield in pp collisions at = 7 TeV has also been studied as a function of the charged particle multiplicity and first results are presented.

J/ψ p_T spectra in 200 GeV p+p and Au+Au collisions at STAR within the p_T range of 3–10 GeV/c are presented. Nuclear modification factor of high-p_T J/ψ is found to be consistent with no suppression in peripheral Au+Au collisions and significantly smaller than unity in central Au+Au collisions. The J/ψ elliptic flow is measured to be consistent with no flow at p_T < 10 GeV/c in 20–60% Au+Au collisions.

Charmonia suppression is one of the highly cited signatures of quark–gluon plasma (QGP) formation in relativistic heavy-ion collisions (Matsui and Satz 1986 Phys. Lett.178 416). PHENIX observed a large suppression of J/Ψ production in Au+Au collisions at GeV. This suppression is in fact similar to that observed at lower energies at CERN-SPS and there is currently no theoretical consensus explaining both data sets in terms of cold nuclear matter effects, hot nuclear matter suppression and possible regeneration or coalescence mechanisms. In order to separate these effects the PHENIX collaboration also measured J/Ψ production in d+Au collisions at GeV and Au+Au collisions at low energies ( and 39 GeV) to explore the energy dependence of the suppression in an attempt to disentangle the important contributions to the final yield.

We study the charmonium spectral function in the gluon plasma on 64³ × N_τ quenched anisotropic lattices with anisotropy a_σ/a_τ = 4, analyzing correlation functions of charmonia by the maximum entropy method. We focus on finite momentum effects on the charmonium spectral function in order to understand the J/ψ suppression mechanism in relativistic heavy ion collisions from the first principles of QCD.

We study J/ψ production in pp collisions at and 7 TeV using the colour-singlet model (CSM), including next-to-leading order (NLO) corrections and dominant α⁵_S contributions (NNLO^⋆). We find that the CSM reproduces the existing data if the upper range of the NNLO^⋆ is near the actual—but presently unknown—NNLO. The direct yield polarization for the NLO and NNLO^⋆ is increasingly longitudinal in the helicity frame when P_T gets larger. When one combines the direct yield with a data-driven range for the polarization of J/ψ from χ_c, the prompt J/ψ polarization yield polarization in the CSM gets significantly closer to the experimental data from the CDF collaboration.

In the ALICE experiment, at forward rapidity (2.5 < y < 4), the production of heavy quarkonium states is measured via their μ⁺μ⁻ decay channels. We present the first measurement of inclusive J/ψ production, down to p_T = 0, from Pb–Pb data collected at the LHC at TeV. Preliminary results on the nuclear modification factor (R_AA) and the central to peripheral nuclear modification factor (R_CP) show J/ψ suppression with no significant centrality dependence and an integrated .

Transverse momentum (p_T) spectra of charged particles are measured as a function of event centrality in Pb−Pb collisions at TeV with ALICE at the LHC. The spectra are compared to those measured in pp collisions at the same collision energy in terms of the nuclear modification factor R_AA. The high-p_T charge particle production in central Pb−Pb collisions (0–5%) is strongly suppressed by a factor ≈6 at transverse momenta p_T = 6–7 GeV/c as compared to expectation from independent superposition of nucleon–nucleon collisions. Above p_T = 7 GeV/c there is a significant rise in the nuclear modification factor, which reaches R_AA ≈ 0.4 at p_T = 50 GeV/c. The measured suppression of high-p_T particles is stronger than that measured at the RHIC.

The ATLAS experiment at the LHC measures the charged particle spectra and the nuclear modification factor in Pb+Pb collisions at the TeV in a transverse momentum range up to 30 GeV and a pseudorapidity range up to |η| < 2.5. The measurement reveals the strong suppression of charged hadron production in the most central collisions at a p_T of about 7 GeV. A suppression of more than a factor of 2 is also measured at the upper edge of the analyzed p_T range. The suppression does not show any strong η dependence.

We compare fully perturbative and fully nonperturbative pictures of high-p_T energy loss calculations to the first results from the LHC. While oversuppressed compared to published ALICE data, parameter-free pQCD predictions based on the WHDG energy loss model constrained to RHIC data simultaneously describe well the preliminary CMS hadron suppression, ATLAS charged hadron v₂, and ALICE D meson suppression; we also provide for future reference WHDG predictions for B meson R_AA. However, energy loss calculations based on AdS/CFT also qualitatively describe well the RHIC pion and non-photonic electron suppression and LHC charged hadron suppression. We propose the double-ratio of charm to bottom quark R_AA as a qualitative distinguisher between these two energy loss pictures.

The nuclear modification factor R_AA(p_T) for large p_T hadrons in central Pb + Pb collisions at TeV/n is calculated within the next-to-leading order perturbative QCD parton model with medium-modified fragmentation functions and agree well with the new data. The jet transport parameter that controls medium modification is assumed to be proportional to the initial parton density and the coefficient is fixed by the RHIC data. The charged hadron multiplicity dN_ch/dη = 1584 ± 80 in central Pb + Pb collisions from the ALICE experiment at the LHC is used to determine both the jet transport parameter and the initial condition for (3+1)D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of R_AA(p_T).

The transverse momentum (p_T) spectra of charged particles is measured by CMS as a function of collision centrality in PbPb collisions at = 2.76 TeV. The results are compared to a pp reference spectrum, constructed by interpolation between = 0.9 and 7 TeV measurements. The nuclear modification factor (R_AA) is constructed by dividing the PbPb p_T spectrum, normalized to the number of binary collisions (N_coll), by the pp spectrum. Measured R_AA in 0–5% centrality bin is compared to several theoretical predictions.

