Journal of Physics G: Nuclear and Particle Physics

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe standard model (SM) processes and search for physics beyond the standard model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF's physics potential.

This article summarizes our present knowledge about nuclear matter at the highest energy densities and its formation in relativistic heavy ion collisions. We review what is known about the structure and properties of the quark-gluon plasma and survey the observables that are used to glean information about it from experimental data.

The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.

Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments—as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER—to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity 'dark showers', highlighting opportunities for expanding the LHC reach for these signals.

The main objectives of the KM3NeT Collaboration are (i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and (ii) the determination of the mass hierarchy of neutrinos. These objectives are strongly motivated by two recent important discoveries, namely: (1) the high-energy astrophysical neutrino signal reported by IceCube and (2) the sizable contribution of electron neutrinos to the third neutrino mass eigenstate as reported by Daya Bay, Reno and others. To meet these objectives, the KM3NeT Collaboration plans to build a new Research Infrastructure consisting of a network of deep-sea neutrino telescopes in the Mediterranean Sea. A phased and distributed implementation is pursued which maximises the access to regional funds, the availability of human resources and the synergistic opportunities for the Earth and sea sciences community. Three suitable deep-sea sites are selected, namely off-shore Toulon (France), Capo Passero (Sicily, Italy) and Pylos (Peloponnese, Greece). The infrastructure will consist of three so-called building blocks. A building block comprises 115 strings, each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. Two building blocks will be sparsely configured to fully explore the IceCube signal with similar instrumented volume, different methodology, improved resolution and complementary field of view, including the galactic plane. One building block will be densely configured to precisely measure atmospheric neutrino oscillations.

The detection of gravitational waves from mergers of tens of Solar mass black hole binaries has led to a surge in interest in primordial black holes (PBHs) as a dark matter candidate. We aim to provide a (relatively) concise overview of the status of PBHs as a dark matter candidate, circa Summer 2020. First we review the formation of PBHs in the early Universe, focussing mainly on PBHs formed via the collapse of large density perturbations generated by inflation. Then we review the various current and future constraints on the present day abundance of PBHs. We conclude with a discussion of the key open questions in this field.

This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2158 new measurements from 551 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on neutrino mass, mixing, and oscillations, QCD, top quark, CKM quark-mixing matrix, V_ud & V_us, V_cb & V_ub, fragmentation functions, particle detectors for accelerator and non-accelerator physics, magnetic monopoles, cosmological parameters, and big bang cosmology.

A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: pdg.lbl.gov.

The Large Hadron–Electron Collider (LHeC) is designed to move the field of deep inelastic scattering (DIS) to the energy and intensity frontier of particle physics. Exploiting energy-recovery technology, it collides a novel, intense electron beam with a proton or ion beam from the High-Luminosity Large Hadron Collider (HL-LHC). The accelerator and interaction region are designed for concurrent electron–proton and proton–proton operations. This report represents an update to the LHeC's conceptual design report (CDR), published in 2012. It comprises new results on the parton structure of the proton and heavier nuclei, QCD dynamics, and electroweak and top-quark physics. It is shown how the LHeC will open a new chapter of nuclear particle physics by extending the accessible kinematic range of lepton–nucleus scattering by several orders of magnitude. Due to its enhanced luminosity and large energy and the cleanliness of the final hadronic states, the LHeC has a strong Higgs physics programme and its own discovery potential for new physics. Building on the 2012 CDR, this report contains a detailed updated design for the energy-recovery electron linac (ERL), including a new lattice, magnet and superconducting radio-frequency technology, and further components. Challenges of energy recovery are described, and the lower-energy, high-current, three-turn ERL facility, PERLE at Orsay, is presented, which uses the LHeC characteristics serving as a development facility for the design and operation of the LHeC. An updated detector design is presented corresponding to the acceptance, resolution, and calibration goals that arise from the Higgs and parton-density-function physics programmes. This paper also presents novel results for the Future Circular Collider in electron–hadron (FCC-eh) mode, which utilises the same ERL technology to further extend the reach of DIS to even higher centre-of-mass energies.

This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2633 new measurements from 689 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 110 reviews are many that are new or heavily revised including those on CKM quark-mixing matrix, V_ud & V_us, V_cb & V_ub, top quark, muon anomalous magnetic moment, extra dimensions, particle detectors, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology.      A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.

We employ the Polyakov-loop enhanced Nambu–Jona-Lasinio model incorporating the quark anomalous magnetic moment to investigate the anisotropy structure and the renormalized magnetization of magnetized quark matter at finite temperature. The ultraviolet divergences and nonphysical oscillatory behavior are eliminated by the vacuum magnetic regularization scheme. With a parametrization of the anomalous magnetic moment that is proportional to the square of the chiral condensate, the renormalized magnetization is enlarged by the strong magnetic field so that the anisotropy becomes more apparent. The inflection point of the renormalized magnetization indicates the pseudocritical temperature for the chiral crossover. We find that the results with the anomalous magnetic moment are closer to the lattice quantum chromodynamics data. The connection between the paramagnetism and the chiral transition provides new insight into a magnetohydrodynamics description of hot and dense QCD matter produced in heavy-ion collisions.

