Bulletin of the American Physical Society
2021 Fall Meeting of the APS Division of Nuclear Physics
Volume 66, Number 8
Monday–Thursday, October 11–14, 2021; Virtual; Eastern Daylight Time
Session EE: Ultrarelativistic Heavy Ions I |
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Chair: Claudia Ratti, University of Houston Room: Park & Scollay |
Tuesday, October 12, 2021 11:45AM - 11:57AM |
EE.00001: Beam-energy dependence of the longitudinal broadening of two-particle transverse momentum correlations from STAR Niseem M Abdelrahman Two-particle transverse momentum correlator is a powerful technique for understanding the dynamics of relativistic heavy-ion collisions. Among these, the transverse momentum correlator $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ is of particular interest for its potential sensitivity to the shear viscosity per entropy density $\eta/s$ of the quark-gluon plasma formed in the collisions [1,2]. We used the data of Au--Au collisions at $\sqrt{s_{\rm NN}}$ = 200-11.5 GeV to investigate: (i) the longitudinal broadening of the $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ correlator, (ii) the long range azimuthal dependence of the $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ [3]. We will present the centrality and event shape dependence of the longitudinal width $\sigma\left( \Delta\eta \right)$ and the azimuthal harmonics $a^{p_T}_{n}$ of the $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ correlator for different beam energies. Our measurements are compared to similar LHC measurements [4]. The $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ beam-energy dependence is expected to reflect the temperature ($T$) and chemical potential ($\mu_{B}$) dependence of $\eta/s$. |
Tuesday, October 12, 2021 11:57AM - 12:09PM |
EE.00002: Angular Power Spectrum and Elliptic Flow from Event Maps in Heavy Ion Collisions Hannah T Anderson, Shengquan Tuo Azimuthal and polar angle distributions of particles produced in heavy ion collisions carry important information about the evolution of the Quark Gluon Plasma. The pixelization code HEALPix was created to analyze the cosmic microwave background, but its two-dimensional representation of a sphere containing pixels of equal area has a broader application to heavy ion collisions. The angular power spectrum, an application of HEALPix, is directly related to the particle flow and is calculated through the angles θ and φ on the sphere. The elliptic flow is calculated from simulated AMPT events and publicly available CMS data. Beyond the angular power spectrum, the two-dimensional pixelization may also be used with the application of convolutional neural networks (CNN) to differentiate between the presence and absence of elliptic flow. The CNN is trained using two-dimensional pixelated images from HEALPix and images of pT and φ density from AMPT with both string melting and the default setup. We show that HEALPix with CNN can be used to detect the presence of elliptic flow for lead-lead collisions and thus has broader applications in the study of QGP in heavy ion collisions. |
Tuesday, October 12, 2021 12:09PM - 12:21PM |
EE.00003: Flow and transverse momentum correlations in Pb+Pb and Xe+Xe collisions with ATLAS Somadutta Bhatta Fluctuations of event-wise average transverse momentum, [pT], and the harmonic flow, vn, carry important information about initial-state geometry. Collisions of nuclei with large quadrapole deformation are predicted to produce an initial state with enhanced shape and size fluctuations, and result in non-trivial correlation between vn and [pT] in the final state. In particular, the v2 - [pT] correlations are predicted to be different between collisions of spherical 208Pb and collisions of deformed 129Xe. |
Tuesday, October 12, 2021 12:21PM - 12:33PM |
EE.00004: Complementary Correlation Observables for Nuclear Collisions and What They Can Teach Us George S Moschelli, Mark Kocherovsky, Sean Gavin, Zoulfekar Mazloum Two-particle correlations are a widely used tool for studying relativistic nuclear collisions. Multiplicity fluctuations comparing charge and particle species have been studied as a possible signal for Quark-Gluon Plasma (QGP) and the QCD critical point. These fluctuation studies all make use of particle variances which can be shown to originate with a two-particle correlation function. Momentum correlations and covariances of momentum fluctuations, which arise from the same correlation function, have also been used to extract properties of the nuclear collision medium such as the shear viscosity to entropy density ratio, the shear relaxation time, and temperature fluctuations. Searches for critical fluctuations are also done with these correlation observables. We derive a mathematical relationship between several number and momentum density correlation observables and outline the different physics mechanisms often ascribed to each. This set of observables also contains a new multiplicity-momentum correlation. Our mathematical relation can be used as a validation tool for measurements, as a method for interpreting the relative contributions of different physics mechanisms on correlation observables, and as a test for theoretical and phenomenological models to simultaneously explain all observables. We compare an independent source model to simulated events from PYTHIA for all observables in the set. |
