Table of contents

This volume contains selected papers presented at the 9th International Workshop on High-p_T at LHC. The workshop was held from 24-28 September 2013 at LPSC Grenoble, France.

The first workshop of this series was organized in 2006 in Trento, Italy. The purpose of the 9th workshop was to offer an opportunity for experimentalists and theoreticians to get together and discuss experimental results from the first heavy-ion program at LHC, lessons learnt from RHIC and theoretical developments. Focus was also given to high p_T physics in pA collisions, especially at the LHC energies. Emphasis was given on discussion over a large scope: high-p_T physics, jets, photons, correlations, hard scattering and hard probes phenomena.

The main topics of the workshop were:

• Nuclear modifications of the parton distribution functions

• High p_T jet production in pp, pA and AA

• High p_T parton propagation in matter

• Nuclear modifications of the fragmentation functions

• Correlations with leading particles

• Direct photon and heavy flavor tagging

About 50 participants coming from 12 countries participated in the workshop. 6 invited talks and 31 oral contributions were presented at the conference.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Azimuthal particle correlations have been extensively studied in the past at various collider energies in p-p, p-A, and A-A collisions. Hadron-correlation measurements in heavy- ion collisions have mainly focused on studies of collective (flow) effects at low-p_T and parton energy loss via jet quenching in the high-p_T regime. This was usually done without event-byevent particle identification. In this paper, we present two-particle correlations with identified trigger hadrons and identified associated hadrons at mid-rapidity in Monte Carlo generated events. The primary purpose of this study was to investigate the effect of quantum number conservation and the flavour balance during parton fragmentation and hadronization. The simulated p-p events were generated with PYTHIA 6.4 with the Perugia-0 tune = 7 TeV. HIJING was used to generate 0 - 10% central Pb-Pb events at = 2.76 TeV. We found that the extracted identified associated hadron spectra for charged pion, kaon, and proton show identified trigger-hadron dependent splitting. Moreover, the identified trigger-hadron dependent correlation functions vary in different p_T bins, which may show the presence of collective/nuclear effects.

Measurements of jets and heavy-flavor production play an important role in understanding properties of hot and dense nuclear matter created in high energy heavy-ion collisions. As direct measurements of jets are difficult due to large underlying background, jet properties can be also studied via di-hadron and multihadron correlations. In this contribution to these proceedings, an overview of recent STAR results on correlations, jet-hadron correlations, and heavy-flavor production in Au+Au collisions at top RHIC energy ( = 200 GeV) is given. In order to draw quantitative conclusions on properties of the hot and dense nuclear matter created in Au+Au collisions, reference measurements in elementary proton-proton collisions and d+Au collisions are essential to estimate contributions due to cold nuclear matter effects and are discussed as well.

We report results from the ALICE collaboration on jet production and jet properties in pp and Pb-Pb collisions at the LHC. The charged particle jet production cross-section and measurements of jet fragmentation and jet shape in pp collisions at = 7 TeV are presented. The results are confronted with simulations from Monte Carlo event generators. Measurements of charged particle jets in Pb-Pb collisions at = 2.76 TeV with resolution parameter R = 0.2 and R = 0.3 show a strong momentum dependent suppression in central relative to peripheral collisions. The analysis of the semi-inclusive distribution of charged particle jets recoiling from a high-p_T trigger hadron allows an unbiased measurement of the jet structure for larger cone radii. No significant redistribution of the energy flow inside the jet cone is observed. Di-hadron correlations allow to assess jet structure at very low particle momenta. For 2 < p_T,trig < 3 GeV/c and 1 < P_T,assoc < 2 GeV/c, a strongly asymmetric near-side correlation, broader in Δ_η than Δ_ϕ, is observed.

We compute the radiation spectrum of antennas containing a heavy quark, focusing on the case of a massive quark-gluon antenna. We apply the classical Yang-Mills formalism (CYM), and treat the interactions of the jet with the medium in the multiple soft scattering approximation. This work completes the studies of antenna radiation inside a medium. The main conclusion is that decorrelation occurs faster in the case in which at least one of the emitters is a heavy quark, and is faster in the massive quark-gluon case.

Event-by-event reconstruction of the collision geometry using some incarnation of the Glauber-model is a widely accepted method in studying heavy ion collisions. While there is no known problem with the procedure when applied to the collision of two large ions, we will argue that in very asymmetric collisions, like p(d)+A with at least one hard scattering process occuring the event geometry deduced with the simple Glauber-model may be biased.

Angular correlations with high transverse momentum hadrons have become an established tool for studying properties of the medium created in ultra-relativistic heavy ion collisions. When investigated in two dimensions, relative azimuth and relative pseudorapidity, the small-angle part of such correlations is commonly attributed to QCD jets, while long-range correlations, specifically ridge observed at low- and intermediate momentum, are often described as manifestations of hydrodynamic expansion of the system, e.g. flow. Comparative analysis of both features, presented in this work for correlations with identified leading hadrons, explores the particle type dependence of the jet structures, and challenges the flow-only hypotheses behind the ridge.