The ALICE Collaboration at the LHC has measured the transverse momentum spectra of neutral pions via their two-photon decay in pp and Pb−Pb collisions at TeV over a broad transverse momentum range with different subsystems: with the electromagnetic calorimeters PHOS and EMCAL and with photon conversions in the inner material of the detectors using e⁺e⁻ pairs reconstructed with the central tracking system. In this paper, neutral pion production is compared between pp and Pb−Pb collisions measured with conversion photons in terms of the nuclear modification factor, R_AA, for different centrality selections of the Pb−Pb data sample.

The suppression of high-p_T single-hadron spectra in heavy-ion collisions is usually interpreted as due to parton energy loss of high-momentum quarks and gluons propagating in the plasma. Here, we discuss to what extent this partonic picture must be complemented by a picture of medium-modified hadronization. In particular, we show how color exchange with the medium modifies the properties of color-singlet clusters arising from the parton branchings, producing a softening of the hadron spectra.

The influence of the damping of radiation on the radiative energy loss spectrum of a relativistic charge in an infinite, absorptive plasma is studied. We find increasing reduction of the spectrum with increasing damping. Our studies, which represent an Abelian approximation for the colour charge dynamics in the quark–gluon plasma, may influence the analysis of jet quenching phenomena observed in high-energy nuclear collisions. Here, we focus on a formal discussion of the limiting behaviour with increasing radiation frequency. In an absorptive (and polarizable) medium, this is determined by the behaviour of the exponential damping factor entering the spectrum and the formation time of radiation.

We summarize recent significant progress in the development of a first-principle formalism to describe the formation and evolution of matter in very high energy heavy ion collisions. The key role of quantum fluctuations both before and after a collision is emphasized. Systematic computations are now feasible to address early time dynamics essential for quantifying properties of strongly interacting quark–gluon matter.

We present measurements by the PHENIX experiment at the RHIC of di-hadron pair production in d +Au collisions where the particles in the pair are varied across a wide range of pseudorapidity, out to η = 3.8. With di-hadrons, varying the p_T and rapidity of the particles in the di-hadron pair allows studying any effects as a function of partonic x in the nucleus. These di-hadron measurements might probe down to parton momentum fractions x ∼ 10⁻³ in the gold nucleus, where the interesting possibility of observing gluon saturation effects at the RHIC is the greatest. Our measurements show that the correlated yield of back-to-back pairs in d +Au collisions is suppressed by up to an order of magnitude relative to p+p collisions, and increases with greater nuclear path thickness and with a selection for lower x in the Au nucleus.

Triangular flow is a new observable in heavy ion collisions that provides interesting information about the structure of the initial state that enters (viscous) hydrodynamic simulations. The longitudinal correlations of the initial state triangularity is investigated to evaluate the potential to distinguish different energy deposition scenarios. We find that radiation processes in a parton cascade introduce similar longitudinal structures as the string fragmentation processes in a hadron cascade. The weak centrality dependence of triangular flow is reproduced within an event-by-event hybrid approach and confirms its sensitivity to initial state fluctuations. The transverse momentum-dependent triangular flow of identified particles at LHC energies is predicted and shows a mass splitting similar to the one observed for elliptic flow.

A unified description of the near-side and away-side structures observed in heavy-ion collisions in two-particle correlations as a function of Δη − Δϕ is proposed for low to moderate transverse momentum. It is based on the combined effect of tubular initial conditions and hydrodynamical expansion.

We present a global analysis of available data on inclusive structure functions measured in electron–proton scattering at small values of Bjorken-x, including the latest data from the combined HERA analysis on reduced cross sections. Further, we discuss the kinematical domain where significant deviations from NLO-DGLAP should be expected and the ability of nonlinear physics to account for such deviations.

In order to describe forward hadron production in high-energy nuclear collisions, we propose a Monte Carlo implementation of the Dumitru–Hayashigaki–Jalilian-Marian formula with the unintegrated gluon distribution obtained numerically from the running-coupling BK equation. We discuss the influence of initial conditions for the BK equation by comparing a model constrained by a global fit of small-x HERA data and a newly proposed one from the 'running-coupling MV model'.

The probing of nuclei and nucleons via deep-inelastic and diffractive processes in the high-energy (low-x) regime will open a new window for the investigation of the gluonic structure of matter. Studies of ep collisions at HERA and especially dAu collisions at the RHIC have found tantalizing hints of saturated gluon densities. Unveiling the collective behavior of densely packed gluons under conditions where their self-interactions dominate will require an electron–ion collider, a new facility with capabilities well beyond those of any existing accelerator.

I show the possibilities for electron–ion studies offered by the proposed electron–hadron collider at CERN, the Large Hadron–electron Collider. I first focus on small-x aspects: inclusive measurements and the determination of parton densities, and diffraction. I end by discussing some possibilities for final state studies.

The LHC with its unprecedented energy offers unique opportunities for groundbreaking measurements in p+p, p+A and A+A collisions even beyond the baseline experimental designs. ALICE is setting up a program of detector upgrades, which could to a large extent be installed in the LHC shutdown planned for 2017/18, to address the new scientific challenges. We will discuss examples of the scientific frontiers and will present the corresponding upgrade projects under study for the ALICE experiment.

The first decade of Relativistic Heavy Ion Collider (RHIC) physics and the first heavy ion running at the Large Hadron Collider (LHC) have produced a wealth of data and discoveries. It is timely to now evaluate what has been learned and ask what compelling new questions have been raised. In this talk, several key unanswered questions about the properties of the strongly coupled quark gluon plasma and the distribution of partons inside nucleons and nuclei will be discussed along with how they can be addressed experimentally. The PHENIX Collaboration has developed a plan for upgrading the experiment in order to address these new questions. The current status of these plans will be presented.

