Event-by-event analysis of chiral charge separation in $p^{\uparrow }+$Au collisions within an improved AMPT model

Abstract

We study the charge separation induced by chiral magnetic effect (CME) in $p^{\uparrow }$+Au collisions at $\sqrt{s_{NN}}=200$ GeV event-by-event with an improved string-melting AMPT model. In this model, an impact-parameter-dependent formation-time delay is introduced to reduce the peak Bjorken energy density from $\sim$2 to $\sim$0.3 GeV/fm${}^3$ and reproduces the illiptic flow $v_2(p_T)$ measured by PHENIX in $p+$Au simultaneously. The event-by-event CME source, whose quark momentum-exchange fraction scales with $|\mathbf{B}{|}_{\mathrm{event}}/|\mathbf{B}{|}_{\max }$ and is capped at the 7\% value from Au+Au, transmits the field geometry directly to the observable. We find that the initial correlator shows a clear scheme hierarchy tracking $B^2$. The parton cascade preserves $\sim$80--90\% of the signal, while coalescence and ART hadronic rescattering dissipate the bulk, leaving $\sim$10--30\% in the final state. The ${\gamma }_{OS}/{\gamma }_{SS}$ splitting remains visible. These results support the use of the inter-scheme difference $\Delta {\gamma }_{\mathrm{IV}}-\Delta {\gamma }_{\mathrm{II}}$ as an essentially background-free CME observable as we proposed in previous work.

Comments: 14 pages, 18 figures