This abstract was submitted and accepted for a talk at the 2020 Winter Workshop on Nuclear Dynamics.

### Probing Extreme Electromagnetic Fields with the Breit-Wheeler Process

Ultra-relativistic heavy-ion collisions are expected to produce the strongest magnetic fields ($10^{13}-10^{16}$ Tesla) in the Universe [1]. Recently, there has been increased interest in the magnetic fields produced by heavy-ion collisions and their possible observational impacts through emergent magnetohydrodynamic phenomena in Quantum Chromodynamics [2]. The initial strong electromagnetic fields produced in heavy ion collisions have been proposed as a source of linearly-polarized, quasi-real photons [3] that can interact via the Breit-Wheeler process to produce $e^+e^-$ pairs [4].

\noindent In this talk, I will present STAR measurements of $e^+e^-$ pair production in ultra-peripheral and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. A comprehensive study of the pair kinematics is presented to distinguish the $\gamma\gamma \rightarrow e^+e^-$ process from other possible production mechanisms. Furthermore, I will present and discuss the first observation of a 4th-order azimuthal modulation of $e^+e^-$ pairs produced in heavy-ion collisions. The striking fourth-order angular modulation is a direct result of vacuum birefringence [5], a phenomenon predicted in 1936 in which empty space can split light according to its polarization components when subjected to a strong magnetic field. These measurements and their implications for the magnetic field produced in heavy-ion collisions will be discussed.

[1] V. Skokov, A. Illarionov, and V. Toneev. Int. J. Mod. Phys. A 24 (2009) 5925–32
[2] D. E. Kharzeev, et al. Prog. Part. Nucl. Phys., 88 (2016) 1–28
[3] C. Weizsäcker, Zeitschrift für Physik 88 (1934) 612–25
[4] G. Breit and J. A. Wheeler. Physical Review 46 (1934) 1087
[5] W. Heisenberg and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038