Bulletin of the American Physical Society
Fall 2022 Meeting of the APS Division of Nuclear Physics
Volume 67, Number 17
Thursday–Sunday, October 27–30, 2022; Time Zone: Central Daylight Time, USA; New Orleans, Louisiana
Session JC: Bulk Matter and Chirality in Heavy-ion Collisions |
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Chair: Daniel Cebra, University of California, Davis Room: Hyatt Regency Hotel Celestin A |
Saturday, October 29, 2022 8:30AM - 8:42AM |
JC.00001: Estimate of nonflow baseline for the chiral magnetic effect in isobar collisions at STAR Yicheng Feng Recently, STAR reported the isobar (96Ru+96Ru, 96Zr+96Zr) results for the chiral magnetic effect (CME) search [1]. The Ru+Ru to Zr+Zr ratio of the CME-sensitive observable Δγ, normalized by elliptic anisotropy (v2), is observed to be close to the inverse multiplicity (N) ratio. In other words, the ratio of the NΔγ/v2 observable is close to the naive background baseline of unity. However, nonflow correlations are expected to cause the baseline to deviate from unity. To further understand the isobar results, we study nonflow effects using the isobar data by studying two-particle correlations as functions of pseudorapidity and azimuthal angle differences (Δη, Δφ) of the pairs. We fit this 2D distribution of same-sign pairs and attempt to extract the "true" v2, whose difference from the measured v2 is estimated as the nonflow contribution to the latter. We decompose the nonflow contributions to NΔγ/v2 (isobar ratio) into three terms [2] and quantify each term by using the nonflow in v2 measurement, published STAR data [1] and HIJNG simulations. From these estimates, we arrive at a nonflow baseline of the isobar ratio of NΔγ/v2 for the CME. We report this nonflow baseline and discuss its implications. |
Saturday, October 29, 2022 8:42AM - 8:54AM |
JC.00002: Event Shape Engineering in Search of the Chiral Magnetic Effect in Heavy-ion Collisions Gang Wang, Zhiwan Xu, Huan Z Huang In high-energy heavy-ion collisions, the deconfined quark-gluon plasma could manifest a spontaneous electric-charge separation in the direction of a strong magnetic field through the chiral magnetic effect (CME). Experimentally, the γ112 ≡ 1 + φ2 - 2ΨRP)> correlator measures the charge separation between two particles across the reaction plane, and is the main observable in search of the CME. However, the recent isobar blind analysis by the STAR Collaboration did not observe any pre-defined CME signal [1], indicating that the γ112 results are dominated by the flow-related background. We present a method study of event-shape engineering (ESE) that projects γ112 and its pure-background variation (γ132 ≡ 1 - 3φ2 + 2ΨRP)>) for π-π correlations to a class of events with minimal flow. We explore multiple event-shape variables to realize the zero-flow mode. The sensitivity on the CME signal and the background are studied for Au+Au, Ru+Ru, and Zr+Zr collisions at √sNN = 200 GeV with a multiphase transport (AMPT) model, as well as a new event generator, Event-By-Event Anomalous-Viscous Fluid Dynamics (EBE-AVFD). |
Saturday, October 29, 2022 8:54AM - 9:06AM |
JC.00003: Search for the Chiral Magnetic Effect Using STAR BES-II Data with Event Shape Selection Zhiwan Xu, Gang Wang, Huan Z Huang Heavy-ion collisions may form topological domains with chirality imbalance in quark-gluon plasma medium, which can lead to the chiral magnetic effect (CME) in the presence of the strong magnetic field from spectator protons. Recently, STAR reported precision measurements of the charge-separation difference between Ru+Ru and Zr+Zr isobar collisions at $\sqrt{s_{\rm NN}}=200$ GeV showing the pre-defined CME signatures to be absent. However, these results cannot rule out a possible CME signal in Au+Au collisions owing to the weaker magnetic field and the larger background in isobar collisions. We present new measurements from STAR in Au+Au collisions from the Beam Energy Scan II, where the magnetic field may last longer, and nonflow effects are expected to be smaller than those at the top RHIC energy. We employ an event-shape selection method, aided by the Event Plane Detector (EPD), to measure the CME-sensitive $\Delta\gamma_{112}$ correlator from a class of selected events with minimum flow and finite spectator protons. Results obtained with various event-shape selection techniques are compared to investigate the sensitivity to the CME signal in Au+Au collisions at $\sqrt{s_{\rm NN}} =$ 27 GeV and 19.6 GeV. |
Saturday, October 29, 2022 9:06AM - 9:18AM |
JC.00004: Scaling properties of background- and chiral-magnetically-driven charge separation: Implications for detecting and characterizing the chiral magnetic effect in heavy-ion collisions. ROY LACEY The scaling properties of the Δγ correlator, inferred from the Anomalous Viscous Fluid Dynamics (AVFD) model, are used to investigate a possible chiral-magnetically-driven (CME) charge separation in p+Au, d+Au, Ru+Ru, Zr+Zr, and Au+Au collisions at 200 GeV, and in p+Pb (5.02 TeV) and Pb+Pb collisions at 2.76 and 5.02 TeV. The results indicate that the values of the quotient Δγ/v2 with the elliptic flow coefficient v2 for p+Au, d+Au, p+Pb, and Pb+Pb collisions, scale as 1/Nch consistent with background-driven charge separation. By contrast, the Δγ/v2 values for Ru+Ru, Zr+Zr, and Au+Au collisions show scaling violations consistent with the presence of background plus a CME-driven contribution. Quantifying this CME-driven component indicates that in mid-central collisions, the fraction of the measured Δγ/v2 attributable to the CME is approximately 27 % for Au+Au and roughly a factor of two smaller for Ru+Ru and Zr+Zr, which show similar magnitudes. |