The STAR Collaboration has identified eight key questions that will drive RHIC science during the coming decade, six of which involve ultrarelativistic heavy-ion collisions. The STAR detector, with its large acceptance tracking, calorimetry and particle identification, is an ideal tool to explore these questions. Complete answers nonetheless will require detector upgrades. Near-term upgrades, including the Heavy Flavor Tracker and the Muon Telescope Detector, will enable a rich program of heavy flavor physics. Upgrades to the forward region will enable detailed studies of the partonic structure of nuclei and the onset of gluon saturation. An additional suite of upgrades during the latter part of the decade will position STAR to make crucial measurements in e+p and e+A collisions during the early phase of the eRHIC. This paper provides an overview of the STAR decadal plan.

Measurements of the cross sections of the reference processes seen by the ALICE trigger system were obtained based on beam properties measured from van der Meer (vdM) scans. The measurements are essential for absolute cross-section determinations of physics processes. The paper focuses on instrumental and technical aspects of detectors and accelerators, including a description of the extraction of beam properties from the vdM scan. As a result, cross sections of reference processes seen by the ALICE trigger system are given for proton–proton collisions at two energies, 2.76 and 7 TeV, together with systematic uncertainties originating from beam intensity measurements and other detector effects. Consistency checks were performed by comparing to data from other experiments in the LHC.

The unprecedented center-of-mass energy available at the LHC offers unique opportunities for studying the properties of the strongly interacting QCD matter created in PbPb collisions at extreme temperatures and very low parton momentum fractions. Electroweak boson production is an important benchmark process at hadron colliders. Precise measurements of Z production in heavy-ion collisions can help to constrain nuclear PDFs as well as serve as a standard candle of the initial state in PbPb collisions at the LHC energies. The inclusive and differential measurements of the Z boson yield in the muon decay channel will be presented, establishing that no modification is observed with respect to next-to-leading order pQCD calculations, scaled by the number of incoherent nucleon–nucleon collisions. The status of the Z measurement in the electron decay channel, as well as the first observation of W → μν in heavy ion collisions will be given. The heavy-ion results will be presented in the context of those obtained in pp collisions with the CMS detector.

We report on the first measurements of J/ψ, W and Z boson production in lead–lead collisions measured by the ATLAS detector at the nucleon–nucleon centre of mass energy TeV. These measurements are performed via the J/ψ and electroweak boson decays to muon final states. We present a measurement of W boson yields produced in heavy-ion collisions as function of centrality, ratios of W⁺/W⁻ and W/Z. For a subset of the W → μν decays, we report on muon rapidity and charge asymmetry measurements. These measurements are the first ATLAS results on W measurements in lead–lead collisions at the LHC.

We present the first STAR dielectron measurement in 200 GeV p+p and Au+Au collisions. Results are compared to hadron decay cocktails to search for vector meson in-medium modification in the low mass region and quark–gluon plasma thermal radiation in the intermediate mass region. The ω → e⁺e⁻ spectra and the transverse mass distribution in the intermediate mass region are also discussed.

The PHENIX experiment was upgraded with the Hadron Blind Detector (HBD) for the measurement of low-mass electron pairs. The HBD reduces the combinatorial background by providing additional electron identification in a field-free region close to the vertex and by exploiting the small opening angle of the π⁰ Dalitz and conversion pairs. The HBD was successfully operated during Run 9 and Run 10 at the RHIC in the measurements of e⁺e⁻ pairs in p+p and Au+Au collisions, respectively. This paper reports on the present status of the dielectron analysis in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV.

Event-by-event fluctuations of initial QCD-matter density produced in heavy-ion collisions at the RHIC enhance the production of thermal photons significantly in the region 2 ⩽ p_T ⩽ 4 GeV/c compared to a smooth initial-state averaged profile in the ideal hydrodynamic calculation. This enhancement is an early time effect due to the presence of hotspots or over-dense regions in the fluctuating initial state. The effect of fluctuations is found to be stronger in peripheral than in central collisions.

PHENIX results on direct photon production in p+p, d+Au, Cu+Cu and Au+Au collisions are presented. Direct photon yield at high p_T is consistent with NLO pQCD calculations in all systems. In heavy ion collisions at low p_T direct photons exhibit an order of magnitude exponential shape enhancement consistent with thermal emission from quark gluon plasma (QGP). The thermal nature of this enhancement is further confirmed by the large elliptic flow of direct photons below p_T ∼ 5 GeV/c.

We study real photons produced in heavy-ion collisions at the RHIC, and we calculate their spectrum and its azimuthal momentum anisotropy. The photons from a variety of sources are included, and the interplay and the time evolution of those sources are modelled in a full 3D hydrodynamic simulation. We quantify the v₂ of thermal photons produced in ideal and viscous fluids, and the consequences of using different initial conditions are explored.

The ALICE experiment has measured the D meson production in pp and Pb–Pb collisions at the LHC at and 2.76 TeV and respectively, via the exclusive reconstruction of hadronic decay channels. The analyses of the D⁰ → K⁻π⁺ and D⁺ → K⁻π⁺π⁺ channels will be described and the preliminary results for the D⁰ and D⁺ nuclear modification factor will be presented.

The medium-induced one-gluon radiation spectrum off a massive quark–antiquark () antenna traversing a colored QCD medium is calculated in this contribution. The gluon spectrum off the antenna computed at first order in the opacity expansion is collinear finite but infrared divergent, which is different from the result obtained from an independent emitter which is both infrared and collinear finite. The interference between emitters dominates the soft gluon radiation when the antenna opening angle is small and the emitted gluon is soft, whereas the antenna behaves like a superposition of independent emitters when the opening angle is large and the radiated gluon is hard. As a phenomenological consequence, we investigate the energy lost by the projectiles due to the radiation. In general, the size of the mass effects is similar in both cases.

