Impact of laser focusing and radiation reaction on particle spectra from nonlinear Breit-Wheeler pair production in the nonperturbative regime

Near-future experiments intend to detect strong-field Breit-Wheeler pair creation from the collision between bremsstrahlung bursts containing GeV-$\ensuremath{\gamma}$ quanta and high-intensity laser pulses. In this theoretical study, we investigate the influence of laser focusing, radiation reaction, and a broad bremsstrahlung $\ensuremath{\gamma}$ spectrum on the energy and angular distributions of created pairs. Understanding the role of these inherent reaction attributes provides relevant insights for experimental detection strategies and data interpretation. We show that the inclusion of radiation reaction leads to a narrowing of the particles' energy distribution, whose maximum is shifted to substantially lower energies as compared to the case in which radiative energy losses are ignored. The broad bremsstrahlung spectrum also has a distinct influence on the shape of the particle distributions, whereas the corresponding impact of laser focusing turns out to be rather moderate in the considered parameter regime.