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 RB: The Chiral Magnetic Effect and Strangeness |
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Chair: Peter Steinberg, Brookhaven National Laboratory |
Sunday, November 1, 2020 8:30AM - 8:42AM |
RB.00001: CME search at STAR using the Event Plane Detector Yu Hu Recently Relativistic Heavy Ion Collider (RHIC) collided isobars (Ru+Ru and Zr+Zr) and STAR collaboration is currently performing blind analysis to make a decisive test of the Chiral Magnetic Effect (CME) at the top RHIC energy ($\sqrt{s_{NN}} =$ 200 GeV). Meanwhile, the observability of CME has been conjectured to be dependent on $\sqrt{s_{NN}}$ due to changes in the lifetime of the magnetic field, the strength of CME signal and non-CME background. So what happens at lower energies? The Event Plane Detector (EPD) installed in the year 2018 provides a unique capability for CME search over a wide range of energies. At lower energies the EPD acceptance ($2.1<|\eta|<5.1$) covers the region where particle production is accompanied by a large directed flow of beam fragments, stopped protons and spectators. Therefore, the EPD can measure the event plane associated with the spectators, strongly correlated to the magnetic field, with good precision. This opens up new opportunities to revisit CME search at lower energies with the BES-II data recently collected by STAR. In this presentation, I will focus on the CME search using the EPD and present the first measurements in Au+Au collision at $\sqrt{s_{NN}} =$ 27 GeV. I will also discuss STAR's plan for blind analysis of the isobar. [Preview Abstract] |
Sunday, November 1, 2020 8:42AM - 8:54AM |
RB.00002: Method Study of Azimuthal Correlators in Search ofthe Chiral Magnetic Effect in Heavy-ion Collisions Nanxi Yao In high-energy heavy-ion collisions, the chiral magnetic effect (CME) has been theorized, where quark domains with chirality imbalance can interact with the strong magnetic field generated by spectator protons, inducing a charge transport across the reaction plane ($\Psi_{\rm RP}$). Several experimental observables have been proposed in search of the CME, such as the $\Delta\gamma_{112}$ correlator[1], and the $R_{\Psi_{\rm RP}}$ correlator[2]. We have employed a multiphase transport (AMPT) model and the anomalous-viscous fluid dynamics (AVFD) model to study the relationship between $\Delta\gamma_{112}$ and $R_{\Psi_{\rm RP}}$. These two methods are found to be equivalent to each other: they are subject to the same amount of background contributions and reveal the same amount of the CME signals. With the AVFD calculations, we have verified that the CME signal and the background can linearly add up in the azimuthal correlators. When we deploy the event-shape engineering technique, the background in $\Delta\gamma_{112}$ is suppressed by a factor of 10. [1]S. Voloshin, Phys. Rev. C, 057901 (2004). [2]N. Magdy, S. Shi, J. Liao, N. Ajitanand, and R. A. Lacey, Phys. Rev. C, 061901 (2018). [Preview Abstract] |
Sunday, November 1, 2020 8:54AM - 9:06AM |
RB.00003: Event-by-event correlations between $\Lambda/\bar{\Lambda}$ polarization and CME observables in Au+Au collisions at $\sqrt{s_{NN}} = 27$ GeV from STAR Yicheng Feng Spin-orbit interactions cause a global polarization of $\Lambda/\bar{\Lambda}$ with the vorticity (total angular momentum) in the participant collision zone. The strong magnetic field mainly created by the spectator protons were predicted to lead to difference in the $\Lambda$ and $\bar{\Lambda}$ global polarization ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). On the other hand, the QCD predicts topological charge fluctuation in vacuum, resulting in a chirality imbalance, or parity violation in a local domain. This would give rise to an imbalanced left- and right-handed $\Lambda/\bar{\Lambda}$, $\Delta N= N_{\text{L}} - N_{\text{R}} \neq 0$, and a charge separation along the magnetic field, chiral magnetic effect (CME). The latter is characterized by the parity-even $\gamma$-correlator $\Delta\gamma$ and parity-odd sine coefficient $a_{1}$. While measurements of individual $\Delta P$, $\Delta\gamma$, and $a_{1}$ have not led to affirmative conclusions on the CME or the magnetic field, correlations among these observables may reveal new insights. We report exploratory measurements of event-by-event correlations between $\Delta P$ and $\Delta\gamma$, and between $\Delta N$ and $a_{1}$, by the STAR experiment in Au+Au collision at $27$ GeV. [Preview Abstract] |
Sunday, November 1, 2020 9:06AM - 9:18AM |
RB.00004: ${}^{3}_{\Lambda}$H and ${}^{4}_{\Lambda}$H Lifetime Measurements in Au+Au collisions at $\sqrt{s_{\rm{NN}}}=3$ GeV with the STAR detector Yue Hang Leung The study of hyperon-nucleon interaction (Y-N) is of great interest in recent years because of its connection to high-density matter systems such as neutron stars. The presence of hyperons inside neutron stars softens the equation of state, inhibiting the formation of large mass neutron stars. Hypernuclei, being bound states of nucleons and hyperons, serve as a natural probe to study the Y-N interaction. Precise measurements of the lifetime provide direct information on the Y-N interaction. The data from fixed target Au+Au collisions at $\sqrt{s_{\rm{NN}}}=3$ GeV, taken in 2018 by the STAR detector, is ideal for studying the properties of light hypernuclei, such as ${}^{3}_{\Lambda}$H and ${}^{4}_{\Lambda}$H, due to the large statistics and high production yield. In this talk, lifetime measurements of ${}^{3}_{\Lambda}$H and ${}^{4}_{\Lambda}$H in Au+Au collisions at $\sqrt{s_{\rm{NN}}}=3$ GeV will be presented. The new results will be compared to previous measurements, and physics implications will be discussed. [Preview Abstract] |
Sunday, November 1, 2020 9:18AM - 9:30AM |
