# Bulletin of the American Physical Society

# 2020 Fall Meeting of the APS Division of Nuclear Physics

## Volume 65, Number 12

## Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA

## Session MC: Initial State |
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Chair: Cesar Luiz da Silva, LLNL |

Saturday, October 31, 2020 2:00PM - 2:12PM |
MC.00001: A collision geometry-based 3D initial condition for relativistic heavy-ion collisions} Sahr Alzhrani, Chun Shen We present a simple way to construct 3D initial conditions for relativistic heavy-ion collisions based on the Glauber collision geometry. Local energy and momentum conservation conditions are imposed to set non-trivial constraints on our parameterizations of longitudinal profiles for the system's initial energy density and flow velocity. After calibrating parameters with charged hadron rapidity distributions in central Au+Au collisions, we test model predictions for particle rapidity distributions in d+Au and peripheral Au+Au collisions in the Beam Energy Scan (BES) program at Relativistic Heavy-Ion Collider (RHIC). Simulations and comparisons with measurements are also made for Pb+Pb collisions at Super Proton Synchrotron (SPS) energies. We demonstrate that elliptic flow measurements in heavy-ion collisions at $\sqrt{s} \sim 10$ GeV can set strong constraints on the dependence of Quark-Gluon Plasma shear viscosity on temperature and net baryon chemical potential. [Preview Abstract] |

Saturday, October 31, 2020 2:12PM - 2:24PM |
MC.00002: Gluonic Hot Spot Initial Conditions in Heavy Ion Collisions Ross Snyder, Megan Byres, Sanghoon Lim, James Nagle Experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have measured collective flow coefficients that reflect the geometry of the quark-gluon plasma created in Au-Au and Pb-Pb collisions. The initial conditions for these collisions are calculated in many different frameworks. In particular, the Monte Carlo Glauber (MCG) method of calculating these initial conditions was a major advancement in the field about a decade ago, and it establishes that nucleon position fluctuations are of importance when describing the flow of the quark-gluon plasma. A new model, called MAGMA, was recently proposed that calculates the flow coefficients by accounting for both the smooth nucleus structure and gluon hotspots within the nucleus. The flow coefficients that MAGMA predicted at each centrality percentage were a good fit to experimental data. Also, it may have explained how ultra-central collisions (0-1\% centrality) produce v$_{2}$\{2\} and v$_{3}$\{2\} flow coefficients that are almost identical. We explore whether the MAGMA calculations are justified by modeling the hydrodynamics of ultra-central events. We also compare MAGMA with other MCG calculations that are often used to describe the experimental data. [Preview Abstract] |

Saturday, October 31, 2020 2:24PM - 2:36PM |
MC.00003: Influence of fluctuating initial-state shape deformations in ultra-central collisions Peifeng Liu, Roy Lacey It is a long standing puzzle that hydrodynamic calculations give larger elliptic flow for ultra-central Pb+Pb events than experiments. One explanation is that sampling from a single particle distribution in Glauber model generates too much shape fluctuation, which can be expected to be modified by NN correlations. We study the fluctuation of shape-deformation as currently implemented in MC-Glauber-like models and its consequences on the magnitude of eccentricities. We show that for both deformed and spherical species, there are large fluctuations in the deformation that emerge event-by-event. For each nucleonic configuration, we characterize the deformation with parameters $\beta$ and $\gamma$, calculated using quadruple moments. The second order eccentricity $\varepsilon_2$ shows a strong correlation with the E-by-E deformation $\beta$'s for ultra-central collisions. We further show, via acoustic scaling, with a moderate reduction of $\beta$ by rescaling, the eccentricities we get then agree with the measured values of $v_n$, for all experimentally available centrality bins. Therefore, the model provides eccentricities that describe experimental data, both where geometry dominates and where fluctuation dominates. [Preview Abstract] |

Saturday, October 31, 2020 2:36PM - 2:48PM |
MC.00004: Initial Conditions of Conserved Charges in Heavy-Ion Collisions Matthew Sievert, Mauricio Martinez, Douglass Wertepny, Jacquelyn Noronha-Hostler One of the major goals of the heavy-ion program in nuclear physics is to experimentally determine the transport properties of the quark-gluon plasma (QGP), a novel state of hot and dense nuclear matter. While properties such as the shear and bulk viscosities of the QGP have been extensively studied at top collider energies, the dynamics of charge transport have largely been relegated to lower energies where net baryon stopping becomes important. In this work, we outline a new approach to studying charge transport in the QGP which can be applied even at the LHC. We present a new Monte Carlo event generator called ICCING which constructs the initial state of a heavy ion collision, including the pockets of positive and negative charge produced by quark pair production (even though the total charge is zero). These initial conditions of the conserved charges can then be evolved in hydrodynamics or other transport models to study the role of charge in particle correlations. We also propose sets of charge-conjugation-odd observables that should be most sensitive to the initial-state fluctuations of the conserved charges. [Preview Abstract] |

