Algorithm for computing the electron-positron yield from the linear Breit-Wheeler process in high-intensity laser-plasma interactions.

High-intensity laser-plasma interactions have been shown to generate dense populations of gammarays, so these interactions are expected to generate electron-positron pairs via binary photon collisions (linear Breit-Wheeler process). However, particle-in-cell (PIC) codes that are used for studies of laser plasma interactions are not yet equipped to compute the yield from the linear Breit-Wheeler process. We present a post-processing algorithm that allows one to quickly calculate the yield of the linear BreitWheeler process inside a photon-emitting plasma using PIC simulation data. The algorithm splits the PIC computational domain into smaller sub-domains whose shape and size are determined based on a specific problem. The photons emitted within each sub-domain are grouped into collimated monoenergetic beams called beamlets. The algorithm computes the yield by evaluating beamlet-beamlet collisions without the spatial integration over the interaction region. Presented benchmarking shows that the computational time is reduced by two orders of magnitude compared to the direct approach that involves the spatial integration, while the resulting error in the total yield remains around 10%. We also show how the algorithm can be leveraged to compute the density of the pair-producing events and the positron momentum distribution at the time of their creation. The ability of our algorithm to quickly compute the pair yield makes it a useful tool for studies of high-intensity laser-plasma interactions. It can also be useful for testing future implementations of the linear Breit-Wheeler process into plasma simulation codes.