Throughout the history of the RHIC physics program, questions concerning the dynamics of heavy quarks have generated much experimental and theoretical investigation. A major focus of the PHENIX experiment is the measurement of these quarks through their semi-leptonic decay channels at mid and forward rapidity. Heavy quark measurements in p+p collisions give information on the production of heavy flavor, without complications from medium effects. New measurements in d +Au and Cu+Cu indicate surprising cold nuclear matter effects on these quarks at midrapidity and provide a new baseline for interpretation of the observed suppression in Au+Au collisions. When considered all together, these measurements present a detailed study of nuclear matter across a wide range of system size and temperature. Here, we present preliminary PHENIX measurements of non-photonic electron spectra and their centrality dependence in d+Au and Cu+Cu and discuss their implications on the current understanding of parton energy loss in the nuclear medium.

We present the measurements of D⁰ and D* in p+p and D⁰ in Au+Au collisions via hadronic decays D⁰ → K⁻π⁺, D*⁺ → D⁰π⁺ → K⁻π⁺π⁺ in mid-rapidity |y| < 1 at = 200 GeV, covering p_T from 0.2 to 6 GeV/c in p+p and 0.4 to 5 GeV/c in Au+Au, respectively. The charm pair production cross section per nucleon–nucleon collision at mid-rapidity is measured to be 202 ± 56 (stat.) ± 40 (sys.) ± 20 (norm.) μb in p+p and 186 ± 22 (stat.) ± 30 (sys.) ± 18 (norm.) μb in Au+Au minimum bias collisions. The number of binary collisions scaling of charm cross section indicates that charm is produced via initial hard scatterings. No suppression of D⁰R_AA in Au+Au 0–80% minbias collisions is observed at p_T below 3 GeV/c. Blast-wave predictions with light-quark hadron parameters are different from data, which may indicate that D⁰ decouples earlier from the medium than the light-quark hadrons.

p–A collisions provide an essential reference for the study of J/ψ suppression in heavy-ion interactions since they allow us to evaluate cold nuclear matter (CNM) effects. These include initial state effects, such as shadowing and parton energy loss, and final state effects such as J/ψ breakup in the collisions with nucleons. To evaluate these effects, NA60 has measured J/ψ production in p–A collisions with seven nuclear targets at GeV and, for the first time, at the same of Pb–Pb and In–In SPS data: 17.3 GeV. More recently, open-charm production has also been studied by analysing the dimuon continuum generated by the simultaneous semi-muonic decay of D meson pairs. The comparison of the production of closed charm with its natural reference (open charm) can give some insight on how the interplay of the different CNM effects produces the observed (effective) J/ψ suppression.

The stochastic dynamics of c and b quarks in the fireball created in nucleus–nucleus collisions at the RHIC and LHC is studied employing a relativistic Langevin equation, based on a picture of multiple uncorrelated random collisions with the medium. Heavy-quark transport coefficients are evaluated within a pQCD approach, with a proper HTL resummation of medium effects for soft scatterings. The Langevin equation is embedded in a multi-step setup developed to study heavy-flavor observables in pp and AA collisions, starting from a NLO pQCD calculation of initial heavy-quark yields, complemented in the nuclear case by shadowing corrections, k_T-broadening and nuclear geometry effects. Then, only for AA collisions, the Langevin equation is solved numerically in a background medium described by relativistic hydrodynamics. Finally, the propagated heavy quarks are made hadronize and decay into electrons. Results for the nuclear modification factor R_AA of heavy-flavor hadrons and electrons from their semi-leptonic decays are provided, both for RHIC and LHC beam energies.

A current focus at the RHIC is the beam energy scan to study the QCD phase diagram—temperature (T) versus baryon chemical potential (μ_B). The STAR experiment has collected data for Au+Au collisions at 7.7, 11.5 and 39 GeV in the year 2010. We present midrapidity results on rapidity density, average transverse mass and particle ratios for identified hadrons from the STAR experiment. Collision dynamics are studied in the framework of chemical and kinetic freeze-out conditions.

To explore the QCD phase diagram, the RHIC has embarked on an energy scan program involving gold nuclei at various energies. In this proceeding, flow coefficients (v_n) of inclusive charge hadrons measured at various beam energies are reported for elliptical flow (n = 2) as well as for higher order harmonics (n = 3, 4). In addition, results of particle-identified v₂ at relatively low energies are presented as well.

This paper gives a brief review of recent theoretical developments in using the non-Gaussian measures of event-by-event fluctuations to discover the QCD critical point presented at Quark Matter 2011.

Non-central heavy-ion collisions create an out-of-plane-extended participant zone that expands toward a more round state as the system evolves. The recent RHIC beam energy scan at of 7.7, 11.5 and 39 GeV provides an opportunity to explore the energy dependence of the freeze-out eccentricity. The new low-energy data from STAR complement high statistics data sets at of 62.4 and 200 GeV. Hanbury Brown–Twiss (HBT) interferometry allows us to determine the size of pion-emitting source regions. The dependence of the HBT radius parameters on the azimuthal angle relative to the reaction plane has been extracted. These dependences can be related to the freeze-out eccentricity. The new results from STAR are consistent with a monotonically decreasing freeze-out eccentricity and constrain any minimum, suggested by previously available data, to lie in the range between 11.5 and 39 GeV. Of several models, UrQMD appears to best predict the STAR and AGS data.

One of the questions addressed by the low energy scan program performed at the RHIC is under what conditions the suppression of high-p_T particles becomes dominant [1]. Over the years, the PHENIX collaboration have studied the suppression of inclusive π⁰ yield varying the colliding systems, the collision energy and centrality. So far, suppression has been found for both Cu+Cu and Au+Au at = 62.4 GeV and above, but a moderate enhancement was observed for the Cu+Cu system at = 22.4 GeV [2]. In 2010, RHIC continued the low energy scan program by collecting Au+Au data at = 62.4 and 39 GeV. Results of these systematic studies are presented here.

