Probing Extreme Electromagnetic Fields with the Breit-Wheeler Process

Abstract

The Relativistic Heavy Ion Collider (RHIC) accelerates nuclei to ultra-relativistic velocities, producing some of the strongest known magnetic fields in the Universe (\({10^{14}-10^{15}}\) Tesla). The highly Lorentz-contracted Coulomb fields of the nuclei generate a flux of linearly polarized quasi-real photons that can interact via the Breit-Wheeler process to produce electron-positron pairs (\({\gamma\gamma \rightarrow e^+e^-}\)). Experimental data presented in this article are consistent with Breit-Wheeler theory across all measured differentials. The detected pairs are produced predominantly at low transverse momentum (\(P_{\perp}\)) with a smooth invariant mass distribution, with the individual \({e^\pm}\) preferentially aligned along the beam direction, and with a 4th-order modulation in azimuth between the \({e^+e^-}\) pair and \({e^\pm}\) momenta. The \(P_{\perp}\) spectrum broadens from large to small impact parameters. Our observation opens new opportunities to study Quantum Chromodynamics under extreme conditions and provides a new tool for interdisciplinary study of extreme electromagnetic fields.