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

The Breit-Wheeler process which produces matter and antimatter from photon collisions is experimentally investigated through the observation of 6085 exclusive electron-positron pairs in ultraperipheral $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The measurements reveal a large fourth-order angular modulation of $\cos 4\mathrm{\Delta }\varphi =(16.8\pm 2.5)\%$ and smooth invariant mass distribution absent of vector mesons ($\varphi$, $\omega$, 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 $\sqrt{⟨P_{\perp }^2⟩}=38.1\pm 0.9\mathrm{MeV}$ 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 ultrarelativistic speed. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.

Figure 1

A Feynman diagram for the exclusive Breit-Wheeler process and the related light-by-light scattering process illustrating the unique angular distribution predicted for each process due to the initial photon polarization.

Figure 2

The ${\chi }_{ee}^2$ vs ${\chi }_{\pi \pi }^2$ distribution before (after) applying the $|\mathrm{\Delta }\mathrm{\Delta }\mathrm{TOF}|<0.4$ criteria in black (color). The $e^{+}{e}^{-}$ candidates are shown in color below the red lines.

Figure 3

The fully corrected differential cross sections for exclusively produced $e^{+}{e}^{-}$ pairs with respect to (a) the invariant mass $M_{ee}$ (and predicted vector-meson background from photoproduction [43]), (b) the polar angle distribution $|\cos {\theta }^{\prime }|$, and (c) the pair transverse momentum $P_{\perp }$.

Figure 4

The $\mathrm{\Delta }\varphi ={\varphi }_{ee}-{\varphi }_e$ distribution from UPCs and 60%–80% central collisions for $M_{ee}>0.45\mathrm{GeV}$ with calculations from QED [47], STARLight [43], and from the publicly available superchic3 code [8].