Challenges in flow background removal in search for the chiral magnetic effect

TL;DR

This paper examines how resonance decays influence the charge separation observable in the search for the chiral magnetic effect, revealing that they can mimic signals and complicate background suppression techniques.

Contribution

It demonstrates that resonance decays significantly affect the three-particle correlator and explains why background suppression methods are not fully effective.

Findings

Resonance decays can account for much of the measured charge separation signal.

Background suppression techniques reduce but do not eliminate the resonance decay effects.

Predictions are made for resonance decay contributions in 200 GeV Au+Au collisions.

Abstract

We investigate the effect of resonance decays on the three-particle correlator charge separation observable in search for the chiral magnetic effect, using a simple simulation with realistic inputs. We find that resonance decays can largely account for the measured signal. We suppress the elliptic flow ($v_2$) background by using zero event-by-event $v_2$ (or via the mixed-event technique). We find that the background is suppressed, but not eliminated as naively anticipated. We identify the reason to be the non-identicalness of the resonance and final-state particle's $v_2$ and the induced correlation between the transverse momentum dependent resonance $v_2$ and decay angle. We make predictions for the charge separation signal due to resonance decays in 200~GeV Au+Au collisions.