The effect of the fluctuating proton size on the study of chiral magnetic effect in proton-nucleus collisions

TL;DR

This paper investigates how fluctuations in proton size during high-energy proton-nucleus collisions can induce correlations between magnetic fields and event planes, potentially enabling the study of the chiral magnetic effect in small systems.

Contribution

It introduces the effect of fluctuating proton size into the Monte Carlo Glauber model to explain correlations in pA collisions, a novel approach for this context.

Findings

Fluctuating proton size causes significant correlation between magnetic field and event plane in pA collisions.

Correlation magnitude in pA is smaller than in AA collisions but still notable.

This effect opens new avenues for studying the chiral magnetic effect in small systems.

Abstract

High energy proton-nucleus (pA) collisions provide an important constraint on the study of the chiral magnetic effect in QCD matter. Naively, in pA collisions one expects no correlation between the orientation of event plane as reconstructed from the azimuthal distribution of produced hadrons and the orientation of magnetic field. If this is the case, any charge-dependent hadron correlations can only result from the background. Nevertheless, in this paper we point out that in high multiplicity pA collisions a correlation between the magnetic field and the event plane can appear. This is because triggering on the high hadron multiplicity amounts to selecting Fock components of the incident proton with a large number of partons that are expected to have a transverse size much larger than the average proton size. We introduce the effect of the fluctuating proton size in the Monte Carlo Glauber model and evaluate the resulting correlation between the magnetic field and the second-order event plane in both pA and nucleus-nucleus (AA) collisions. The fluctuating proton size is found to result in a significant correlation between magnetic field and the event plane in pA collisions, even though the magnitude of the correlation is still much smaller than in AA collisions. This result opens a possibility of studying the chiral magnetic effect in small systems.