Measurement of $e^+e^-$ Momentum and Angular Distributions from Linearly Polarized Photon Collisions

Abstract

The Breit-Wheeler process which produces matter and anti-matter from photon collisions is investigated experimentally through the observation of 6085 exclusive electron-positron pairs in ultra-peripheral Au+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The measurements reveal a large fourth-order angular modulation of $\cos{4\Delta\phi}=(16.8\pm2.5)\%$ and smooth invariant mass distribution absent of vector mesons ($\phi$, $\omega$ and $\rho$) at the experimental limit of $\le 0.2\%$ of the observed yields. The differential cross section as a function of $e^+e^-$ pair transverse momentum $P_\perp$ peaks at low value with $\sqrt{ \langle P_\perp^2 \rangle } = 38.1\pm0.9$ MeV and displays a significant centrality dependence. These features are consistent with QED calculations for the collision of linearly polarized photons quantized from the extremely strong electromagnetic fields generated by the highly charged Au nuclei at ultra-relativistic speed. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.