UCLA

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.