Radiation beaming in the quantum regime

Classical theories of radiation reaction predict that the electron motion is confined to the plane defined by the electron's instantaneous momentum and the force exerted by the external electromagnetic field. However, in the quantum radiation reaction regime, where the recoil exerted by individual quanta becomes significant, the electron can scatter `out-of-plane', as the photon is emitted into a cone with finite opening angle. We show that Monte Carlo implementation of an angularly resolved emission rate leads to substantially improved agreement with exact QED calculations of nonlinear Compton scattering. Furthermore, we show that the transverse recoil caused by this finite beaming, while negligible in many high-intensity scenarios, can be identified in the increase in divergence, in the plane perpendicular to the polarization, of a high-energy electron beam that interacts with a linearly polarized, ultraintense laser.