Fourier Coefficients of Asynchronous Collective Motions in Heavy-ion Collisions

We present a novel scenario in heavy-ion collisions where different modes of collective motions evolve asynchronously in the created nuclear medium. Such physics mechanisms could each dominate at a distinct evolution stage, or coexist simultaneously without coordinating with each other. If we employ a separate single-harmonic Fourier expansion to describe how each asynchronous collective motion affects particle emission, the particle azimuthal distribution should be the product of all these expansions. Consequently, cross terms between collectivity modes appear, and their contributions to experimental observables could be significant. In particular, we argue that the chiral magnetic effect (CME) and elliptic flow can develop asynchronously, with their convolution affecting the observable that is sensitive to the shear-induced CME. We will use the event-by-event anomalous-viscous fluid dynamics model to illustrate the effects of this scenario. Besides giving new insights into searches for the CME, we also propose a feasible experimental test based on conventional flow harmonics

Event Shape Selection Method in Search of the Chiral Magnetic Effect in Heavy-ion Collisions

The search for the chiral magnetic effect (CME) in heavy-ion collisions has been impeded by the significant background arising from the anisotropic particle emission pattern, particularly elliptic flow. To alleviate this background, the event shape selection (ESS) technique categorizes collision events according to their shapes and projects the CME observables to a class of events with minimal flow. In this study, we explore two event shape variables to classify events and two elliptic flow variables to regulate the background. Each type of variable can be calculated from either single particles or particle pairs, resulting in four combinations of event shape and elliptic flow variables. By employing a toy model and the realistic event generator, event-by-event anomalous-viscous fluid dynamics (EBE-AVFD), we discover that the elliptic flow of resonances exhibits correlations with both the background and the potential CME signal, making the resonance flow unsuitable for background control. Through the EBE-AVFD simulations of Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV with various input scenarios, we ascertain that the optimal ESS strategy for background control entails utilizing the single-particle elliptic flow in conjunction with the event shape variable based on particle pairs

A high-granularity calorimeter insert based on SiPM-on-tile technology at the future Electron-Ion Collider

We present a design for a high-granularity calorimeter insert for future experiments at the Electron-Ion Collider (EIC). The sampling-calorimeter design uses scintillator tiles read out with silicon photomultipliers. It maximizes coverage close to the beampipe, while solving challenges arising from the beam-crossing angle and mechanical integration. It yields a compensated response that is linear over the energy range of interest for the EIC. Its energy resolution meets the requirements set in the EIC Yellow Report even with a basic reconstruction algorithm. Moreover, this detector will provide 5D shower data (position, energy, and time), which can be exploited with machine-learning techniques. This detector concept has the potential to unleash the power of imaging calorimetry at the EIC to enable measurements at extreme kinematics in electron-proton and electron-nucleus collisions

Search for the Chiral Magnetic and Vortical Effects Using Event Shape Approaches in Au+Au Collisions at STAR

The chiral magnetic/vortical effect (CME/CVE) in heavy-ion collisions probe the topological sector of Quantum Chromodynamics, where P and CP symmetries are violated locally in strong interactions. However, the experimental observables for the CME/CVE are dominated by backgrounds related to elliptic flow and nonflow. We employ event shape variables to mitigate the flow background and event planes based on spectators to minimize the nonflow background. We report on the CME search in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 14.6, 19.6, 27, and 200 GeV, as well as the CVE search at 19.6 and 27 GeV.Comment: Quark Matter Conference 2023 proceedin

Search for the Chiral Magnetic Effect from RHIC Beam Energy Scan II data with STAR

In high-energy heavy-ion collisions, the Chiral Magnetic Effect (CME) offers a unique window to probe several fundamental properties in quantum chromodynamics (QCD): topological vacuum transitions, chiral symmetry restoration at high temperature, and the properties of a new QCD phase, Quark Gluon Plasma. Furthermore, it simultaneously probes the strong magnetic field (B) created by spectator protons in the colliding nuclei at almost the speed of light. The CME describes an electric charge separation of nearly massless quarks, which are at local chirality imbalance, along the B direction, manifestly violating local P and CP symmetries. This charge separation effect is quantified by the Δγ112 correlator between pairs of final-state charged hadrons and the reaction planes of the collision. However, due to the complex dynamics of the expanding fireball, the major background in CME search comes from the elliptic flow coupling with physics such as resonance decay, local charge conservation, and local momentum conservation. In order to mitigate the flow background in CME measurement, a novel event shape selection (ESS) approach is developed that manages to classify events based on their emission pattern shapes and determines Δγ112_ESS at the zero-flow limit. Furthermore, the spectator protons collected by STAR EPD detector are used to reconstruct the reaction plane correlated with B direction, while minimizing nonflow backgrounds. The search for the CME in the RHIC Beam Energy Scan phase II (BES-II) carries great scientific impact. It promises a thorough systematic investigation by the newly developed methods to mitigate all known backgrounds and by utilizing the stronger magnetic field provided by larger collision systems of Au+Au. With significantly higher data quality compared to BES-I and the successful development of a new ESS methodology, we observed a positive charge separation of 3-sigma significance in the 20-50% centrality range of Au+Au collisions at each of the three center-of-mass energies, 11.5, 14.6, and 19.6 GeV. The findings and physics implications will be discussed