We introduce four supersymmetric (SUSY) axion models in which the strong CP problem and the μ problem are solved with the help of the Peccei–Quinn mechanism and the Kim-Nilles mechanism, respectively. The axion physics enriches the SUSY model by introducing axion as a dark matter candidate and, therefore, the lightest supersymmetric particle (LSP) could just be a part of the total dark matter. For this reason, axion relieves the tensions between SUSY models and numerous experimental measurements, such as the dark matter direct detection experiments and the precise measurements of anomalous magnetic moment of the muon a_μ. In the present paper, we investigate the constraints imposed by the latest a_μ measurements and LUX-ZEPLIN (LZ) experiment on the relic density of the Higgsino-like LSP. Additionally, we consider the constraints arising from the cosmology of saxions and axinos, and their impacts on the parameter space of our models are carefully examined. For the axion constituting the remaining portion of dark matter, we find that the conventional misalignment mechanism can successfully account for the correct dark matter relic density observed by the Planck satellite.

We present a state-of-the-art calculation of the unpolarized pion valence-quark distribution in the framework of large-momentum effective theory (LaMET) with improved handling of systematic errors as well as two-loop perturbative matching. We use lattice ensembles generated by the MILC collaboration at lattice spacing a ≈ 0.09 fm, lattice volume 64³ × 96, N_f = 2 + 1 + 1 flavors of highly-improved staggered quarks and a physical pion mass. The LaMET matrix elements are calculated with pions boosted to momentum P_z ≈ 1.72 GeV with high-statistics of O(10⁶) measurements. We study the pion PDF in both hybrid-ratio and hybrid-regularization-independent momentum subtraction (hybrid-RI/MOM) schemes and also compare the systematic errors with and without the addition of leading-renormalon resummation (LRR) and renormalization-group resummation (RGR) in both the renormalization and lightcone matching. The final lightcone PDF results are presented in the modified minimal-subtraction scheme at renormalization scale μ = 2.0 GeV. We show that the x-dependent PDFs are compatible between the hybrid-ratio and hybrid-RI/MOM renormalization with the same improvements. We also show that systematics are greatly reduced by the simultaneous inclusion of RGR and LRR and that these methods are necessary if improved precision is to be reached with higher-order terms in renormalization and matching.

In this paper we present the results of a new kaonic helium-4 measurement with a 1.37 g l⁻¹ gaseous target by the SIDDHARTA-2 experiment at the DAΦNE collider. We measured, for the first time, the energies and yields of three transitions belonging to the M-series. Moreover, we improved by a factor about three, the statistical precision of the 2p level energy shift and width induced by the strong interaction, obtaining the most precise measurement for gaseous kaonic helium, and measured the yield of the L_α transition at the employed density, providing a new experimental input to investigate the density dependence of kaonic atoms transitions yield.

Baryon number conservation is not guaranteed by any fundamental symmetry within the standard model, and therefore has been a subject of experimental and theoretical scrutiny for decades. So far, no evidence for baryon number violation has been observed. Large underground detectors have long been used for both neutrino detection and searches for baryon number violating processes. The next generation of large neutrino detectors will seek to improve upon the limits set by past and current experiments and will cover a range of lifetimes predicted by several Grand Unified Theories. In this White Paper, we summarize theoretical motivations and experimental aspects of searches for baryon number violation in neutrino experiments.

Nuclear double-beta decays are a unique probe to search for new physics beyond the standard model. Hypothesized particles, non-standard interactions, or the violation of fundamental symmetries would affect the decay kinematics, creating detectable and characteristic experimental signatures. In particular, the energy distribution of the electrons emitted in the decay gives an insight into the decay mechanism and has been studied in several isotopes and experiments. No deviations from the prediction of the standard model have been reported yet. However, several new experiments are underway or in preparation and will soon increase the sensitivity of these beyond-the-standard-model physics searches, exploring uncharted parts of the parameter space. This review brings together phenomenological and experimental aspects related to new-physics searches in double-beta decay experiments, focusing on the testable models, the most-sensitive detection techniques, and the discovery opportunities of this field.

This article summarizes our present knowledge about nuclear matter at the highest energy densities and its formation in relativistic heavy ion collisions. We review what is known about the structure and properties of the quark-gluon plasma and survey the observables that are used to glean information about it from experimental data.

