Lowest laser intensity for observable dynamically assisted Schwinger pairs

The Schwinger rate in a constant field, $w = (e E)^2/(4\pi^3 \hbar^2 c)\exp(-\pi E_S/E)$ with $E_S \sim 4.4 \times 10^{13}\,\mathrm{statvolt}/\mathrm{cm}$, is negligible at the $E/E_S \sim 10^{-3}$ of present peak laser fields (peak intensity about $10^{30}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, approximate), where $\exp(-\pi E_S/E) \sim \exp(-3000)$. Superposing a weak high-frequency field (dynamically assisted Schwinger mechanism, Schuetzhold, Gies and Dunne 2008) reduces the exponent. For realistic focused optical pulses combined with XFEL or gamma-ray photons, determine the minimal strong-field intensity at which the nonperturbative assisted yield reaches 1 pair per shot, and specify an observable that separates it from perturbative multiphoton Breit-Wheeler pairs.

Calculations of assisted pair yields in realistic 3D focused field geometries beyond the locally constant field approximation, followed by a measurement at the predicted threshold.