Session MF: Mini-Symposium: The Initial State of Heavy-Ion Collisions II

Adiabatic hydrodynamization in the bottom-up thermalization scenario

In this talk we demonstrate that the early stages of the bottom-up thermalization scenario are well described by the adiabatic hydrodynamization framework. All of the qualitative features exhibited in QCD effective kinetic theory (EKT) simulations at weak coupling are captured by the emergence of an effective low-energy instantaneous ground state for the 1-particle gluon distribution function, which defines the early-time kinetic theory attractor. This ground state may be pulled back to arbitrarily early times, where it represents a free-streaming solution, and at later times it integrally describes the BMSS fixed point, including the recently observed deviations from the original predictions for the scaling exponents.

We proceed by studying scaling in the small-angle scattering approximation of QCD EKT. We find that a momentum rescaling allows the scaling distribution to be identified as the instantaneous ground state of the evolution of the distribution function, and the approach to the scaling function is described by the decay of the excited states. Corrections to the BMSS fixed point exponents agree quantitatively with previous numerical studies.   

Initializing Conserved Charges in the Initial State of Heavy-Ion Collisions

While it is well known that there is a significant amount of quark anti-quark pairs in the initial state of nuclear collisions, the production of these due to gluon splitting has yet to be thoroughly investigated. The ICCING (Initial Conserved Charges in Nuclear Geometry) algorithm reconstructs these quark distributions, providing conserved baryon, strange, and electric charges, by sampling a given model for the gluon to quark/antiquark splitting function over the initial energy density, which is valid at top collider energies, even when baryon chemical potential equals zero. There is a substantial difference between the charge and energy densities that indicates sensitivity to different geometries of the initial state. The ICCING algorithm includes fluctuations in the gluon longitudinal momenta, a structure that supports the implementation of dynamical processes, and the c++ version is now open-source. Further development is done through the inclusion of pre-hydrodynamic evolution that introduces locally dependent density fluctuations that propagate spatially.   

Two-Particle Azimuthal Correlations at Forward Rapidity in d+Au Collisions at √sNN=200 GeV.

Gluons dominate the nuclear wavefunction at low x, where x is the parton's momentum fraction of a nucleon within the nucleus. It is now well established that fewer gluons per nucleon are present in the nucleus than in a bare proton. This reduction may be the result of gluon saturation within the nucleus as described by the Color Glass Condensate (CGC) model where an equilibration between gluon splitting and gluon fusion at low x is established leading to a saturation of gluons within the nucleus. Two-particle azimuthal correlations at forward rapidity in d+Au collisions are sensitive to both the number and the kinematics of low-x gluon kinematics in the gold nucleus making them useful to study the CGC. In 2016, PHENIX took data with the MPC-EX detector, which consists of eight finely segmented Si-W presampling layers in front of the MPC electromagnetic calorimeter located at 3.1<η<3.8. The goal of the MPC-EX is to study gluon saturation. In this talk we present the current status of the analysis of π⁰-π⁰ correlations measured in the MPC+MPC-EX acceptance of the PHENIX detector at 200 GeV d+Au collisions and discuss their implications for understanding gluon saturation.    

Studies of low-x phenomena with the LHCb detector

With a unique geometry covering the forward rapidity region, the LHCb detector provides unprecedented access to the low Bjorken-x down to x~10^-5 or lower. The excellent momentum resolution, vertex reconstruction and particle identification allow precision measurements down to very low hadron transverse momentum. In this contribution we present the latest studies of the relatively unknown low-x region using the LHCb detector, and discuss their unique constraints these measurements provide on nuclear parton distribution functions. Comparisons to various theoretical model calculations and implications on our understanding of gluon saturation in the nucleus will also be discussed.   

Diffractive photo-nuclear production of $\rho^0$ mesons in peripheral Au+Au collisions at STAR

Highly Lorentz-contracted electromagnetic fields from fast-moving nuclei can be quantized as linearly polarized quasi-real photons. Photons from one nucleus may scatter coherently off of the other nucleus, and produce vector mesons, such as $\rho^0$. This diffractive interaction creates vector mesons which are (like their parent photon) polarized along the direction of the field and radially away from the nucleus. Coherently produced vector mesons have been previously measured in p+p and Ultra-Peripheral A+A Collisions (UPCs). The $\rho^0$ in UPCs is expected to exhibit quantum interference between the wave functions of the $\rho^0$ from either nucleus. A recent measurement demonstrates this effect in UPCs directly by measuring the angular distribution of the daughter pions. In this talk, we present a complementary measurement by looking for the same photoproduction and quantum interference of $\rho^0$ mesons in 200 GeV peripheral Au+Au collisions at STAR. It is not known to what degree (if at all) the quantum interference might survive the hadronic interactions occurring concurrently in the same Au+Au collision. By measuring such an interference in peripheral Au+Au collisions, one can test the limits of the ``coherence'' of the diffractive process.   

Measurements of J/?? production vs event multiplicity in the forward rapidity in p+p and p+A?? collisions in the PHENIX experiment

J/?? production has been studied extensively in high energy ??+?? and heavy ion collisions at RHIC and other facilities, and several promising pQCD based production models have been investigated in details, however, our understanding of the production mechanisms still remain largely uncertain. The recent observations of enhanced particle yields, including ??/??, in high event multiplicity ??+?? collisions at RHIC and LHC suggest strong contributions from possible semi-hard Multi-Parton Interactions (MPI) as well as from other effects including gluon saturation in the initial state. These new results challenge not only the traditional “single hard-scattering” pQCD description, but also the interpretation of other important observables, such as event centrality in heavy ion collisions and certain transverse spin asymmetries in polarized ??+??(??) interactions. To gain further insight into particle production mechanisms, we study the ??/?? yield in the forward (and backward) rapidity in p+p and p+Au collisions as a function of event multiplicity determined over a broad range of rapidity, considering both possible global and local correlations. The latest results from PHENIX will be presented.

    

Φ-Meson Production at Forward Rapidity in Au + Au collisions at √sNN = 200 GeV.

A major objective in the field of high-energy nuclear physics is to quantify and characterize the quark-gluon plasma formed in relativistic heavy-ion collisions. The ? meson is an excellent probe for studying this hot and dense state of nuclear matter because of its very short lifetime, and the absence of strong interactions between its associated decay muons and the surrounding hot hadronic matter makes the Φ→µ⁺µ^- decay channel particularly interesting. Since ?-meson is composed of a strange and anti-strange quark, its nuclear modification in heavy-ion collisions may provide an insight into strangeness enhancement in-medium. PHENIX has measured ? meson cold nuclear modification in a variety of small systems, and the same measurement in Au + Au collisions may provide useful information about the hot nuclear matter effects. In this talk, we will present the analysis status of ? meson production at forward rapidity in Au + Au collisions at √s_NN = 200 GeV.

    

ALICE Forward Calorimeter (FoCal) Upgrade

The Forward Calorimeter (FoCal) is proposed as an upgrade for the ALICE experiment for Run 4 at the LHC to provide new constraints on the small-x gluon structure of the protons and nuclei. Its main goal is the measurement of direct photon production from QCD Compton scattering, while also providing very precise data in neutral meson and jet measurements. FoCal consists of an electromagnetic (FoCal-E) and hadronic calorimeter (FoCal-H). The FoCal-E is a Si-W calorimeter combining digital pixel and analog pad readout in order to maximize both the energy and position resolution of the individual showers. The FoCal-H is a spaghetti-style calorimeter with Cu absorbers and scintillating fibers. In this presentation we report results from beam tests of the prototype detector at the SPS at CERN in 2021 and 2022.