Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

TLDR

Parity‑odd domains arising from QCD topological solutions may form in relativistic heavy‑ion collisions, leading to quark charge separation along the system’s orbital momentum axis. The study investigates a P‑even three‑particle azimuthal correlator to directly probe charge separation. Measurements of charged hadrons near mid‑rapidity in Au+Au and Cu+Cu collisions at 200 GeV with the STAR detector were performed, and potential non‑parity‑violating contributions were examined. A signal consistent with theoretical expectations of charge separation was observed.

Abstract

Parity-odd domains, corresponding to nontrivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the system's orbital momentum axis. We investigate a three-particle azimuthal correlator which is a $\mathcal{P}$ even observable, but directly sensitive to the charge separation effect. We report measurements of charged hadrons near center-of-mass rapidity with this observable in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$ using the STAR detector. A signal consistent with several expectations from the theory is detected. We discuss possible contributions from other effects that are not related to parity violation.