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 SB: Heavy Ions and Electromagnetic and Global Observables |
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Chair: Ramona Vogt, UC Davis, LLNL |
Sunday, November 1, 2020 10:30AM - 10:42AM |
SB.00001: Dielectron production in Au+Au collisions at $\sqrt{s_{\rm {NN}}}=54$ GeV at STAR Zhen Wang Dielectrons are excellent probes of the Quark-Gluon Plasma (QGP) created in high-energy heavy-ion collisions. Because they can be produced at all stages of the collision system evolution and do not interact with the medium strongly, which indicates that dielectrons carry the information from the initial stage to the final stage. In the low mass region (LMR, $\rm M_{ee} < M_{\phi}$), the mass spectra of vector mesons will be modified by the hot and dense medium which is related to the chiral symmetry restoration in the medium. In the intermediate mass region (IMR, $\rm M_{\phi} < \rm M_{ee} < \rm M_{J/\psi}$), QGP thermal radiation can be used as a QGP thermometer. However, it is complicated to measure the QGP thermal radiation because of the heavy flavor semi-leptonic decay contributions. In this presentation, I will present the dielectron production in Au$+$Au collisions at $\sqrt{s_{\rm {NN}}}=54$ GeV at STAR. With a 10 times larger data sample than that at 62 GeV from the first phase of the STAR Beam Energy Scan (BES-I) program, in-medium $\rho$ modification can be studied with better precision and compared to different theoretical predictions. Furthermore, the physics implications of these measurements will be discussed. [Preview Abstract] |
Sunday, November 1, 2020 10:42AM - 10:54AM |
SB.00002: Low-$p_T$ $e^+e^{-}$ pair production in Au+Au collisions at $\sqrt{s_{\rm {NN}}}=54.4$~GeV~at~STAR Xiaofeng Wang In high-energy heavy-ion collision, strong electromagnetic fields arising from the Lorentz-contraction of large amounts of charge in nuclei generate a large flux of high-energy quasi-real photons. Dielectrons can be produced via the interaction of these photons. Dielectron production from photon-photon scattering are distinctly peaked at very low transverse momentum. Traditionally these photon-photon proccesses were expected to exist only in Ultra-Peripheral Collisions (UPC). However, it has been recently realized that even in peripheral collisions, the dielectron production at very low transverse momentum mainly originates from the two photon interactions, which provides a possible tool to directly measure the giant magnetic field created in heavy-ion collisions. In this presentation, we will present measurements of dielectron production at low transverse momentum in peripheral (80-100\%) Au+Au collisions at $\sqrt{s_{\rm {NN}}}=54.4$ GeV at STAR. [Preview Abstract] |
Sunday, November 1, 2020 10:54AM - 11:06AM |
SB.00003: Insight into the direct photon puzzle at PHENIX Michael Giles Direct photons provide valuable insight into the collective properties of Quark Gluon Plasma (QGP). They are emitted throughout the evolution of a heavy ion collision and do not interact strongly with the medium as they leave it. The PHENIX experiment has observed both a large yield and a large elliptic flow of low p$_{\mathrm{T}}$ direct photons in Au$+$Au collisions at $\sqrt s_{NN} =$200GeV. This presents a challenge to the theoretical models of QGP, since a large direct photon yield requires photons emitted in the early stages of the collisions where the temperature is high, while a large elliptic flow requires photons emitted in the later stages of the collisions where the collective motion of the system is well developed. Furthermore, an intriguing scaling behavior is observed between the low p$_{\mathrm{T}}$ direct photons production and the charged particle multiplicity across all A$+$A collision systems and different beam energies from 39 GeV to 2760 GeV, indicating similar photon sources for all these systems. To provide more insight into photon production mechanisms and more constraints on theoretical models, precise measurement of the direct photon anisotropy is needed. In this talk, measurements of spectra and the flow coefficients of low-momentum photons will be presented with the high statistics Au$+$Au data taken in the year 2014.~ [Preview Abstract] |
Sunday, November 1, 2020 11:06AM - 11:18AM |
SB.00004: Measurement of Low-Momentum Direct Photons in Small System Collisions at PHENIX Arun Kingan Results from small collision systems at RHIC and LHC have shown interesting evidence of collective behavior, hinting at the onset of QGP in small collision systems. Photons do not interact strongly with the medium produced in high-energy heavy-ion collisions. By measuring the photons emitted early on in the collision (i.e. direct photons or photons not produced by hadron decays) information about the environment in which they were produced can be obtained. With access to large datasets of p+p and p/d/3He+Au collisions at 200 GeV, PHENIX is in an ideal position to search for indications of thermal photon emissions in these systems. In this talk, the measurements of low-momentum direct photons from p+p and p+Au collisions reaching down to 0.6 GeV/c will be presented and the status of the measurement in d/3He+Au will be discussed [Preview Abstract] |
Sunday, November 1, 2020 11:18AM - 11:30AM |
