  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.