RB.00005: $\phi$ Meson production at forward rapidity in Au + Au collisions at $\sqrt{s_{NN}}$= 200 GeV Uttam Acharya 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 $\phi$ 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 $\phi \rightarrow \mu^{+}\mu^{-}$ decay channel particularly interesting. Since $\phi$ meson is composed of a strange and anti-strange quark, its nuclear modification in heavy-ion collisions may provide insight on strangeness enhancement in-medium. PHENIX has measured $\phi$ meson cold nuclear modification in a variety of small systems, and the same measurement in Au + Au collisions may provide insights on the hot nuclear matter effects. In this talk, we present the analysis status of $\phi$ meson production at forward rapidity in Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. [Preview Abstract] |
Sunday, November 1, 2020 9:30AM - 9:42AM |
RB.00006: $\phi$-meson production in Au + Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 3\ GeV from STAR Guannan Xie $\phi$-meson is the lightest bound state of strange quarks($s\overline{s}$). It has relatively small hadronic interaction cross sections, therefore $\phi$-meson is considered to be a sensitive probe of the earlier dynamics in the heavy-ion collision. Recent measurements by HADES and FOPI on subthreshold $\phi$-meson production show a larger $\phi/K^{-}$ ratio compared to the results at higher energies[1,2], and this larger $\phi/K^{-}$ ratio cannot be described by thermal model calculations with Grand Canonical Ensemble for strangeness. In this presentation, we will report on our first measurements of $\phi$-meson production in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 3$\ $GeV with energy just above the NN threshold. The data were taken in 2018 by the STAR experiment with Fixed Target configuration. $\phi$-mesons are measured through their hadronic decay channel, $\phi\rightarrow K^-+K^+$. After being corrected for the detector acceptance and tracking efficiencies, invariant yields of $\phi$-mesons as well as $\phi/K^{-}$ ratio are presented in several centrality intervals and the results will compared to model calculations. \\ [1] HADES Collaboration, Phys. Let. {\bf B 778}, (2018) 403-407. \\ [2] FOPI Collaboration, Eur. Phys. J. {\bf A 52}, (2016) 177. [Preview Abstract] |
Sunday, November 1, 2020 9:42AM - 9:54AM |
RB.00007: Pentaquark Search in the Strange Sector at ALICE at the LHC Jacobb Martinez The recent discovery of the hidden charm pentaquarks $P_{c}(4312)^{+}, P_{c}(4440)^{+},$ and $P_{c}(4457)^{+}$ by LHCb has reopened the question of whether pentaquarks exist in the strange sector. The strangeness enhancement, measured by ALICE as a function of increasing charged particle multiplicity even in $\rm{\textit{p}}$-$\rm{\textit{p}}$ collisions, further adds to the likelihood of observing a strange pentaquark state. Following analogous decay channels for the $P_{c}^{+}$ states into the strange sector, results for ${P_{s}}\rightarrow\phi\rm{\textit{p}}$, ${P_{s}}\rightarrow\Lambda{K}$, ${P_{s}}\rightarrow\Lambda{K}^{*}$, and ${P_{s}}\rightarrow\Sigma^{*}{K}$ through the invariant mass analysis in $\rm{\textit{p}}$-$\rm{\textit{p}}$ collisions at $\sqrt{s} = 13$ TeV are presented. Upper limits to the yields of ${P_{s}}$ states will be determined. [Preview Abstract] |
Sunday, November 1, 2020 9:54AM - 10:06AM |
RB.00008: Study of Baryon Fluctuations in Azimuthal Phase Space and Search for Critical Phenomena at STAR Dylan Neff Divergence of correlation length is a universal feature of critical phenomena in phase transitions. In the search for a critical point in the QCD phase diagram, such a divergence may be reflected in particle yield fluctuations in phase space via the coalescence formation mechanism. Fluctuations of baryon multiplicities in heavy-ion collisions within a limited pseudo-rapidity range have been used to search for signs of a critical point in the STAR Beam Energy Scan (BES) Phase I data. Particle fluctuations arising from critical phenomena are expected to span both longitudinal and azimuthal phase space. Leveraging the azimuthal symmetry of RHIC collisions with respect to the reaction plane along with utilizing mixed events as a baseline may allow for the measurement of correlation length proxies that are less sensitive to common experimental complications. Measurements of proton multiplicity fluctuations in azimuthal partitions of Au+Au collisions from the STAR BES-I program are presented along with methods for analyzing these distributions and their moments. These results are also compared with AMPT model calculations. [Preview Abstract] |
Sunday, November 1, 2020 10:06AM - 10:18AM |
RB.00009: Search for clustering of strange quarks in relativistic heavy ion collision at the LHC Surya Prakash Pathak ~ Heavy ion collisions at the LHC provide a tool to study the phase transition from the hadronic matter to a deconfined phase of quarks and gluons. This state of strongly interacting QCD matter produced in high energy collision, revealed an enhance production of strange hadrons. Taking this result as a basis, we search for clusters of strange quarks in heavy ion collision by studying the azimuthal distribution of particles on an event by event basis. We will present experimental result of the mean and variance of the probability of event clustering $P(E)$ and $N_{min}/N_{max}$ where $N_{min}$ and $N_{max}$ are the smallest and largest number of particles produced per event in the respective azimuthal bins. The data are measured as a function of centrality for six different azimuthal bins in Pb-Pb collision at 5.02 TeV using the ALICE experiment at the LHC. The measurement of kaons, which serves as a proxy of strange quarks, is then compared with the result for pions, which represent the light quark distribution. [Preview Abstract] |
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