Design an event-shape selection that nullifies flow-driven backgrounds without degrading CME sensitivity

Determine an event-shape selection observable or method that effectively drives the resonance-related elliptic-flow background v2,res to zero in γ-correlator measurements, while preserving the correlation between the estimated event plane and the magnetic-field direction in heavy-ion collisions, so that the chiral magnetic effect signal can be isolated in a fully data-driven way.

Background

The event-shape-engineering (ESE) strategy aims to suppress flow-driven backgrounds in the CME-sensitive γ-correlator by selecting events with small elliptic flow. However, eliminating v2 of inclusive final-state particles does not necessarily reduce the resonance-related component v2,res that drives much of the background. Moreover, overly aggressive selection can randomize the event-plane proxy and simultaneously diminish both background and potential signal.

The authors explicitly note that there is no definitive prescription for an experimental quantity that guarantees v2,res → 0 without compromising sensitivity to the CME, highlighting the need for a principled, data-driven event-shape selector.