Saturday, October 31, 2020 2:48PM - 3:00PM |
MC.00005: Octupole deformation of $^{208}$Pb does not resolve the ultracentral $v_2$ to $v_3$ puzzle Patrick Carzon, Skanda Rao, Matthew Sievert, Matthew Luzum, Jacquelyn Noronha-Hostler Recent measurements establish the sensitivity of ultracentral heavy-ion collisions to deformation parameters of non-spherical nuclei. In ${}^{ 129}$Xe collisions, a quadrupole deformation of nuclear profile led to an enhancement of elliptic flow in the most central collisions. In ${}^{ 208}$Pb collisions a discrepancy exists in similar centralities, where either elliptic flow is over-predicted or triangular flow is under-predicted by hydrodynamic models; this is known as the $v_2$-to-$v_3$ puzzle in ultracentral collisions. Motivated by low-energy nuclear structure calculations, we consider the possibility that $^{208}$Pb nuclei could have an octupole deformation, which has the effect of increasing triangular flow in central PbPb collisions. Using data from ALICE and ATLAS, we revisit the $v_2$-to-$v_3$ puzzle in ultracentral collisions, including new constraints from recent measurements of triangular cumulant ratio $v_3\{4\}/v_3\{2\}$ and comparing two different hydrodynamic models. We find that data is consistent with an octupole deformation $\beta_3$ of ${}^{ 208}$Pb less than $0.0375$ and that addition of an octupole deformation does not significantly improve the agreement with data. [Preview Abstract] |

Saturday, October 31, 2020 3:00PM - 3:12PM |
MC.00006: New constraints of initial states in PbPb collisions with Z boson yields and azimuthal anisotropy at $\sqrt{s_{NN}}$ = 5.02 TeV Austin Baty The differential yields of Z bosons decaying to a pair of leptons are measured in PbPb collisions collected by the CMS experiment at the LHC. The measurements are performed for collisions at 5.02 TeV, using both the muon and electron decay channels. The yields in various centrality bins are compared to Glauber model predictions of the production rates of hard probes not modified by the presence of a hot medium. For the first time, Z boson yields in peripheral collisions are found to deviate from the canonical scaling expected for colorless hard probes, indicating the presence of initial collision geometry and centrality selection biases. Because the measurement uncertainties are comparable to the uncertainties of a Glauber-scaled reference, Z boson yields can now be used as an experimental measure of the effective nucleon-nucleon luminosity without loss of precision. A high precision measurement of the Z boson azimuthal anisotropy ($v_2$) is also found to be compatible with zero, showing that Z bosons do not experience significant final-state modification in heavy ion collisions. [Preview Abstract] |

Saturday, October 31, 2020 3:12PM - 3:24PM |
MC.00007: Pre-hydrodynamic evolution and its signatures in final-state heavy-ion observables Tiago Jose Nunes da Silva, David Chinellato, Mauricio Hippert, Willian Serenone, Jun Takahashi, Gabriel S. Denicol, Matthew Luzum, Jorge Noronha We investigate the effects of pre-hydrodynamic evolution on final-state observables in heavy-ion collisions, including results in small systems, using state-of-the art event simulations coupled to different pre-hydrodynamic scenarios, which include the recently-developed effective kinetic transport theory evolution model KoMPoST. While we found flow observables to be insensitive to the details of pre-hydrodynamic evolution, we observe an effect in the transverse momentum spectra. We point out that at least part of this effect is a consequence of the underlying conformal invariance assumption currently present in pre-equilibrium approaches, which leads to an artificially large out-of-equilibrium bulk pressure when switching from (conformal) pre-hydrodynamic evolution to hydrodynamics (using the non-conformal QCD equation of state). These results indicate that a consistent treatment of pre-hydrodynamic evolution in heavy-ion collisions requires the use of non-conformal models of early-time dynamics. [Preview Abstract] |

Saturday, October 31, 2020 3:24PM - 3:36PM |
MC.00008: Probing the Structure of the Initial State of Heavy-Ion Collisions with $p_T$-Dependent Flow Fluctuations Mauricio Hippert, Joao Gabriel Barbon, David Dobrigkeit Chinellato, Matthew Luzum, Jorge Noronha, Tiago Nunes da Silva, Willian Serenone, Jun Takahashi The connection between initial-state geometry and anisotropic flow can be quantified through a well-established mapping between $p_T$-integrated flow harmonics and cumulants of the initial transverse energy distribution. In this paper we successfully extend this mapping to also include $p_T$-differential flow. In doing so, we find that subleading principal components of anisotropic flow can reveal previously unobserved details of the hydrodynamic response, in both the linear and the nonlinear regimes. Most importantly, we show that they provide novel information on the small-scale structures present in the initial stage of relativistic heavy-ion collisions. [Preview Abstract] |

Saturday, October 31, 2020 3:36PM - 3:48PM |
MC.00009: Pseudorapidity fluctuations in relativistic heavy-ion collisions with the ALICE detector Raquel Quishpe Anisotropic flow in relativistic heavy-ion collisions has been studied as a signature of the quark gluon plasma (QGP). For different colliding systems, anisotropic flow has been described by the decomposition of azimuthal correlations into Fourier coefficients. Furthermore, it has been observed that longitudinal correlations also show anisotropies in the medium, which may give further constrains to the QGP and the initial state. These longitudinal correlations can be analyzed and described by orthogonal polynomials, such as Legendre Polynomials. Results from longitudinal correlations coefficients, a_n, are presented for Xe-Xe (√sNN = 5.44 TeV) and Pb-Pb (√sNN = 5 TeV) collisions at the LHC. [Preview Abstract] |

Saturday, October 31, 2020 3:48PM - 4:00PM |
MC.00010: 3-D Glasma initial state: Breaking boost-invariance by collisions of extended shock waves in classical Yang-Mills theory Pragya Singh, Soeren Schlichting We perform classical Yang-Mills simulations of the 3+1D structure of the initial state, based on the Color-Glass Condensate framework beyond the boost-invariant approximation. We demonstrate how the boost invariant case is recovered in the high-energy limit and study violations of boost invariance at smaller center of lower energies. By including (semi-) realistic color charge distributions, based on a simple parameterization of small-x TMDs, we further investigate the longitudinal fluctuations which emerge naturally within our framework. [Preview Abstract] |

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