Using analytic solutions of the Yang-Mills equations we calculate the initial flow of energy of the classical gluon field created in collisions of large nuclei at high energies. We find radial and elliptic flow which follows gradients in the initial energy density, similar to a simple hydrodynamic behavior. In addition we find a rapidity-odd transverse flow field which implies the presence of angular momentum and should lead to directed flow in final particle spectra. We trace those energy flow terms to transverse fields from the non-abelian generalization of Gauss' Law and Ampere's and Faraday's Laws.

After a short presentation of the event generator EPOS, we discuss the production of heavy quarks and prompt photons which has been recently implemented. Whereas we have satisfying results for the charm, work on photons is still in progress.

Preliminary results from the STAR collaboration for the azimuthal elliptic anisotropy (v₂) of high transverse momentum (p_T) direct photons (γ_dir) produced at midrapidity (|η^γ_dir| < 1) in Au+Au collisions at center-of-mass energy = 200 GeV are presented, and compared to the measured (v₂) of neutral pions (π⁰) in the same kinematic range. The electromagnetic transverse shower profile is used to distinguish π⁰ from direct photons. The measured v₂^γ_dir(p_T) at high p_T (8 < p_T^γ_dir < 20 GeV/c) is found to be smaller than that of π⁰ and consistent with zero when using the forward detectors in determining the event plane.

The centrality dependencies of the inclusive neutral pion and prompt photon nuclear modification factors for p+Pb collisions at the LHC are studied using a spatially dependent set of nuclear PDFs, EPS09s. The calculations are performed at mid- and forward rapidities searching for an observable which would optimally probe the spatial dependence of the nuclear PDFs. In addition, we discuss to which x values of the nucleus the different observables are sensitive.

Energy loss models based on leading order strong-coupling derivations oversuppress high-pT particles compared to RHIC and LHC data whereas energy loss models based on leading order weak-coupling derivations robustly describe qualitatively known RHIC and LHC results. Currently there are large theoretical uncertainties in both calculations and next-to-leading order corrections will be necessary for 1) a definitive falsification of the strong-coupling calculations and 2) a quantitative comparison of pQCD-based calculations to data.

We demonstrate that not only the production of virtual photons decaying into low- mass lepton pairs, but also the one of weak bosons at large transverse momenta is dominated by quark-gluon scattering. Measurements of these processes at the LHC can therefore provide useful constraints on the parton densities in the proton, in particular the one of the gluon, and their nuclear modifications.

The Cronin effect, which is nuclear enhancement of high-p_T hadron production in pA collisions was successfully predicted prior the measurements at RHIC and LHC. The restrictions imposed by energy conservation lead to spectacular effects. Energy deficit becomes an issue for hadron production in pA collisions at large x_L and/or large x_T towards the kinematic bounds x_L,T = 1. It leads to a suppression, which has been indeed observed for hadrons produced at forward rapidities and large p_T. Intensive energy dissipation via gluon radiation by a highly virtual parton produced with large pT, makes impossible this process to continue long. Color neutralization and creation of a colorless dipole must occur promptly. When this happens inside a hot medium created in AA collisions, attenuation of dipoles, rather than induced energy loss, becomes a dominant mechanism for suppression of high-p_T hadrons.

ALICE at the LHC, collecting data in Pb-Pb, p-Pb and pp collisions, aims for the characterisation of the strongly interacting matter at unprecedented energy densities. While the first running period until 2018 can provide detailed description of the global and bulk phenomena and a first set of intriguing results on rare probes of heavy-ion collisions, many important questions involving rare processes cannot be addressed in details with the current experimental setup. In this talk, we give an overview of the ALICE Upgrade program focusing on the physics prospects related to heavy-quarks, quarkonia, low-mass dileptons, jets and exotica searches achievable in high luminosity Pb-Pb collisions at the LHC. We also discuss the technological challenges and choices of the detector upgrades to be installed during the LHC Long Shutdown 2.

Partonic energy loss in the medium formed in heavy ion collisions results in significant modifications of jet spectra. Quantitative understanding of these modifications can constrain models for partonic energy loss in heavy ion collisions. The ALICE detector is capable of unique measurements of jets due to its low momentum tracking and particle identification capabilities. The ALICE Electromagnetic Calorimeter (EMCAL) is a key element for the measurement of fully reconstructed jets in ALICE due to its measurement of neutral particles and its triggering capabilities. Measurements of fully reconstructed jet spectra and the nuclear modification factor, R_AA, in Pb-Pb collisions at = 2.76 TeV are presented.

In the standard pQCD picture particles are produced via the parton jet fragmentation process. However, there are also other production mechanisms like higher twist (HT) processes. A usual example of a HT process is a direct production of an outgoing hadron, where the hadron is produced in the hard subprocess without fragmentation.