We explore the potential of fluctuation measurements to investigate the properties of hot and dense matter created in relativistic heavy-ion collisions to investigate signals expected from the critical point and the first-order phase transition. Contrary to calculations in a grand canonical ensemble we combine the dynamics of the systems with the nonequilibrium propagation of the order parameter of chiral symmetry. We find an enhancement of the fluctuations of the order parameter at the phase transition. Furthermore, we study the effect of exact baryon charge conservation on the net-baryon and the net-proton kurtosis for central (b ⩽ 2.75 fm) Pb+Pb/Au+Au collisions from E_lab = 2A GeV to GeV from the UrQMD model. The results are compared to experimentally applied cuts and corrections.

In the particle-flow approach, information from all available sub-detector systems is combined to reconstruct all stable particles. The global event reconstruction has been shown to improve, in particular, the resolution of jet energy and missing transverse energy in pp collisions compared to purely calorimetric measurements. This improvement is achieved primarily by combining the precise momentum determination of charged hadrons in the silicon tracker with the associated energy depositions in the calorimeters. By resolving individual particles inside jets, particle flow reduces the sensitivity of the jet energy scale to the jet fragmentation pattern, which is known to be one of the largest sources of systematic uncertainty in jet reconstruction. Particle flow reconstruction is thus potentially well suited for the study of potential modifications to jet fragmentation in heavy-ion collisions. The particle flow algorithm has been adapted to the heavy-ion environment. The performance of jet reconstruction from particle flow objects in Pb–Pb collisions using the anti-k_T jet reconstruction algorithm is presented.

The description of jet quenching and elliptic flow at the RHIC and the LHC within a common dynamical framework has been notoriously difficult. In this paper, an approach using the perturbative QCD-based partonic transport model BAMPS (Boltzmann approach to multi-parton scatterings) is presented that has recently been extended to include light quarks. The investigations are complemented by a study on the suppression of D-mesons at the LHC based on elastic interactions with the medium.

We compute the path-length dependence of energy loss for higher azimuthal harmonics of jet fragments in a generalized model of energy loss that can interpolate between pQCD and AdS/CFT limits and compare results with Glauber and CGC/KLN initial conditions. We find, however, that even the high-p_T second moment is most sensitive to the poorly known early-time evolution during the first fm/c. Moreover, we demonstrate that quite generally the energy and density dependence leads to an overquenching of high-p_T particles relative to the first LHC R_AA-data, once the parameters of the energy-loss model are fixed from R_AA-data at RHIC.

We present preliminary results of a di-hadron correlation analysis with identified triggers, measured as a function of relative azimuth (Δϕ) and pseudorapidity (Δη). The relativistic rise in the momentum dependence of the ionization energy loss of particles measured in the STAR TPC is used to statistically separate charged pions from kaons and protons. On the near side, the long range Δη correlation known as the ridge is revealed to have a strong trigger-type dependence, providing new constraints for its theoretical description. The jet-like cone structure above the ridge, while also trigger-dependent, shows little modification compared to d+Au measurements, challenging expectations from recombination models.

Hadron production consists traditionally of two main channels in the intermediate-p_T region at RHIC and LHC energies: parton coalescence/recombination and jet fragmentation. We have studied a new channel, namely hadron production connected to the appearance of strong time-dependent coherent field in heavy ion collisions. In the momentum window of 5 GeV/c <p_T < 25 GeV/c, this channel could yield an anomalous baryon enhancement. This extra yield is able to explain the measured structure of nuclear modification factors for identified hadrons at RHIC energies. We give predictions for LHC energies.

Jets, jet–medium interaction and hydrodynamic evolution of fluctuations in initial parton density all lead to the final anisotropic dihadron azimuthal correlations in high-energy heavy-ion collisions. We remove the harmonic flow background and study the net correlations from different sources with different initial conditions within a multi-phase transport model. We also study γ-hadron correlations which are only influenced by jet–medium interactions.

The energy loss of partons in high-energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. Analyzing data from Pb–Pb collisions at a center-of-mass energy of 2.76 TeV, recorded with the compact muon solenoid (CMS) detector, parton energy loss is observed as a significant imbalance of dijet transverse momentum. To gain further understanding of the parton energy loss mechanism and how it manifests itself in the distribution of final state particles, fragmentation functions are studied for the leading and subleading jet in bins of dijet imbalance.

We study the evolution of a partonic jet shower propagating through a quark–gluon plasma. Combining the in-medium evolutions of the leading parton and shower gluons, we compute the depletion of the energy from the jet cone by dissipation through elastic collisions with medium constituents, by scattering of shower partons to larger angles and by radiation outside the jet cone. Numerical results are presented for the nuclear modification of dijet asymmetry in Pb+Pb collisions at the large hadron collider (LHC).

Advancements in full jet reconstruction have made it possible to use jets as triggers in azimuthal angular correlations to study the modification of hard-scattered partons in the medium created in ultrarelativistic heavy-ion collisions. This increases the range of parton energies accessible in these analyses and improves the signal-to-background ratio compared to dihadron correlations. Results of a systematic study of jet–hadron correlations in central Au–Au collisions at GeV are indicative of a broadening and softening of jets which interact with the medium. Furthermore, jet–hadron correlations suggest that the suppression of the associated hadron yield at high-p_T is balanced in large part by low-p_T enhancement.

We have developed a Monte Carlo simulation describing the 2 → 2 scatterings of perturbatively produced, non-eikonally propagating high-energy partons with the quarks and gluons of the expanding QCD medium created in ultrarelativistic heavy-ion collisions. The partonic scattering rates are computed in leading-order perturbative QCD (pQCD), while three different hydrodynamical scenarios are used to model the strongly interacting medium. We compare our results and tune the model with the neutral pion suppression observed in GeV Au+Au collisions at the BNL-RHIC. We find the incoherent nature of elastic energy loss incompatible with the measured angular dependence of the suppression. The effects of the initial state density fluctuations of the bulk medium are found to be small. Also the extrapolation from RHIC to the LHC is discussed.

