Dominance of $gamma$-$gamma$ electron-positron pair creation in a plasma driven by high-intensity lasers

Creation of electrons and positrons from light alone is a basic prediction of quantum electrodynamics, but yet to be observed. Here we show that it is possible to create ${>}10^8$ positrons by dual laser irradiation of a structured plasma target, at intensities of $2 times 10^{22} mathrm{W}mathrm{cm}^{-2}$. In contrast to previous work, the pair creation is primarily driven by the linear Breit-Wheeler process ($gammagamma to e^+ e^-$), not the nonlinear process assumed to be dominant at high intensity, because of the high density of $gamma$ rays emitted inside the target. The favorable scaling with laser intensity of the linear process prompts reconsideration of its neglect in simulation studies, but also permits positron jet formation at intensities that are already experimentally feasible. Simulations show that the positrons, confined by a quasistatic plasma magnetic field, may be accelerated by the lasers to energies $> 200$ MeV.