In the past twenty years, hadron spectroscopy has made immense progress. Experimental facilities have observed different multiquark states during these years. There are different models and phenomenological potentials to study the nature of interquark interaction. In this work, we have reviewed different quark potentials and models used in hadron spectroscopy.

Les Houches activities in 2021 were truncated due to the lack of an in-person component. However, given the rapid progress in the field and the restart of the LHC, we wanted to continue the bi-yearly tradition of updating the standard model precision wishlist. In this work we therefore review recent progress (since Les Houches 2019) in fixed-order computations for LHC applications. In addition, necessary ingredients for such calculations such as parton distribution functions, amplitudes, and subtraction methods are discussed. Finally, we indicate processes and missing higher-order corrections that are required to reach the theoretical accuracy that matches the anticipated experimental precision.

We present a state-of-the-art calculation of the unpolarized pion valence-quark distribution in the framework of large-momentum effective theory (LaMET) with improved handling of systematic errors as well as two-loop perturbative matching. We use lattice ensembles generated by the MILC collaboration at lattice spacing a ≈ 0.09 fm, lattice volume 64³ × 96, N_f = 2 + 1 + 1 flavors of highly-improved staggered quarks and a physical pion mass. The LaMET matrix elements are calculated with pions boosted to momentum P_z ≈ 1.72 GeV with high-statistics of O(10⁶) measurements. We study the pion PDF in both hybrid-ratio and hybrid-regularization-independent momentum subtraction (hybrid-RI/MOM) schemes and also compare the systematic errors with and without the addition of leading-renormalon resummation (LRR) and renormalization-group resummation (RGR) in both the renormalization and lightcone matching. The final lightcone PDF results are presented in the modified minimal-subtraction scheme at renormalization scale μ = 2.0 GeV. We show that the x-dependent PDFs are compatible between the hybrid-ratio and hybrid-RI/MOM renormalization with the same improvements. We also show that systematics are greatly reduced by the simultaneous inclusion of RGR and LRR and that these methods are necessary if improved precision is to be reached with higher-order terms in renormalization and matching.

In this paper we present the results of a new kaonic helium-4 measurement with a 1.37 g l⁻¹ gaseous target by the SIDDHARTA-2 experiment at the DAΦNE collider. We measured, for the first time, the energies and yields of three transitions belonging to the M-series. Moreover, we improved by a factor about three, the statistical precision of the 2p level energy shift and width induced by the strong interaction, obtaining the most precise measurement for gaseous kaonic helium, and measured the yield of the L_α transition at the employed density, providing a new experimental input to investigate the density dependence of kaonic atoms transitions yield.

We perform an extensive survey of rare and exclusive few-body decays ---defined as those with branching fractions $\mathcal{B} \lesssim 10^{-5}$ and two or three final particles--- of the Higgs, Z, W bosons, and the top quark. Such rare decays can probe physics beyond the Standard Model (BSM), constitute a background for exotic decays into new BSM particles, and provide precise information on quantum chromodynamics factorization with small nonperturbative corrections. We tabulate the theoretical $\mathcal{B}$ values for almost 200 rare decay channels of the four heaviest elementary particles, indicating the current experimental limits in their observation. Among those, we have computed for the first time ultrarare Higgs boson decays into photons and/or neutrinos, H and Z radiative decays into leptonium states, radiative H and Z quark-flavour-changing decays, and semiexclusive top-quark decays into a quark plus a meson, while updating predictions for a few other rare H, Z, and top quark partial widths. The feasibility of measuring each of these unobserved decays is estimated for p-p collisions at the high-luminosity Large Hadron Collider (HL-LHC), and for $e^+e^-$ and p-p collisions at the future circular collider (FCC).

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) is a neutral-current low-energy 
electro-weak reaction-channel detected recently by the COHERENT experiment at the Oak Ridge National 
Laboratory (ORNL), USA, in the Spallation Neutron Source facility. The extremely weak signal on the 
$CsI$ detector of the first experiment and on the liquid $Ar$ of the repeated COHERENT experiment is 
the energy-recoil due to the neutrino-nucleus interaction, where the nucleus is elastically scattered 
as a whole while simultaneously the neutrino goes out. Today, several promising nuclear detectors 
are on the way to be employed in designed and ongoing experiments. In our present work, we provide 
predictions for incoherent scattering cross sections of low-energy neutrinos on $^{98,100}$Mo isotopes 
obtained with the Deformed Shell Model employed previously for similar predictions in other electroweak 
processes. We mention that, Mo detector medium has been used previously in the MOON and NEMO double 
beta decay experiments.

