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Abstract

Electron beam longitudinal polarization during the interaction with
counterpropagating circularly-polarized ultraintense laser pulses is
investigated, while accounting for the anomalous magnetic moment of the
electron. Although it is known that the helicity transfer from the laser
photons to the electron beam is suppressed in linear and nonlinear Compton
scattering processes, we show that the helicity transfer nevertheless can
happen via an intermediate step of the electron radiative transverse
polarization, phase-matched with the driving field, followed up by spin
rotation into the longitudinal direction as induced by the anomalous magnetic
moment of the electron. With spin-resolved QED Monte Carlo simulations, we
demonstrate the consequent helicity transfer from laser photons to the electron
beam with a degree up to 10%, along with an electron radial polarization up to
65% after multiple photon emissions in a femtosecond timescale. This effect is
detectable with currently achievable laser facilities, evidencing the role of
the leading QED vertex correction to the electron anomalous magnetic moment in
the polarization dynamics in ultrastrong laser fields.