Bright γ rays source and nonlinear Breit-Wheeler pairs in the collision of high density particle beams

The collision of ultrashort high-densitye−ore−ande+beams at 10s of GeV, to be available at the FACET II and in laser wakefield accelerator experiments, can produce highly collimatedγrays (few GeVs) with peak brilliance of1027ph/smm2mrad20.1%BWand up to105nonlinear Breit-Wheeler pairs. We provide analytical estimates of the photon source properties and of the yield of secondary pairs, finding excellent agreement with full-scale 3D self-consistent particle-in-cell simulations that include quantum electrodynamics effects. Our results show that beam-beam collisions can be exploited as secondary sources ofγrays and provide an alternative to beam-laser setups to probe quantum electrodynamics effects at the Schwinger limit.