Electromagnetic fields in small systems from a multiphase transport model

TL;DR

This study uses a multiphase transport model to analyze electromagnetic fields in small collision systems at RHIC and LHC energies, revealing their non-negligible magnitude and complex azimuthal correlations, impacting CME detection strategies.

Contribution

It provides the first detailed event-by-event analysis of electromagnetic fields and their azimuthal correlations in small systems, challenging previous assumptions about their suitability for CME studies.

Findings

Electromagnetic fields are significant in small systems like p+Au, d+Au, and p+Pb collisions.

Azimuthal correlation between magnetic field and participant plane is nearly zero in high-multiplicity small systems.

Charge azimuthal correlation is not a reliable CME probe in high-multiplicity small systems.

Abstract

We calculate the electromagnetic fields generated in small systems by using a multiphase transport (AMPT) model. Compared to $A+A$ collisions, we find that the absolute electric and magnetic fields are not small in $p$+Au and $d$+Au collisions at energies available at the BNL Relativistic Heavy Ion Collider and in $p$+Pb collisions at energies available at the CERN Large Hadron Collider. We study the centrality dependencies and the spatial distributions of electromagnetic fields. We further investigate the azimuthal fluctuations of the magnetic field and its correlation with the fluctuating geometry using event-by-event simulations. We find that the azimuthal correlation $\left\langle \cos2(\Psi_B - \Psi_{2}) \right\rangle$ between the magnetic field direction and the second harmonic participant plane is almost zero in small systems with high multiplicities, but not in those with low multiplicities. This indicates that the charge azimuthal correlation, $\left\langle \cos(\phi_{\alpha}+\phi_{\beta} - 2\Psi_{RP}) \right\rangle$, is not a valid probe to study the chiral magnetic effect (CME) in small systems with high multiplicities. However, we suggest searching for possible CME effects in small systems with low multiplicities.