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A bstract We examine the origins of azimuthal correlations observed in high energy proton-nucleus collisions by considering the simple example of the scattering of uncorrelated partons off color fields in a large nucleus. We demonstrate how the physics of fluctuating color fields in the color glass condensate (CGC) effective theory generates these azimuthal multiparticle correlations and compute the corresponding Fourier coefficients v n within different CGC approximation schemes. We discuss in detail the qualitative and quantitative differences between the different schemes. We will show how a recently introduced color field domain model that captures key features of the observed azimuthal correlations can be understood in the CGC effective theory as a model of non-Gaussian correlations in the target nucleus.

Thermal noise is present in any viscous fluid, making the simulation of relativistic noise in heavy ion collisions a necessity. It is likely possible to use it to make an independent measurement of viscosity in heavy ion collisions. The size, energy densities, and time scales of the collisions determine the relative importance of thermal noise. This causes a non-trivial contribution to two-particle correlations as well as event-by-event fluctuations in observables.

Time dependent medium modifications of low mass vector mesons are investigated within a nonequilibrium quantum field theoretical description on the basis of the Kadanoff-Baym equations. Time scales for the adaption of the spectral properties to changing self energies are given and, under use of a model for the fireball evolution, nonequilibrium dilepton yields from the decay of ρ- and ω-mesons are calculated. Comparison of the results with calculations that assume instantaneous adaption to the changing medium show that the consideration of memory effects is important.

We study initial state momentum correlations and event-by-event geometry in p+Pb collisions at \\(s=5.02~ TeV\\) by following the approach of extending the IP-Glasma model to 3D using JIMWLK rapidity evolution. On examining the non-trivial rapidity dependence of the observables, we find that the geometry is correlated over large rapidity intervals, while the initial state momentum correlations have a relatively short range in rapidity. Based on our results, we discuss implications for the relevance of both effects in explaining the origin of collective phenomena in small systems.

We employ the SMASH transport model to provide event-by-event initial conditions for the energy-momentum tensor and conserved charge currents in hydrodynamic simulations of relativistic heavy-ion collisions. We study the fluctuations and dynamical evolution of three conserved charge currents (net baryon, net electric charges, and net strangeness) with a 4D lattice-QCD-based equation of state, NEOS-4D, in the hydrodynamic phase. Out-of-equilibrium corrections at the particlization are generalized to finite densities to ensure the conservation of energy, momentum, and the three types of charges. These theoretical developments are integrated within the X-SCAPE code as a unified framework for studying the nuclear matter properties in the Beam Energy Scan program.

The physics case for the operation of high-luminosity proton-nucleus (\\(pA\\)) collisions during Run 3 and 4 at the LHC is reviewed. The collection of \\(O\\)(1-10 pb\\(^-1\\)) of proton-lead (\\(p\\)Pb) collisions at the LHC will provide unique physics opportunities in a broad range of topics including proton and nuclear parton distribution functions (PDFs and nPDFs), generalised parton distributions (GPDs), transverse momentum dependent PDFs (TMDs), low-\\(x\\) QCD and parton saturation, hadron spectroscopy, baseline studies for quark-gluon plasma and parton collectivity, double and triple parton scatterings (DPS/TPS), photon-photon collisions, and physics beyond the Standard Model (BSM); which are not otherwise as clearly accessible by exploiting data from any other colliding system at the LHC. This report summarises the accelerator aspects of high-luminosity \\(pA\\) operation at the LHC, as well as each of the physics topics outlined above, including the relevant experimental measurements that motivate -- much -- larger \\(pA\\) datasets.

We constrain the average density profile of the proton and the amount of event-by-event fluctuations by simultaneously calculating the coherent and incoherent exclusive diffractive vector meson production cross section in deep inelastic scattering. Working within the Color Glass Condensate picture, we find that the gluonic density of the proton must have large geometric fluctuations in order to describe the experimentally measured large incoherent cross section.

We present predictions and postdictions for a wide variety of hard jet-substructure observables using a multistage model within the JETSCAPE framework. The details of the multistage model and the various parameter choices are described in [A. Kumar et al., arXiv:2204.01163]. A novel feature of this model is the presence of two stages of jet modification: a high virtuality phase [modeled using the modular all twist transverse-scattering elastic-drag and radiation model (MATTER)], where modified coherence effects diminish medium-induced radiation, and a lower virtuality phase [modeled using the linear Boltzmann transport model (LBT)], where parton splits are fully resolved by the medium as they endure multiple scattering induced energy loss. Energy-loss calculations are carried out on event-by-event viscous fluid dynamic backgrounds constrained by experimental data. The uniform and consistent descriptions of multiple experimental observables demonstrate the essential role of modified coherence effects and the multistage modeling of jet evolution. Using the best choice of parameters from [A. Kumar et al., arXiv:2204.01163], and with no further tuning, we present calculations for the medium modified jet fragmentation function, the groomed jet momentum fraction \\(z_g\\) and angular separation \\(r_g\\) distributions, as well as the nuclear modification factor of groomed jets. These calculations provide accurate descriptions of published data from experiments at the Large Hadron Collider. Furthermore, we provide predictions from the multistage model for future measurements at the BNL Relativistic Heavy Ion Collider.

Quark-gluon plasma produced at the early stage of ultrarelativistic heavy ion collisions is unstable, if weakly coupled, due to the anisotropy of its momentum distribution. Chromomagnetic fields are spontaneously generated and can reach magnitudes much exceeding typical values of the fields in equilibrated plasma. We consider a high energy test parton traversing an unstable plasma that is populated with strong fields. We study the momentum broadening parameter \\( q\\) which determines the radiative energy loss of the test parton. We develop a formalism which gives \\( q\\) as the solution of an initial value problem, and we focus on extremely oblate plasmas which are physically relevant for relativistic heavy ion collisions. The parameter \\( q\\) is found to be strongly dependent on time. For short times it is of the order of the equilibrium value, but at later times \\( q\\) grows exponentially due to the interaction of the test parton with unstable modes and becomes much bigger than the value in equilibrium. The momentum broadening is also strongly directionally dependent and is largest when the test parton velocity is transverse to the beam axis. Consequences of our findings for the phenomenology of jet quenching in relativistic heavy ion collisions are briefly discussed.

Heavy-ion collisions are well described by a dynamical evolution with a long hydrodynamical phase. In this phase the properties of the strongly coupled quark-gluon plasma are reflected in the equation of state (EoS) and the transport coefficients, most prominently by the shear and bulk viscosity over entropy density ratios \\(\\)/s(T) and \\(\\)/s(T), respectively. While the EoS is by now known to a high accuracy, the transport coefficients and in particular their temperature and density dependence are not well known from first-principle computations yet, as well as the possible influence they can have once used in hydrodynamical simulations. In this work, the most recent QCD-based parameters are provided as input to the MUSIC framework. A ratio \\(\\)/s(T) computed with a QCD based approach is used for the first time Haas:2013hpa,Christiansen:2014ypa. The IP-Glasma model is used to describe the initial energy density distribution, and UrQMD for the dilute hadronic phase. Simulations are performed for Pb--Pb collisions at \\(s_ NN\\) = 2.76 TeV, for different centrality intervals. The resulting kinematic distributions of the particles produced in the collisions are compared to data from the LHC, for several experimental observables. The high precision of the experimental results and the broad variety of observables considered allow to critically verify the quality of the description based on first-principle input to the hydrodynamic evolution.