Energy Dependence of Polarized $\mathbf{\gamma\gamma\rightarrow e^{+}e^{-}}$ in Peripheral Au+Au Collisions at RHIC

TL;DR

This study measures the energy dependence of the polarized Breit-Wheeler process in peripheral gold-gold collisions at RHIC, revealing how yields and pair momentum vary with collision energy and providing insights into nuclear structure.

Contribution

First measurement of the energy dependence of polarized gamma-gamma to electron-positron pairs in heavy-ion collisions at RHIC, linking experimental data to nuclear form factors and photon polarization effects.

Findings

Yields increase with collision energy at low transverse momentum.

Pair transverse momentum decreases as energy increases.

Data used to extract the Au nucleus charge radius.

Abstract

We report the differential yields at mid-rapidity of the Breit-Wheeler process ($\gamma\gamma\rightarrow e^{+}e^{-}$) in peripheral Au+Au collisions at $\sqrt{s_{_{\rm{NN}}}} = $ 54.4 GeV and 200 GeV with the STAR experiment at RHIC, as a function of energy $\sqrt{s_{_{\rm{NN}}}}$, $e^{+}e^{-}$ transverse momentum $p_{\rm T}$, $p_{\rm T}^{2}$, invariant mass $M_{ee}$ and azimuthal angle. In the invariant mass range of 0.4 $<$ $M_{ee}$ $<$ 2.6 GeV/$c^{2}$ at low transverse momentum ($p_{\rm T}$ $ < $0.15 GeV/$c$), the yields increase while the pair $\sqrt{\langle p_{\rm T}^{2} \rangle}$ decreases with increasing $\sqrt{s_{_{\rm{NN}}}}$, a feature is correctly predicted by the QED calculation. The energy dependencies of the measured quantities are sensitive to the nuclear form factor, infrared divergence and photon polarization. The data are compiled and used to extract the charge radius of the Au nucleus.