CME search at STAR

TL;DR

This paper reports on the search for the Chiral Magnetic Effect (CME) in heavy-ion collisions at STAR, using azimuthal correlators and various collision energies, providing preliminary estimates of CME fraction and discussing methodological advances.

Contribution

It introduces a novel approach to measure CME fraction via azimuthal correlators relative to the spectator plane and discusses results across different energies and collision systems.

Findings

Preliminary CME fraction estimate at 8% with large uncertainties.

No significant CME signal observed at lower energy (27 GeV) with current data.

Methodological advancements with the Event Plane Detector and isobar analysis are discussed.

Abstract

The hot and dense medium produced in relativistic heavy-ion collisions has been conjectured to be accompanied by an axial charge asymmetry that may lead to a separation of electric charges in the direction of the extremely strong magnetic field, also known as the Chiral Magnetic Effect (CME). The measurement of azimuthal correlator ($\Delta\gamma$) with respect to the spectator plane, gives us an opportunity to measure the possible CME fraction beyond the flow background. Preliminary results using this approach with combined Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV and U+U at $\sqrt{s_{NN}} =$ 193 GeV show $f_{CME}$ at $(8\pm4\pm8)$ $\%$. 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 strengths of CME signal and non-CME background. At lower energies, the Event Plane Detector (EPD) installed in the year 2018 provides a unique capability for CME search. The background scenario test at Au+Au $\sqrt{s_{NN}} =$ 27 GeV by using $\Delta\gamma$ with respect to TPC and the new installed EPD shows a consistency with no-CME scenario in the current statistics. The method of the ongoing isobar blind analysis, and the latest sensitivity check with the event-by-event AVFD model on the different observables between Ru+Ru and Zr+Zr are also briefly discussed.