We perform a joint jet tomographic analysis of the PHENIX RHIC and ALICE LHC data on the nuclear modification factor R_AA within the light-cone path integral approach to induced gluon emission. Our results show that variation of R_AA from RHIC to LHC energies indicates that the QCD coupling constant is suppressed in the quark–gluon plasma produced at the LHC.

The propagation of hard partons through the strongly interacting matter created in high-energy heavy-ion collisions involves widely separated scales. The methods of effective field theories can provide a factorized description at lowest nontrivial order, and a formalism where the corrections to this factorization are calculable systematically order by order in the small ratios between the different scales. We present our preliminary results on the medium-induced collinear radiation by using the methods of soft collinear effective theory (SCET). The radiated gluon is collinear with the incoming hard parton and gets an arbitrary fraction of its energy.

The measurement of elliptic flow of charged particles in Pb+Pb collisions with the ATLAS detector at the Large Hadron Collider (LHC) is presented in a wide range of transverse momentum (p_T), pseudorapidity (η) and collision centrality. For the first time at this energy, the elliptic flow v₂ is measured over five units of pseudorapidity, from −2.5 to 2.5, and over a broad range in transverse momentum, 0.5–20 GeV. Measurement of higher order flow harmonics, up to v₆, using both the event plane and the two-particle correlation methods over a broad p_T and centrality range is also presented.

We present an event-by-event hydrodynamical framework which takes into account the initial density fluctuations arising from a Monte Carlo Glauber model. The elliptic flow is calculated with the event plane method and a one-to-one comparison with the measured event plane v₂ is made. Both the centrality- and p_T-dependence of v₂ are remarkably well reproduced. We also find that the participant plane is a quite good approximation for the event plane.

Separation of charges along the extreme magnetic field created in non-central relativistic heavy-ion collisions is predicted to be a signature of local parity violation in strong interactions. We report on results for charge-dependent two-particle azimuthal correlations with respect to the reaction plane for Pb–Pb collisions at TeV recorded in 2010 with ALICE at the LHC. The results are compared with measurements at RHIC energies and against currently available model predictions for LHC. Systematic studies of possible background effects including comparison with conventional (parity-even) correlations simulated with Monte Carlo event generators of heavy-ion collisions are also presented.

Parity-odd domains from QCD are predicted to cause charge separation of quarks across the reaction plane created in non-central relativistic heavy-ion collisions—the chiral magnetic effect. Here, we present several measurements to search for charge separation across the reaction plane at STAR. We use three different observables: a three-point correlator, a multiplicity asymmetry correlator as well as a reaction-plane-dependent balance function. The correlations are studied differentially and are presented for several Au+Au collision energies: 200, 62.4, 39, 11.5 and 7.7 GeV. We will discuss the sensitivities of these latest measurements to possible parity odd signals and parity even backgrounds.

Charged particle directed flow at midrapidity, |η| < 0.8, and forward rapidity, 1.7 < |η| < 5.1, is measured in Pb–Pb collisions at = 2.76 TeV with ALICE at the LHC. Directed flow is reported as a function of collision centrality, charged particle transverse momentum and pseudorapidity. Results are compared to measurements at RHIC and recent model calculations for LHC energies.

The Large Hadron Collider experiments have revealed that the predictions of the color glass condensate (CGC) tend to underestimate the multiplicity at mid-rapidity. We develop and estimate a full second-order viscous hydrodynamic model for the longitudinal expansion to find that the CGC rapidity distributions are visibly deformed during the hydrodynamic stage due to the interplay between the entropy production and the entropy flux to forward rapidity. The results indicate the importance of considering viscous hydrodynamic evolution with non-boost invariant flow for understanding the CGC in heavy-ion collisions.

Using a 3+1D viscous hydrodynamic model of relativistic heavy ion collisions, we show that event-by-event fluctuation is essential in understanding the quark–gluon plasma produced in collisions at the RHIC and the LHC.

An observable that can be used to better constrain the mechanism responsible for parton energy loss is the elliptic azimuthal event anisotropy, v₂. We report on measurements of v₂ for inclusive and identified charged particles in Pb–Pb collisions at TeV recorded by the ALICE experiment at the LHC. v₂ is presented for a wide range of particle transverse momentum up to p_T = 20 GeV/c within the pseudo-rapidity region |η| < 0.8. The particle v₂ is finite, positive and approximately constant for p_T > 8 GeV/c. The proton v₂ is higher than that of the pion up to about p_T = 8 GeV/c. The results are compared to the measurements at lower energy reported by RHIC experiments.

We study the evolution of local event-by-event deviations from smooth average fluid dynamic fields, as they can arise in heavy ion collisions from the propagation of fluctuating initial conditions. Local fluctuations around Bjorken flow are found to be governed by nonlinear equations whose solutions can be characterized qualitatively in terms of Reynolds numbers. Perturbations at different rapidities decouple quickly and satisfy (after suitable coordinate transformations) an effectively two-dimensional Navier–Stokes equation of non-relativistic form. We discuss the conditions under which nonlinearities in these equations cannot be neglected and turbulent behavior is expected to set in.

Analytical results for the anisotropic collective flow of a Lorentz gas of massless particles scattering on fixed centres are presented.

Identified particle observables from viscous hydrodynamics are sensitive to the fluid-to-particle conversion. Instead of the commonly assumed 'democratic' Grad ansatz for phase space corrections δf, we utilize corrections calculated from linearized covariant transport theory. Estimates based on a π–p system with binary collisions indicate that protons are much closer to equilibrium than pions, significantly affecting the dissipative reduction of differential elliptic flow in Au+Au at the RHIC. In addition, we test the linear response against fully nonlinear transport for a two-component massless system in a Bjorken scenario. Strikingly, we find that, while linear response accounts well for the dynamical sharing of shear stress, the momentum dependence of phase space corrections is best described by Grad's quadratic ansatz, and not the linear response solution.

