11th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions

The forward geometry of the LHCb detector provides unprecedented access to both the very high and very low regions of Bjorken $x$ inside the nucleon. With full particle ID and a fast DAQ, LHCb is able to fully reconstruct plentiful charged particles and $\pi^{0}$ mesons, as well as relatively rare probes such as $Z$ bosons and heavy quarks, providing a unique set of constraints on nucleon...

In the early stages of heavy-ion collisions, at the highest energies, the system begins in a highly anisotropic state which is far from equilibrium. At later times, the dynamic evolution of the system is well described in the framework of relativistic hydrodynamics which requires local thermodynamic equilibrium. The KoMPoST framework has had some success in bridging the gap between these...

High energy nuclear collisions produce far-from-equilibrium matter with a high density of gluons at early times. We identify for the first time two local order parameters for condensation, which can occur as a consequence of the large density of gluons. We demonstrate that an initial over-occupation of gluons can lead to the formation of a macroscopic zero mode towards low momenta that scales...

Within a microscopic kinetic description based on the Boltzmann equation, we evaluate the importance of the pre-equilibrium stage in high-energy heavy-ion collisions for final state observables over a large range of viscosity and system size. We use our results to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics...

Measurements of quarkonia production in peripheral and ultra-peripheral heavy-ion collisions are sensitive to photon-photon and photon-nucleus interactions, the partonic structure of nuclei, and to the mechanisms of vector-meson production. LHCb has studied production of the $J/\psi$ and $\psi(2S)$ charmonium states in peripheral and ultra-peripheral collisions using PbPb data at forward...

Minijets, created by perturbative hard QCD collisions at moderate energies, can represent a significant portion of the total multiplicity of a heavy-ion collision event. Since their transverse momenta are larger than the typical saturation scale describing the bulk of the equilibrating QGP, they do not in general hydrodynamize at the same pace than the bulk of the collision. In this work we...

In relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Thus, photon-induced processes are the dominant interaction mechanism when the colliding nuclei have a transverse separation larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs), the photon provides a clean, energetic probe of the partonic structure of the nucleus,...

We construct an improved 3+1D version of the Trento initial state model, which includes rapidity-dependent fluctuations. The correlation between the fluctuations at different rapidities is controlled by a new parameter. We then use this improved model to study rapidity-dependent observables for ultracentral collisions. It it known that ultracentral flow at midrapidity is sensitive to...

We study the impact of the Glasma fields, used to describe the very-early stage of heavy-ion collisions, on the transport of hard probes, namely heavy quarks and jets. We perform numerical simulations of the strong classical fields using techniques from real-time lattice gauge theory. The resulting fields are used as background for the classical transport of ensembles of particles, described...

Photonuclear reaction is induced by the strong electromagnetic field generated by ultrarelativistic heavy ion collisions. This process has been extensively studied in ultra-peripheral collisions (UPC). Photoproduced quarkonia are used to probe the nuclear gluon distributions at low Bjorken-$x$. In recent years, the coherent photoproduction of the J/$\psi$ vector meson has also been observed in...

We discuss the evolution of initial momentum anisotropy in the early-stage quark-gluon plasma. We use kinetic theory to study the far-from-equilibrium evolution of an expanding plasma with an anisotropic momentum-space distribution. We identify slow and fast degrees of freedom in the far-from-equilibrium plasma from the evolution of moments of this distribution. At late times, the slow modes...

Proton-lead collisions at LHC energies offer unique possibilities to investigate the nuclear modifications of parton distribution functions (PDF) over a wide kinematic range. Several probes can be measured to characterize these effects in different kinematic regimes. The top-quark production is expected to be sensitive to effects at high Bjorken-x values, which are hard to access...

Small systems display large anisotropic flow coefficients that can potentially be interpreted as a hydrodynamic signal. At these moderate multiplicities anisotropic flow is however relatively sensitive to subtle effects. These include the precise experimental procedure, rapidity coverage and gaps as well as effects due to resonance decays. In this talk we quantify these effects for pPb, OO and...

The early stages of heavy-ion collisions are largely unexplored experimentally, despite great theory progresses. In such collisions, electromagnetic radiation such as dileptons are produced throughout the history of the medium and probe its quark content. Hence, they are useful tools to investigate the early stages of the quark-gluon plasma, allowing to better understand its chemical and...

In ultraperipheral collisions (UPCs) of relativistic heavy ions, the coherent heavy-flavor vector meson production via photon-nuclear interactions is of particular interest, since its cross section is directly sensitive to the nuclear gluon density. However, in experimental measurements, because each of the two nuclei in symmetric UPCs can serve both as a photon-emitter projectile and a...

Viscous hydrodynamics serves as a successful mesoscopic description of the Quark-Gluon Plasma (QGP) on large time and distance scales. Since highly energetic Jets deposit part of their energy into the QGP in a very localized fashion, it is important to understand to what extent the propagation of the deposited energy can be described within hydrodynamics. We investigate this problem by...

We investigate the early time dynamics of heavy ion collisions studying the time evolution of the energy-momentum tensor as well as energy-momentum correlations within a uniformly thermalizing holographic QGP. From these quantities, we suggest a far-from equilibrium definition of shear viscosity, which is a crucial property of QCD matter as it significantly determines the generation of...

We demonstrate the unique opportunities for small systems studies offered by complementing the future run of $^{16}$O+$^{16}$O collisions at the CERN LHC with collisions of bowling-pin-shaped $^{20}$Ne isotopes.

1. Origin of collectivity: A comprehensive campaign of hydrodynamic calculations (~20 million simulated events) demonstrates that the impact of the extreme shape of neon on...

We present a new 3+1D resolved model for the initial state of ultrarelativistic Heavy-Ion collisions, based on the $k_\perp$-factorized Color Glass Condensate hybrid approach [1-4]. This new model responds to the need for a rapidity-resolved initial-state Monte Carlo event generator which can deposit the relevant conserved charges (energy, charge and baryon densities) both in the midrapidity...

Heavy-ion collisions at $\sqrt{s_{\rm NN}} \sim 10$ GeV probes the QCD phase diagram at large baryon densities. However, because the longitudinal Lorentz contraction is small at these collision energies, understanding the dynamics during the early phase of the collision is essential for the subsequent modeling of the system evolution, and for constraining the QGP transport properties at finite...

I am going to present a state-of-the-art computation for the production of forward dijets in proton-proton and proton-lead collisions at the LHC, in rapidity domains covered by the ATLAS calorimeter and the planned FoCal extension of the ALICE detector. We use the small-x improved TMD (ITMD) formalism, together with collinearly improved TMD gluon distributions and full b-space Sudakov...

Nuclear Parton Distribution Functions (nPDF) are an essential tool to predict hard QCD processes in nuclear collisions. Recently various nPDF sets have been extracted using heavy flavour data (D mesons, quarkonia) in pA collisions in the global fits. However, these measurements should be affected by fully coherent energy loss (FCEL) in nuclear matter, which entails a careful treatment in the...

Since 2011 a wide variety of measurements suggest the existence of strong collectivity in collisions of small systems such as proton-proton (pp) and proton-nucleus (pPb) with hydrodynamic models and gluon saturation in the initial state as two theory alternatives showing consistency with the observations. These results raise the question as to whether such phenomena may be present in even...

Analyzing data from nuclear lepton Deep-Inelastic Scattering, Drell- Yan processes, and W and Z boson production, we show that factorizing nuclear structure into quasi-free nucleons and universally modified close-proximity Short Range Correlated (SRC) nucleon pairs allows us to fully describe the quark-gluon structure of nuclei down to very-low momentum fractions. This is the first combined...