SB.00005: Systematic Glauber Model Study of Multiplicity Distributions from $\sqrt{s_{NN}}=3.0$ GeV to $200$ GeV at STAR Zachary Sweger The Glauber Model has long been used in heavy-ion collisions as a method of determining centrality by simulating nucleus-nucleus collisions and producing particles using a Negative Binomial Distribution (NBD). The core task is to determine what values of the negative binomial parameters and the hardness scaling reproduce the observed distributions from data, all of which contribute to the shape of the simulated distribution. Particle production in heavy-ion collisions scales with the number of binary nucleon collisions for hard processes and with the number of participant or wounded nucleons for soft processes; the hardness scaling is an essential part of the Glauber Model and determines what fraction of produced particles are generated from soft versus hard processes. Using this hardness scaling, there is potential to learn about nucleon stopping and the energy deposition in the interaction region. STAR has collected many data sets at various energies, ranging from $\sqrt{s_{NN}}=3.0$ GeV to $200$ GeV, which provide a unique opportunity to perform a systematic scan of the hardness as a function of energy. STAR data from the RHIC Beam Energy Scans I and II, the STAR Fixed Target program, and additional energies covering the full range of collisions from RHIC will be presented. [Preview Abstract] |
Sunday, November 1, 2020 11:30AM - 11:42AM |
SB.00006: Multiplicity distributions from heavy-ion collisions at the Large Hadron Collider negin alizadehvandchali Collisions of relativistic heavy-ions at the Large Hadron Collider (LHC) are carried out in order to create a unique state of matter known as the Quark Gluon Plasma (QGP). In this state, hadronic matter melts, and quarks and gluons become the relevant degrees of freedom. It is believed that the universe temporally existed in such a state shortly after the big bang. The distribution of produced charged particles (multiplicity) provides critical information regarding the initial state of these collisions. I will present the latest multiplicity distributions and mean transverse momentums measured by the ALICE collaboration at the LHC. I will discuss comparisons of these distributions to theoretical models, the measurements of the speed of sound experimentally and detail future plans regarding new measurement techniques. [Preview Abstract] |
Sunday, November 1, 2020 11:42AM - 11:54AM |
SB.00007: Centrality Determination and Study of Pileup Effects in Au+Au Collisions at $\sqrt{s_{NN}}$ = 3 GeV from STAR Yu Zhang In fixed target heavy ion experiments, multiple collision events may pile up in a recorded event due to the finite thickness of the target. This imposes a serious challenge to the analysis of higher cumulants of proton multiplicity distributions. In this talk we will present the result of the collision centrality determination and the study of pileup effect in Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV with the fixed-target mode (beam energy of 3.85 GeV/u) at STAR. We will utilize different sub-detectors to separate single and pileup events. The multiplicity distributions will be compared to MC Glauber model calculations for centrality determination and the remaining event pileup fraction can be evaluated. Furthermore, the influence of the pileup events on higher-order proton cumulants will be discussed using transport model calculations. [Preview Abstract] |
Sunday, November 1, 2020 11:54AM - 12:06PM |
SB.00008: Charged Hadron Production from Au+Au Collisions at $\sqrt{ s_{NN} } $ = 27 GeV at STAR Matthew Harasty The Relativistic Heavy Ion Collider (RHIC) beam energy scan I (BES-I) covered a range of energies from $\sqrt{ s_{NN} } $ = 62.4 to 7.7 GeV and ran from 2010 to 2014. Only mid-rapidity spectra for $\pi$, $K$, and p have been published from those energies. The BES-I results have justified a new beam energy scan (BES-II) with higher statistics and a series of detector upgrades. The first collider energy from BES-II, 27 GeV, was run in 2018. This analysis will address the spectra and yields of $\pi$, $K$, and p as a function of rapidity and centrality from the 27 GeV Au+Au collisions from 2018 at RHIC. The relative yields of the various particle species allow for a measurement of the chemical freeze-out temperature and baryon chemical potential. The $K^{+}$ to $K^{-}$ ratio gives insight into associated production as an alternative to thermal production. The $p$ to $\bar{p}$ ratio determines the baryon chemical potential, pinning down the collision's location on the QCD phase diagram. The parameters extracted from the 4$\pi$ yields in the current analysis are compared to experimental results extracted from previous mid-rapidity particle yields. [Preview Abstract] |
Sunday, November 1, 2020 12:06PM - 12:18PM |
SB.00009: Low-$p_{T}$ $\mu^{+}\mu^{-}$ pair production in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV at STAR Jian Zhou Recently, significant enhancements of $e^{+}e^{-}$ pair at very low transverse momentum ($p_{T}$) were observed by the STAR collaboration in peripheral Au+Au collisions. The excess can be explained by photon-photon interactions induced by the extremely strong electromagnetic field produced by the fast-moving heavy ions. The photon interaction was usually studied in ultra-peripheral collisions without any nuclear overlaps. However, the photon interaction in peripheral collisions may provide a novel probe of QGP because the very-low-$p_{T}$ dileptons are produced in the early stage of the collisions and there could be QGP produced in the nuclear overlapping region in peripheral collisions. In such collisions, the photon-photon interactions could be further used to probe the possible existence of strong magnetic fields trapped in a conducting QGP medium. Measurements with $\mu^{+}\mu^{-}$ pairs provide a complementary channel to investigate these phenomena. In 2014 and 2016, the STAR experiment at RHIC recorded large samples of Au+Au collisions at $\sqrt{s}$ = 200 GeV. In this talk, we will present invariant mass and yield distributions for $\mu^{+}\mu^{-}$ pair production at $p_{T} < 0.15$ GeV/$c$.The $p_{T}^{2}$ distribution of the excess yields will also be shown. [Preview Abstract] |
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