Tuesday, October 12, 2021 12:33PM - 12:45PM |
EE.00005: Strangeness and electric charge dependent splitting of the rapidity-odd directed flow between quarks and anti-quarks in Au+Au collisions Ashik Ikbal Sheikh We report the first measurement of the rapidity-odd directed flow ($v_1$) of multi-strange baryons ($\Xi$ and $\Omega$) in Au+Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider. |
Tuesday, October 12, 2021 12:45PM - 12:57PM |
EE.00006: TRENTo-3D: Modeling the Rapidity Evolution of Ultrarelativistic Heavy-Ion Collision Geometry Derek Soeder, Weiyao Ke, J-F Paquet, Steffen A Bass We present TRENTo-3D, a new extension of the TRENTo initial-conditions ansatz for heavy-ion collisions that computes a fully three-dimensional energy density profile based on the nuclear thicknesses generated by TRENTo. The TRENTo-3D model is designed to ensure energy conservation among a central fireball at midrapidity and two fragmentation regions at forward and backward rapidities, while apportioning the energy deposited by the collision according to the limiting fragmentation hypothesis and appropriate longitudinal fluctuation. The design exploits accepted scaling relations as well as indications from past TRENTo-based analyses in order to achieve considerable flexibility with a modest number of parameters. We demonstrate that TRENTo-3D produces initial conditions that enable good agreement with rapidity-dependent experimental data across an array of collision systems and energies, and through closure testing we explore the potential for experimental data to constrain the model's parameters. |
Tuesday, October 12, 2021 12:57PM - 1:09PM |
EE.00007: Study of the nuclear deformation at RHIC Chunjian Zhang Collective phenomena in heavy-ion collisions are very sensitive to initial geometry including nuclei deformation effects. In the hydrodynamic model description of heavy ion collisions, the final-state anisotropic flow $v_n$ are linearly related to the strength of the multi-pole shape of the nucleon density distribution in the transverse plane $\epsilon_n$, $v_n \propto \epsilon_n$. The $\epsilon_n$ are sensitive to the shape of the colliding ions, characterized by nuclear deformation. Results on the $v_n$ from various collision systems with the STAR detector will be presented. The precise calculations with Monte-Carlo Glauber and a multi-phase transport (AMPT) model could be helpful to understand the role of the shape of atomic nuclei in heavy-ion collisions. |
Tuesday, October 12, 2021 1:09PM - 1:21PM |
EE.00008: Probe charm quark dynamics via multiparticle azimuthal correlations in PbPb collisions at 5.02TeV with the CMS detector Yousen Zhang The anisotropy at the final state of the expansion of the quark-gluon plasma (QGP) in relativistic heavy ion collisions originates from the initial eccentric collision geometry. And it is characterized by the so-called elliptic flow. Besides the hydrodynamic evolution, the energy loss during particles traversing through QGP also has an impact on the elliptic flow. The event-by-event fluctuations of initial geometry and energy loss lead to the variations of elliptic flow at the final state. Measuring fluctuations of elliptic flow will provide insights into dynamics inside QGP. Charm quarks are mostly created at the initial stage of collisions because of their large masses. Recent measurements at the LHC show that they are strongly coupled with QGP. Thus they are perfect probes to both initial geometry of collisions and the inner workings of the quark-gluon plasma. In this contribution, the elliptic flow of neutral D meson (consisting of a charm quark) using 2- and 4-particle technique will be presented. And the fluctuations of the elliptic flow of neutral D meson will also be compared with charged particles in the same kinematic range. |
Tuesday, October 12, 2021 1:21PM - 1:33PM |
EE.00009: Baryon Stopping in pp and AA at high energies Flemming Videbaek Baryon stopping and transport have been investigated both experimentally and theoretically in elementary p+p and heavy ion collisions from AGS to LHC energies. In this talk an overview in particular for data relevant for RHIC and SPS energies will be given. Both published and preliminary data from BRAHMS for p+p and Au+Au collisions will be presented, and comparisons to comparisons to models and centrality dependence will be discussed. Even with these data at fairly high energy and rapidity the lack of data near beam rapidities at higher energies leaves open questions for complete understanding of baryon transport need to understand initial state conditions in relativistic heavy ion collisions. |
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