Continuous-radiative-loss Model for Electron-Spin Dynamics in the Radiation-Dominated Regime

Spin dynamics in the constant homogeneous magnetic field is studied analytically and numerically in the regime when the radiative losses are strong. Analytically we treat photon emission as a continuous process, which allows us to describe system dynamics with a set of differential equations for both energy and spin of the electrons. The developed model predicts that the radiative losses significantly damps polarization buildup in the classical regime, i.e., when Lorentz invariant quantum parameter chi << 1. The spin-resolved Monte Carlo-based algorithm for simulating spin dynamics is developed and verified. As expected, for the classical regime, the theoretical solution is in very good agreement with numerical simulations. In the strong-field QED regime chi > 1 the model predictions and the numerical results coincide to each other only in order of magnitude.