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Abstract

When dense high-energy lepton bunches collide, the beam particles can
experience rest-frame electromagnetic fields which greatly exceed the QED
critical one. Here it is demonstrated that beamstrahlung efficiently converts
lepton energy to high-energy photons in this so-called supercritical QED
regime, as the single-photon emission spectrum exhibits a pronounced peak close
to the initial lepton energy. It is also shown that the observation of this
high-energy peak in the photon spectrum requires one to mitigate multiple
photon emissions during the interaction. Otherwise, the photon recoil induces
strong correlations between subsequent emissions which soften the photon
spectrum and suppress the peak. The high-energy peak in the photon spectrum
constitutes a unique observable of photon emission in the supercritical QED
regime, and provides decisive advantages for the realization of an efficient
multi-TeV laserless gamma-gamma collider based on electron-electron collisions.