Signatures for strong-field QED physics in the quantum limit of beamstrahlung

Collisions between round ultrarelativistic leptonic beams are proposed to probe strong-field quantum electrodynamics (SF-QED) in the quantum limit of beamstrahlung. The collective radiation spectrum from a leptonic collision is derived. A characteristic spectral peak close to the beam energy is identified, as a signature predicted by SF-QED theory. The dependences of the spectral peak and beamstrahlung on the collision parameters are determined, paving the way to experimentally verify SF-QED. Ultrashort, ultrabright, and high-luminosity colliding gamma-ray beams are generated. The theoretical results are confirmed by self-consistent 3-dimensional QED particle-in-cell simulations.