Saturday, October 29, 2022 9:18AM - 9:30AM |
JC.00005: Study chirality imbalance with quantum algorithms Fanyi Zhao, Alexander M Czajka, Zhongbo Kang, Henry Ma, Yuxuan Tee As a result of the chiral magnetic effect, the chiral chemical potential is introduced to imitate the impacts of the topological charge changing transitions in the quark-gluon plasma under an external magnetic field. We apply the (1+1) dimensional Nambu–Jona-Lasinio (NJL) model to study the chiral phase structure and chirality charge of strongly interacting matter with finite chiral chemical potential μ5 in a quantum simulator. By performing the Quantum imaginary time evolution QITE algorithm, we simulate the (1+1) dimensional NJL model at various temperature T and chemical potential μ, μ5 and find that the quantum simulations are in good agreement with analytical calculations as well as exact diagonalizations. |
Saturday, October 29, 2022 9:30AM - 9:42AM |
JC.00006: Identified hadron spectra and baryon stopping in $\gamma+\rm{Au}$ collisions at STAR Nicole A Lewis Photonuclear collisions are one of the simplest possible processes that can occur in a heavy-ion collision. In these collisions, one nucleus emits a quasi-real photon which interacts with the other colliding nucleus, similar to an $e+A$ collision except that the photon tends to have a much smaller virtuality. Photonuclear collisions can be used to study bulk properties of the medium created in these collisions, such as collectivity due to initial-state effects and hadron chemistry. Results are presented for identified $\pi^\pm$, $K^\pm$, and $p(\bar{p})$ spectra in photonuclear collisions at STAR for $\rm{Au}\rm{+}\rm{Au}$ collisions at $\sqrt{s_{_{NN}}} = 54.4~\rm{GeV}$. Significant baryon stopping and rapidity asymmetry are observed at low transverse momentum. These measurements constitute an important step in the search for the existence of a baryon junction within the nucleon, i.e., a nonperturbative Y-shaped configuration of gluons which carries the baryon number and is attached to all three valence quarks~\cite{Kharzeev:1996sq,OurParpar}. Measuring the same spectra using the 2019 $\rm{Au}\rm{+}\rm{Au}$ dataset at $\sqrt{s_{_{NN}}} = 200~\rm{GeV}$ shows how these effects change as a function of beam energy. Measurements of particle spectra and their rapidity dependence in photonuclear events will give insight into the origin of baryon stopping and the gluon structure of the nucleon. They will also help inform future measurements of identified particles at the Electron Ion Collider. |
Saturday, October 29, 2022 9:42AM - 9:54AM |
JC.00007: Fluctuations in Lambda Multiplicity Distribution in Au+Au collisions at √sNN = 3 GeV at STAR Jonathan Gonzalo Ball Cap The study of nuclear matter over a wide range of collision energy is provided by the RHIC Beam Energy Scan (BES). One focus of the program, namely to locate the critical point (CP) in the QCD phase diagram, is closely tied to the measurement of kurtosis in net-proton multiplicity distribution as a function of √sNN . Previous results from BES-I showing non-monotonic energy dependence with 3.1σ significance motivated us to increase the statistics and to extend the collision energy down to √sNN =3.0 GeV in the BES-II. |
Saturday, October 29, 2022 9:54AM - 10:06AM |
JC.00008: Measurement of η' meson in pp collisions at the LHC with ALICE Ewa Glimos With a mass of nearly 1 GeV, the η' is the heaviest amongst the pseudoscalar mesons composed of the lightest quarks. This anomalously large mass is believed to be induced by the gluonic interactions and the axial U(1) anomaly of QCD. However, its exact production cross-section is still not well understood for heavy ion collisions, therefore the measurement of $\eta'$ can provide deeper insight into perturbative chromodynamics in this regime. Moreover, its measurements in proton-proton collisions at ultrarelativistic energies provide a baseline for heavy-ion collisions, where a large fraction of the mesons will decay within the produced quark-gluon plasma, which is anticipated to lead to the lowering of its mass with respect to its value in vacuum. |
Saturday, October 29, 2022 10:06AM - 10:18AM |
JC.00009: Bayesian Parameter Estimation of Relativistic Heavy Ion Collisions Simulation with Viscous Anisotropic Hydrodynamics Modeling Dan P Liyanage, Ulrich W Heinz, Ozge Surer, Matthew Plumlee, Stefan Wild State-of-the-art hydrodynamic simulation models for relativistic heavy ion collisions are only applicable after 1 fm/c or so after the collision due to large pressure gradients that are present at the early times. As a solution, pre-hydrodynamic stage which models the early stage evolution as a conformal, weekly interacting gas is usually used before the hydrodynamic stage. The transition from pre-hydrodynamic to the hydrodynamic stage is discontinuous and introduce a considerable theoretical ambiguity to the model. Recently a novel hydrodynamic model, Viscous Anisotropic Hydrodynamics (VAH) which can handle large pressure anisotropies has been introduced as a promising solution. VAH is applicable at very early times of the collision and it smoothly matches to conventional second-order viscous hydrodynamics at late times. In this work we present a Bayesian parameter estimation study for VAH model using the experimental data for Pb-Pb collisions at LHC ($\mathrm{s}\sqrt{NN}$=2.76 TeV). We find that the VAH model can fit the experimental data well and we also present the novel physics insights based on the model parameters inferred from the experimental data using Bayesian statistical methods. |
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