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

Signatures for strong-field quantum electrodynamics are determined for collisions between round ultra-relativistic leptonic beams in the quantum limit of beamstrahlung. In the low-disruption regime, we derive the integrated beamstrahlung photon spectrum that features a characteristic peak close to the beam energy. The conditions to precisely observe this peak experimentally are given regarding the beam parameters. Moreover, the effects of electron-positron pair creation and beam disruption on the photon spectrum are discussed and explored with three-dimensional particle-in-cell QED simulations. The photon spectrum is associated with the emission of ultrashort and highly collimated gamma-ray beams with a peak spectral brightness exceeding 10(30) photons/(s mm(2) mrad(2) 0.1% BW) at the 100-GeV level of photon energy (close to the beam energy).