Electromagnetic dissociation of heavy nuclei in ultra-peripheral interactions at high energies can be used to monitor the beam luminosity at colliders. In ALICE, neutrons emitted by the excited nuclei close to beam rapidity are detected by the zero-degree calorimeters (ZDCs), providing a precise measurement of the event rate. During the 2010 Pb run, a dedicated data taking was performed triggering on electromagnetic processes with the ZDCs. These data, combined with the results from a Van der Meer scan, allowed us to measure the electromagnetic dissociation cross section of Pb nuclei at = 2.76 TeV. Experimental results on various cross sections are presented together with a comparison to the available predictions.

We give a simple overview and perspective on our recent results (Arnold and Vaman 2010 J. High Energy Phys. JHEP10(2010)099, Arnold and Vaman 2011 J. High Energy Phys. JHEP04(2011)027) on jet stopping in strongly coupled plasmas using gauge-gravity duality. There is an important distinction between typical and maximum stopping distances, and we find that the typical stopping distance scales with energy as E^1/4.

We use our non-conformal holographic bottom-up model for QCD described in Ficnar et al (2011 Nucl. Phys. A 855 372) to further study the effect of the QCD trace anomaly on the energy loss of both light and heavy quarks in a strongly coupled plasma. We compute the nuclear modification factor R_AA for bottom and charm quarks in an expanding plasma with Glauber initial conditions. We find that the maximum stopping distance of light quarks in a non-conformal plasma scales with the energy with a temperature- (and energy)-dependent effective power.

We investigate the microscopic origin of the relaxation time coefficient in relativistic fluid dynamics. We show that the extraction of the shear viscosity relaxation time via the gradient expansion is ambiguous and in general fails to give the correct result. The correct value for the shear viscosity relaxation time is extracted from the slowest non-hydrodynamic pole of the corresponding retarded Green's function, if such a pole is purely imaginary. According to the AdS/CFT correspondence, in strongly coupled SYM, the non-hydrodynamic poles of the shear stress tensor nearest to the origin have a nonzero real part, which implies that the transient fluid-dynamical equations for this gauge theory are not equivalent to the well-known Israel–Stewart equations.

We report on a continuum extrapolation of the vector current correlation function for light valence quarks in the deconfined phase of quenched QCD. This is achieved by performing a systematic analysis of the influence of cutoff effects on light quark meson correlators at T ≃ 1.45 T_c using clover improved Wilson fermions (Ding et al 2011 Phys. Rev. D 83 034504). We discuss resulting constraints on the electrical conductivity and the thermal dilepton rate in a quark–gluon plasma. In addition, new results at 1.2 and 3.0 T_c will be presented.

In studies of the dense medium produced in ultra-relativistic heavy ion collisions, photons are important hard probes, since they are not expected to be modified by the medium. The measurement of isolated prompt photon production in Pb–Pb collisions provides a test of perturbative quantum chromodynamics and the information to constrain the nuclear parton distribution functions. CMS has shown photon purity measurement capabilities in pp collisions using the shower shape templates. In Pb–Pb collisions at CMS, this technique was applied for the first time in heavy ion collisions. We report the first measurement of the transverse momentum spectra of isolated photons with pT from 20 to 80 GeV/c in Pb–Pb collisions at = 2.76 TeV. The centrality dependence of the nuclear modification factor is also reported by comparing the result to the photon spectrum of pp reference which is computed from NLO calculations.

The NA60 experiment has studied low-mass muon pair production in proton–nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/c proton beam at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, η' and ϕ. A new high-precision measurement of the electromagnetic transition form factors of the η and ω mesons is presented, complemented with a measurement of the temperature parameter of the ρ meson in cold nuclear matter. The p_T spectra for the ω and ϕ mesons are extracted in the full p_T range accessible, up to p_T = 2 GeV/c. The nuclear dependence of the production cross sections for the η, ω and ϕ mesons has been investigated in terms of the power law σ_pA∝A^α, and the α parameter was studied as a function of p_T.

Electron–positron pairs are effective probes for investigating the hot, dense matter created in RHIC collisions because they carry no color charge and therefore, once created, do not interact strongly with the medium. As a result, they retain characteristics of the full time evolution and dynamics of the system. The PHENIX experiment has presented dielectron continuum spectra for p+p, Cu+Cu and Au+Au collisions at GeV. PHENIX has recently measured dielectrons in d+Au collisions to serve as a control experiment to the heavy ion collisions allowing us to disentangle any cold nuclear effects from those due to the quark gluon plasma. First results are presented here.

We report a few selected results from systematic studies of e⁺ e⁻ production in C+C, Ar+KCl, p+p, d+p and p+Nb collisions performed by the High-Acceptance Di-Electron Spectrometer (HADES) in the energy domain of 1–3.5 GeV kinetic energy per nucleon. The comparison of a N+N reference spectrum with the di-electron invariant-mass distribution measured in Ar+KCl collisions shows pronounced excess radiation. Medium effects have been further investigated in p+Nb interactions at 3.5 GeV by a direct comparison to p+p reactions measured at the same beam energy.

The mechanism of quark–gluon plasma (QGP) formation in pp collisions is discussed in the framework of the EPOS model. The enhancement of direct photons at low p_t is predicted as a sensitive QGP signal in pp collisions at 7 TeV. Experimental measurements to test this prediction and to cross-check from other QGP signals are advocated.

We propose a novel relation between the low-mass enhancement of dielectrons observed at PHENIX and transport coefficients of QGP such as the charge diffusion constant D and the relaxation time τ_J. We parameterize the transport peak in the spectral function using the second-order relativistic dissipative hydrodynamics by Israel and Stewart. Combining the spectral function and the full (3+1)-dimensional hydrodynamical evolution with the lattice EoS, theoretical dielectron spectra and the experimental data are compared. Detailed analysis suggests that the low-mass dilepton enhancement originates mainly from the high-temperature QGP phase where there is a large electric charge fluctuation as obtained from lattice QCD simulations.

