# Initial transverse-momentum broadening of Breit-Wheeler process in relativistic heavy-ion collisions

## TL;DR

This paper investigates the initial transverse-momentum broadening of the Breit-Wheeler process in relativistic heavy-ion collisions, showing how impact parameter influences the $P_{ot}$ distribution and explaining recent experimental observations.

## Contribution

It introduces a generalized differential Equivalent Photon Approximation and compares it with a baseline QED calculation to analyze impact-parameter dependence of $P_{ot}$ broadening.

## Key findings

- $P_{ot}$ broadening depends strongly on impact parameter.
- The model quantitatively explains recent RHIC and LHC experimental data.
- Impact-parameter sensitive observables can probe extreme electromagnetic fields.

## Abstract

We calculate the cross section and transverse-momentum ($P_{\bot}$) distribution of the Breit-Wheeler process in relativistic heavy-ion collisions and their dependence on collision impact parameter ($b$). To accomplish this, the Equivalent Photon Approximation (EPA) was generalized in a more differential way compared to the approach traditionally used for inclusive collisions. In addition, a lowest-order QED calculation with straightline assumption was performed as a standard baseline for comparison. The cross section as a function of $b$ is consistent with previous calculations using the equivalent one-photon distribution function. Most importantly, the $P_{\bot}$ shape from this model is strongly dependent on impact parameter and can quantitatively explain the $P_{\bot}$ broadening observed recently by RHIC and LHC experiments. This broadening effect from the initial QED field strength should be considered in studying possible trapped magnetic field and multiple scattering in a Quark-Gluon Plasma (QGP). The impact-parameter sensitive observable also provides a controllable tool for studying extreme electromagnetic fields.