Ultra-relativistic heavy ion collisions are expected to produce some of 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 magnetohydrodynamical phenomena in Quantum Chromodynamics, like the Chiral Magnetic Effect[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].

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 measured distribution of electron-positron pairs reveals a striking fourth-order angular modulation which is a direct result of vacuum birefringence[7], 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 provide the first direct experimental evidence that ultra-relativistic heavy ion collisions are capable of produce ultra-strong magnetic fields approximately 10,000 times stronger than the magnetic fields found in the magnetosphere of magnetars (inferred to be $\approx 10^{10}-10^{12}$ Tesla), the strongest magnetic fields in the known Universe until now. These measurements provide constraints on existing models and provide an important experimental tool and baseline for the measurement of possible medium effects driven by strong final state magnetic fields or from Coulomb multiple scattering through the QGP[5,6]. If time permits I’ll further discuss the application of these discoveries to one of the “most intellectually pressing” questions that an electron-ion collider will address[8].

[1] V. Skokov, A. Illarionov, and V. Toneev. International Journal of Modern Physics A 24 (2009): 5925–32.
[2] Kharzeev, D. E., 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] STAR Collaboration, Phys. Rev. Lett. 121 (2018) 132301
[6] ATLAS Collaboration, Phys. Rev. Lett. 121 (2018) , 212301
[7] Heisenberg, W., and H. Euler. Zeitschrift für Physik, (1936) arXiv: physics/0605038
[8] EIC White Paper https://www.bnl.gov/npp/docs/EIC_White_Paper_Final.pdf