Stochastic effects in the spin (de)polarization of an ultrarelativistic electron beam during photon emissions in a counterpropagating ultrastrong focused laser pulse in the quantum radiation reaction regime are investigated. We employ a Monte Carlo method to describe the electron dynamics semiclassically and photon emission and electron radiative polarization quantum mechanically. While in the latter the photon emission is inherently stochastic, we are able to identify its imprints in comparison with the semiclassical stochasticity-free method of radiative polarization applicable in the quantum regime. With an initially-spin-polarized electron beam, the impact of stochastic effects of photon emissions on the spin observable is demonstrated in the dependence of the depolarization degree on the electron scattering angle and the final electron energy (spin stochastic diffusion). With an initially unpolarized electron beam, the stochastic effects on the spin are exhibited in enhancing the known effect of splitting of the electron beam along the propagation direction into two oppositely polarized parts by an elliptically polarized laser pulse. The considered stochastic effects for the spin are observable with currently achievable laser and electron-beam parameters.

• Received 25 March 2020
• Accepted 1 September 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.033483

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Published by the American Physical Society