Theory of Radiative Electron Polarization in Strong Laser Fields

Recent high-intensity laser-plasma experiments provided evidence for quantum radiation reaction effects due to hard photon emission. In this talk I will focus on the influence of electron spin polarization on the non-linear Compton photon emission, and radiative spin-polarization as a manifestation of quantum radiation reaction affecting the spin-dynamics of electrons. We recently showed that a build-up of spin-polarization of electrons---analogous to the Sokolov-Ternov effect---is possible for electrons in counter-propagating laser pulses with intensities exceeding 5 × 10^22 W/cm^2 on femtosecond timescales. I will present a density matrix approach for describing the radiative beam polarization in strong electromagnetic fields. The local constant crossed field approximation (LCFA) for the polarization density matrix is derived, which is a generalization of the well known LCFA scattering rates. We find spin-dependent expressions that may be included in electromagnetic charged-particle simulation codes, such as particle-in-cell plasma simulation codes, using Monte-Carlo modules. The validity of the LCFA is confirmed by explicit comparison with an exact QED calculation of electron polarization in an ultrashort laser pulse.