We present a lattice QCD analysis of the Δ-baryon spectrum, with the goal of finding the position of the 2s radial excitation of the Δ(1232) ground state. Using smeared three-quark operators in a correlation matrix analysis, we report masses for the ground, first and second excited states of the J^P = 3/2⁺ spectrum across a broad range of m_π². We identify the lowest lying state as being a 1s state, consistent with the well known Δ(1232). The first excitation is identified as a 2s state, but is found to have a mass of approximately 2.15 GeV on our ∽ 3 fm lattice, which does not appear to be associated with the Δ(1600) resonance in a significant manner. We also report on the spin-1/2 and odd-parity states accessible via our methods. The large excitation energies of the radial excitations provide a potential resolution to the long-standing missing baryon resonances problem.

In recent years, there has been tremendous progress in the investigation of bound systems of quarks with multiplicities beyond the more usual two- and three-quark systems. Experimental and theoretical progress has been made in the four-, five- and even six-quark sectors. In this paper, we review the possible lightest six-quark states using a simple ansatz based on SU(3) symmetry and evaluate the most promising decay branches. The work will be useful to help focus future experimental searches in this six-quark sector.

True muonium (TM) (μ⁺μ⁻) is the heaviest and smallest bound state not containing hadrons, after TM (τ⁺τ⁻) and mu-tauonium (μ^±τ^∓). One of the proposed methods to observe the spin 1 fundamental state of TM, which has the smallest lifetime among TM spin 1 states, was to build an e⁺e⁻ collider with a large crossing angle (θ ∼ 30°) in order to provide TM with a large boost and detect its decay vertex in e⁺e⁻. The following paper will instead show that TM excited states can be observed in relatively large quantities (${ \mathcal O }$(10)/month) at a e⁺e⁻ collider with standard crossing angle, after setting their center-of-mass energy to the TM mass (∼2m_μ = 211.4 MeV).

We have studied a flavor symmetry-based extended left–right symmetric model (LRSM) with a dominant type-II seesaw mechanism and have explored the associated neutrino phenomenology. The particle content of the model includes usual quarks and leptons along with additional sterile fermion per generation in the fermion sector while the scalar content contains Higgs doublets and scalar bidoublet. Realization of this extension of LRSM has been done by using A₄ × Z₄ discrete symmetries. In this work, we have also included the study of sterile neutrino dark matter phenomenology along with neutrinoless double beta decay within the framework.

A potential for the vertex and self-energy correction is derived from the first-order Born theory. The inclusion of this potential in the Dirac equation, together with the Uehling potential for vacuum polarization, allows for a nonperturbative treatment of these quantum electrodynamical effects within the phase-shift analysis. Investigating the ¹²C and ²⁰⁸Pb targets, a considerable deviation of the respective cross section change from the Born results is found for the heavier target. It is shown that at low impact energies the dispersion effects play no role. Estimates for the correction to the beam-normal spin asymmetry and its accuracy at 5 MeV (for ²⁰⁸Pb and ¹⁹⁷Au) are also provided.

Simulations to evaluate the feasibility of antineutron identification and kinematic characterization via the hadronic charge exchange (CEX) interaction $n\,+\,\bar{n}\,\to \,p\,+\,\bar{p}$ are reported. The target neutrons are those composing the silicon nuclei of which inner tracking devices present in the Large Hadron Collider experiments ALICE, ATLAS, and CMS. Simulations of pp collisions in PYTHIA were carried out at different energies to investigate $\bar{n}$ production and energy spectra. These simulations produced a decreasing power-law $\bar{n}$ energy spectra. Then, two types of GEANT4 simulations were performed, placing an $\bar{n}$ point source at the ALICE primary vertex, as a working example. In the first simulation, the kinetic energy E_k was kept at an arbitrary (1 GeV) fix value to develop an $\bar{n}$ identification and kinematics reconstruction protocol. The second GEANT4 simulation used the resulting PYTHIA at $\sqrt{{s}_{{pp}}}\,=\,13$ TeV $\bar{n}$ energy spectra. In both GEANT4 simulations, the occurrence of CEX interactions was identified by the unique outgoing $\bar{p}$. The simplified simulation allowed to estimate a 0.11% CEX-interaction identification efficiency at E_k = 1 GeV. The p CEX-partner identification is challenging because of the presence of silicon nucleus-fragmentation protons. Momentum correlations between the $\bar{n}$ and all possible $\bar{p}p$ pairs showed that p CEX-partner identification and $\bar{n}$ kinematics reconstruction corresponds to minimal momentum-loss events. The use of inner tracking system dE/dx information is found to improve $\bar{n}$ identification and kinematic characterization in both GEANT4 simulations. The final protocol applied to the realistic GEANT4 simulation resulted in a $\bar{n}$ identification and kinematic reconstruction efficiency of 0.006%, based solely on $\bar{p}p$ pair observable. If applied to the ALICE minimum-bias RUN2 pp at $\sqrt{{s}_{{pp}}}=13$ TeV data sample, this technique is found to have the potential to identify and reconstruct the kinematics of $4.3\times {10}^{8}\bar{n}$'s, illustrating the feasibility of the method.