Suppression of quarkonia in heavy-ion collisions with respect to proton–proton collisions due to the Debye screening of the potential between the heavy quarks was hypothesized to be a signature of the quark–gluon plasma (QGP) (Matsui and Satz 1986 Phys. Lett. B 178 146). However, other effects besides Debye screening, such as the statistical recombination of heavy-flavor pairs, or co-mover absorption can also affect quarkonia production in heavy-ion collisions. Quantifying the suppression of an entire family of quarkonium mesons can give us a model-dependent constraint on the temperature. The suppression of ϒ can be quantified by calculating the nuclear modification factor, R_AA, which is the ratio of the production in Au+Au collisions to the production in p+p scaled by the number of binary collisions. We present our results for mid-rapidity ϒ(1S+2S+3S) production in p+p and Au+Au collisions at √s_NN = 200 GeV. The centrality dependence of R_AA will be shown for the combined ϒ(1S+2S+3S) yield.

We present results on inclusive electrons for 1.5 <p_T < 6 GeV/c in Pb–Pb collisions at = 2.76 TeV measured with ALICE at the LHC and compare these to a cocktail of background electron sources. The excess of electrons beyond the cocktail at high momenta (p_T > 3.5 GeV/c) is attributed to electrons from heavy-flavour decays. The corresponding nuclear modification factor indicates heavy-flavour suppression by a factor of 1.5–4.

We present a detailed phenomenological study of the associated production of a prompt photon and a heavy-quark jet (charm or bottom) in proton–nucleus (p–A) and nucleus–nucleus (A–A) collisions. We show that future p–A data to be collected at the LHC should allow one to disentangle the various sets currently available. In A–A collisions, the photon transverse momentum can be used to gauge the initial energy of the massive parton which is expected to propagate through the dense QCD medium produced in those collisions, as shown by the present phenomenological analysis carried out in Pb–Pb collisions at the LHC.

STAR has recently reported charge-dependent azimuthal correlations that are sensitive to the charge separation effect in Au+Au collisions at = 200 GeV. Qualitatively, these results agree with some of the theoretical predictions for local parity violation in heavy-ion collisions. However, a study using reaction-plane-dependent balance functions shows an alternative origin of this signal. The balance function, which measures the correlation between oppositely charged pairs, is sensitive to the mechanisms of charge formation and the subsequent relative diffusion of the balancing charges. The reaction-plane-dependent balance function measurements can be related to STAR's charge-dependent azimuthal correlations. We report reaction-plane-dependent balance functions for Au+Au collisions at = 200, 62.4, 39, 11.5 and 7.7 GeV using the STAR detector. The model of Schlichting and Pratt incorporating local charge conservation and elliptic flow reproduces most of the three-particle azimuthal correlation results at 200 GeV. The experimental charge-dependent azimuthal charge correlations observed at 200 GeV can be explained in terms of local charge conservation and elliptic flow.

We present first results on the production of nuclei and antinuclei such as (anti)deuterons, (anti)tritons, (anti)³He and (anti)⁴He in pp collisions at = 7 TeV and Pb–Pb collisions at = 2.76 TeV. These particles are identified using their energy loss (dE/dx) information in the Time Projection Chamber of the ALICE experiment. The inner tracking system gives a precise determination of the event vertex, by which primary and secondary particles are separated. The high statistics of over 360 million events for pp and 16 million events for Pb–Pb collisions give a significant number of light nuclei and antinuclei (Pb–Pb collisions: ∼30 000 antideuterons () and ∼4 antialpha ()). The predictions of various particle ratios from the THERMUS model are also discussed.

We present results for lattice QCD in the limit of infinite gauge coupling, obtained from a worm-type Monte Carlo algorithm on a discrete spatial lattice but with continuous Euclidean time. This is obtained by sending both the anisotropy parameter γ² ≃ a/a_t and the number of time-slices N_τ to infinity, keeping the ratio γ²/N_τ ≃ aT fixed. The obvious gain is that no continuum extrapolation N_τ → ∞ has to be carried out. Moreover, the algorithm is faster and the sign problem disappears. We compare our computations with those on discrete lattices. We determine the phase diagram as a function of temperature and baryon chemical potential.

Although the RHIC experiments have measured the inclusive J/ψ and open heavy-flavor productions at = 200 GeV, no data are available yet for the contribution of secondary J/ψ from the B-hadron decays in heavy-ion collisions. Since the B-hadron and the prompt J/ψ interact with the medium differently, subtracting the non-prompt J/ψ contribution is crucial for our understanding of modifications in the prompt J/ψ production in heavy-ion collisions. It should also be a measure of b-quark energy loss. CMS is able to separate the prompt and the non-prompt J/ψ in PbPb collisions performing an unbinned two-dimensional maximum-likelihood fit to the mass and pseudo-proper decay length distributions. In this paper, the first measurement of non-prompt J/ψ production in the dimuon channel, as a function of centrality in PbPb collisions at = 2.76 TeV, is presented.

A listing of members of the ALICE Collaboration is given in the PDF file.

A listing of members of the ATLAS Collaboration is given in the PDF file.

A listing of members of the CMS Collaboration is given in the PDF file.

A listing of members of the HADES Collaboration is given in the PDF file.

A listing of members of the NA49 Collaboration is given in the PDF file.

A listing of members of the NA60 Collaboration is given in the PDF file.

A listing of members of the NA61/SHINE Collaboration is given in the PDF file.

A listing of members of the PHENIX Collaboration is given in the PDF file.

A listing of members of the STAR Collaboration is given in the PDF file.

The PDF file provided contains web links to all articles in this issue.

The PDF file provided contains web links to all articles in this volume.