Session 1WDB: Chirality and Vorticity II

Recent results of CME search from isobar and AuAu collisions at RHIC

Relativistic heavy ion collisions serve as a unique testing ground for Quantum Chromodynamics (QCD) at high energies, generating the densest known matter and the strongest electromagnetic fields. Among the phenomena observed, the Chiral Magnetic Effect (CME) arises from the violation of local P and CP symmetries in the presence of strong fields. It is associated with the creation of an electric current along the direction of magnetic field -- leading to the separation of positively and negatively charged particles across the plane determined by the impact parameter and collision directions of the two heavy ions. Signatures of CME have been extensively searched in heavy ion collisions. However, most of the previous measurements of the CME have been inconclusive due to significant background contributions. To overcome this challenge, the STAR collaboration conducted a blind analysis on a large dataset comprising approximately 3.8 billion isobar Ru+Ru and Zr+Zr collisions at a center-of-mass energy of 200 GeV at the Relativistic Heavy Ion Collider in the year 2018. This analysis aimed to better control the influence of signal and backgrounds, providing more robust insights into the CME. In this presentation, I provide a brief historical overview of the search for the CME in HICs and discuss the findings obtained from the blind analysis of the isobar data.

Additionally, I will explore the future directions of CME research in heavy ion collisions, building upon the success of the isobar program. Promising avenues include the analysis of high-statistics data from the second phase of the RHIC beam energy scan program using new detector upgrades of the STAR experiment. Such an initiative offers unique opportunities to investigate the collision energy dependence of CME. Additionally, the anticipated run of RHIC in the years 2023-2025, coupled with innovative analysis techniques, provides a chance to extract upper limits on the observability of the CME in Au+Au collisions.

The implications of CME search go beyond the community of nuclear physics and opens avenues for investigating the early universe and provides a deeper understanding of the strong interaction dynamics under extreme conditions.   

Search for the Chiral Magnetic Effects Using Event Shape Selection with BES-II data at STAR

Heavy-ion collisions provide unique opportunities to probe the topological sector of Quantum Chromodynamics via chiral anomaly. However, experimental observables related to the predicted chiral magnetic effect (CME) are intrinsically convoluted with backgrounds related to elliptic flow and nonflow. Recently, the STAR experiment has revealed significant background contributions in the CME search using isobar collisions, preventing an unambiguous observation of the CME signal. We have developed a new analysis technique based on event shape selection (ESS) relying on variations of particle emission pattern to suppress the flow background in Δγ measurements and applied it to the STAR Beam Energy Scan (BES) II data.  The Au+Au system produces more intense magnetic fields than the isobars, while collisions at lower beam energies provide a longer-lasting magnetic field. The STAR Event Plane Detector enables us to construct the spectator plane with a good resolution, which better represents the magnetic field direction and effectively suppress the nonflow background. Using particles of interest to build the ESS variable, we project the Δγ correlator to the apparent zero-flow limit. We will contrast our ESS with the method of Event Shape Engineering proposed in the literature[1]. Besides the single particle information, we also utilize particle pair momentum to systematically evaluate the residual background.  We report the Δγ measurements using h-h (excluding protons) in Au+Au collisions at √s_NN = 7.7, 11.5, 14.6, 19.6, and 27 GeV, and demonstrate a five-fold reduction of background. We will discuss the physics implications of the new results for the CME searches.

[1]J. Schukraft, A. Timmins, S. A. Voloshin, Phys. Lett. B {f 719}, 394 (2013).   

Recent Developments in Strong Field Physics

I will discuss recent developments in strong-field physics with some reviews. Especially, I will discuss the QCD phase diagram in strong magnetic fields including implications from recent lattice QCD simulations developed after excitement on the inverse magnetic catalysis. Finally, I will discuss the phases of the Kondo effect induced by strong magnetic fields.