We study the HT phenomena using a shape analysis (x_T scaling) of the inclusive invariant cross sections of charged hadrons, measured by the ALICE collaboration at center-of-mass energies = 2.76 TeV and 7 TeV. The data is compared to PYTHIA8 event generator and to a phenomenological model for HT. Using PYTHIA8, we explore a possible enhancement of HT phenomena for isolated particles, by comparing the shapes of the isolated distributions to inclusive distributions. The results from the standard PYTHIA8, without HT, is compared to a PYTHIA8 where we had included a HT process.

Finally, we found out that the effects observed in the x_T spectra originate from kinematic biases posed by the isolation cuts, rather than from an enrichment of the HT hadrons at the observed cross sections. A more detailed data analysis is ongoing.

We present an overview of the current status of the recent jet measurements in PbPb and pp collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV that are studied with the CMS detector at the LHC. Jets are reconstructed using the anti-k_T sequential reconstruction algorithm on particle flow objects. To distinguish between different possible energy loss mechanisms and to provide an important experimental check on the validity of the assumed jet fragmentation, inclusive jets, flavor tagged jets and jet shapes are studied.

Particle production in heavy-ion collisions at RHIC and the LHC reveals an enhancement of the baryon-to-meson ratios with respect to pp collisions. The observed suppression of high-p_T particles raises the question of how the jet fragments may (re)distribute in ultra-relativistic heavy-ion collisions. With two-hadron correlations, we can study the particle composition in jets in order to get a better understanding of the aforementioned observables and the particle production mechanisms in the dense medium. In these proceedings, we present the status of the current analysis using two-hadron angular correlations for Pb-Pb collisions at = 2.76 TeV using the ALICE detector. We study the two different possibilities, considering the strange particle as either the trigger particle or the associated particle. Results on K⁰_s-h^±, (Λ A)-h^± and h^±-K⁰_S correlations are shown for central collisions.

RHIC introduced the method of hard scattering of partons as an in-situ probe of the the medium produced in A+A collisions. A suppression, R_AA ≈ 0.2 relative to binaryscaling, was discovered for π⁰ production in the range 5 < p_T < 20 GeV/c in central Au+Au collisions at = 200 GeV, and surprisingly also for single-electrons from the decay of heavy quarks. Both these results have been confirmed in Pb+Pb collisions at the LHC at = 2.76 TeV. Interestingly, in this p_T range the LHC results for pions nearly overlap the RHIC results. Thus, due to the flatter spectrum, the energy loss in the medium at LHC in this p_T range must be ~ 40% larger than at RHIC. Unique at the LHC are the beautiful measurements of the fractional transverse momentum imbalance of di-jets in Pb+Pb collisions. At the Utrecht meeting in 2011, I corrected for the fractional imbalance of di-jets with the same cuts in p-p collisions and showed that the relative fractional jet imbalance in Pb+Pb/p-p is ≈ 15% for jets with 120 ≤ ≤ 360 GeV/c. CMS later confirmed this much smaller imbalance compared to the same quantity derived from two-particle correlations of di-jet fragments at RHIC corresponding to jet ≈ 10 - 20 GeV/c, which appear to show a much larger fractional jet imbalance ≈ 45% in this lower range. The variation of apparent energy loss in the medium as a function of both p_T and is striking and presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues, for instance, the absence of evidence for a effect, due to momentum transferred to the medium by outgoing partons, which would widen the away-side di-jet and di-hadron correlations in a similar fashion as the k_T-effect. Another issue well known from experiments at the CERN ISR, SpS and SpS collider is that parton-parton hard-collisions make negligible contribution to multiplicity or transverse energy production in p-p collisions-soft particles, with p_T ≤ 2 GeV/c, predominate. Thus an apparent hard scattering component for A+A multiplicity distributions based on a popular formula, dN^AA_ch/dη = [(1 - x) ⟨N_part⟩ dN^pp_ch/dη/2 + x ⟨N_coll⟩dN^pp_ch/dη], seems to be an unphysical way to understand the deviation from N_part scaling. Based on recent p-p and d+A measurements, a more physical way is presented along with several other stimulating results and ideas from recent d+Au (p+Pb) measurements.

We present the recent results for D-meson production measured by the ALICE collaboration in pp collisions at = 7 TeV, Pb-Pb collisions at = 2.76 TeV and p-Pb collisions at = 5.02 TeV.

The recent proton-lead run at the LHC shall provide new information on the partonic behaviour within the nuclear medium. At LHC energies the dominant contribution comes from gluon-initiated processes that is, the least well constrained parton density. Therefore it is important to profit from any information that new data can provide us. A time-saving alternative to performing a global fit is the use of Bayesian inference, a powerful tool to realize the impact of data into a set of PDFs independently of the original fitters. In this work we apply the Bayesian re-weighting technique to analyze pseudo data for LHC kinematics in Drell-Yan and hadro-production processes. A set of Monte Carlo replicas for EPS09 is released in a public code for general use.