Anomalous lepton acceleration in the radiation reaction dominated reflection regime

Relativistic electrons colliding with intense counterpropagating laser pulses are expected to lose energy through radiation reaction. However, we reveal a counterintuitive regime where reflected leptons (including incident electrons, generated electrons, and positrons) gain significant energies when an ultraintense laser pulse interacts with counterpropagating electrons. Because of strong radiation reaction, these particles can be halted and reflected near the laser peak. The subsequent asymmetric laser field then accelerates the reflected leptons to energies far exceeding their initial values. Using three-dimensional particle-in-cell simulations, we demonstrate the generation and acceleration of quasimonoenergetic positrons to multi-GeV energies with a high number conversion efficiency employing forthcoming multipetawatt lasers, which provide conceptually a simple single-stage solution for positron creation and acceleration. The proposed mechanism of lepton acceleration is scalable to the astrophysical scenario and can be related to the origin of ultrahigh-energy cosmic rays in environments with intense fast radio bursts.