Search for CME in U+U and Au+Au collisions in STAR with different approaches of handling backgrounds

Abstract

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field between left- and right-handed quarks, caused by interactions with topological gluon fields from QCD vacuum fluctuations. We present two approaches to handle the dominant elliptic flow (v2) background in the three-particle correlator (Δγ112), sensitive to CME.In the first approach, we present the Δγ112 and Δγ123 measurements in U+U and Au+Au collisions. While hydrodynamic simulations including resonance decays and local charge conservation predict that Δγ112 scaled by Npart/ν2 will be similar in U+U and Au+Au collisions, the projected B-field exhibits a distinct difference between the two systems and with varying Npart. Therefore, U+U and Au+Au collisions provide configurations with different expectations for both CME signal and background. Moreover, the three-particle observable Δγ123 scaled by Npart/ν3 provide baseline measurement for only the background.In the second approach, we handle the ν2 background by measuring Δγ112 with respect to the planes of spectators measured by Zero Degree Calorimeters and participants measured by Time Projection Chamber. These measurements contain different amounts of contributions from CME signal (along B-field, due to spectators) and ν2 background (determined by the participant geometry). With the two Δγ112 measurements, the possible CME signal and the background contribution can be determined. We report such a measurement in Au+Au collisions at sNN=27GeV with the newly installed event plane detector, and report the new findings in U+U system where the spectator-participant plane correlations are expected to differ from those in Au+Au collisions.