Session LL: Mini-Symposium: Chirality and Vorticity in High-energy Nuclear Collisions

Chirality, Polarization, and Vorticity in heavy-ion collisions

The strong magnetic field generated in non-central heavy-ion collisions could create an electric current with non-zero axial charge, due to so-called Chiral Magnetic Effect (CME). Searching for such possible chiral phenomena has been actively performed at both RHIC and LHC, but the measurements are likely dominated by the background such as the elliptic flow in conjunction with local charge conservation. A lot of studies are ongoing, e.g. by improving the analysis technique to reduce and possibly disentangle the background effect and by investigating p+A collisions where the magnetic field is randomly oriented unlike A+A collisions and therefore one does not expect the CME. A global rotation of the system is also closely related to the chiral phenomena, e.g. Chiral Vortical Effect. The global polarization of $\Lambda$ hyperons was observed for Au+Au collisions at $\sqrt{s_{NN}} = 7.7 — 200$GeV at RHIC, indicating the system vorticity. The global polarization might differ between particles and antiparticles, due to the opposite sign of the magnetic moments, which could be a direct tool to study the magnetic field in heavy-ion collisions. In this talk, the current status of the CME search in heavy-ion collisions will be presented. Recent results on global and local polarization caused by anisotropic flow will be also presented.   

Virtual photon polarization in the presence of vorticity

We study virtual photon polarization in the presence of vorticity. For this purpose, we analyze the angular dependence of dilepton production rate from a quark gluon plasma, in which we semi-classically evaluate the production rate in the fluid background. We also discuss the effect of chiral anomaly on the virtual photon polarization, when an axial chemical potential is nonzero.   

Thermalization and viscosity of classical fields

Thermalization time in high-energy heavy-ion collisions is found to be unexpectedly short. Since the systems can be approximately described by the classical Yang-Mills (CYM) fields that are called ”glasma” in the early stages of relativistic heavy ion collisions, classical fields should play important roles to realize early thermalization. Actually, glasma instabilities and chaoticity have been found to promote rapid pressure isotropization and entropy production [1]. Thus it is valuable to study the relaxation of the classical fields around the equilibrium.
In this study, we investigate the viscosity of classical scalar fields [2] by using the Green-Kubo formula. From the correlation function of the energy-momentum tensor in the classical statistical approach, we obtain the shear viscosity using the Green-Kubo formula. We also plan to discuss entropy production of classical fields.

[1] P. Romatschke and R. Venugopalan, Phys. Rev. D 74, 045011 (2006); T. Epelbaum and F. Gelis, Phys. Rev. Lett. 111, 232301 (2013); H. Tsukiji, T. Kunihiro, A. Ohnishi, and T. T. Takahashi, PTEP 2018, 013D02 (2018).
[2] M. M. Homor and A. Jakovac, Phys. Rev D 92, 105011 (2015).   

Particle production in high energy hadronic/nuclear collisions at all p_t

We describe a new unified formalism which contains both large and small x gluons in a target proton or nucleus wave function. It generalizes the Color Glass Condensate formalism (appropriate for low pt only) and therefore can be applied to particle production in high energy hadronic/nuclear collisions in both small and large transverse momentum regions and all rapidities.   

Azimuthal angle dependence of pion femtoscopy in &[root]sNN = 200 GeV Cu+Au collisions at STAR

HBT measurements provide opportunity to investigate the source shape at freeze-out and have access to its spatial and temporal development in relativistic heavy ion collisions. Especially, azimuthal dependence of HBT radii with respect to the n^th-order event plane reveals the property of the expanding medium and the difference between initial and final source eccentricities.
 In Cu+Au collisions, there is an initial asymmetric density gradient due to the asymmetry in number of participants. It generates a difference of source expansion in forward and backward directions. Comparing the HBT radii by such an initial shape will help in further understanding of the expansion dynamics.
 In this talk, we will present the results of azimuthal dependence of HBT radii in Cu+Au collisions as well as two-particle average momentum and centrality dependences. The results will be compared to those obtained for Au+Au collisions.   

Ultra-peripheral collisions in STAR

Ultra-relativistic heavy ions generate strong electromagnetic fields, which
offer the opportunity to study exclusive photon-photon and photon-nucleus processes at RHIC
in ultra-peripheral collisions (UPC). While exclusive photoproduction of vector
mesons is sensitive to the gluon distribution of the interacting target nucleus,
photoproduction of light vector mesons is also a probe to photon-pomeron coupling.
Exclusive production of lepton pairs in the photon-photon interactions provides
a test to quantum electrodynamics with a large coupling constant.

We present STAR measurements on light $\rho^0$ mesons and heavy $J/\psi$ mesons
in photon-nucleus processes
and exclusive electron-positron pair production in photon-photon processes
in Au+Au UPC at $\sqrt{s_{NN}}$ = 200 GeV.
The photoproduction processes
are tagged at the trigger level by forward neutrons emitted by mutual
Coulomb excitations of interacting nuclei.
All these measurements are performed
at central rapidities of about $|y|<1$.