Dilepton photoproduction measures the fluctuations of initial electromagnetic fields in nuclear collisions

TL;DR

This paper investigates dilepton photoproduction in nuclear collisions as a novel method to measure the initial electromagnetic field fluctuations, which are otherwise difficult to observe directly, by analyzing the sensitivity of dilepton yields to these fields.

Contribution

It introduces dilepton photoproduction as a new probe for initial electromagnetic field fluctuations in nuclear collisions, linking electromagnetic field dynamics to measurable dilepton spectra.

Findings

Dilepton production is proportional to the square of electromagnetic fields.

The method is sensitive to the magnitude and lifetime of initial EM fields.

Event-by-event fluctuations in nuclear charge affect dilepton mass spectra.

Abstract

Dilepton production from two photon interactions $\gamma\gamma\rightarrow l^+l^-$ are studied in semi-central and peripheral nuclear collisions. Based on Weizs\"acker-Williams approach, it is shown that the dilepton photoproduction is proportional to the electromagnetic (EM) fields $\sim E^2B^2$ and therefore sensitive to the magnitude and lifetime of initial EM fields which last only for a short time and are hard to be measured in experiments directly. We propose dilepton photoproduction as a probe for the nuclear charge fluctuations, which are crucial for the electric/magnetic field induced chiral and charge particle evolutions. We calculate the relative standard deviation of dilepton mass spectrum with event-by-event fluctuating nuclear charge distributions (and EM fields).