Theory of radiative electron polarization in strong laser fields

Abstract

Radiative polarization of electrons and positrons through the Sokolov-Ternov effect is important for applications in high-energy physics. Radiative spin polarization is a manifestation of quantum radiation reaction affecting the spin dynamics of electrons. We recently proposed that an analog of the Sokolov-Ternov effect could occur in the strong electromagnetic fields of ultra-high-intensity lasers, which would result in a buildup of spin polarization in femtoseconds. In this paper, we develop a density matrix formalism for describing beam polarization in strong electromagnetic fields. We start by using the density matrix formalism to study spin flips in nonlinear Compton scattering and its dependence on the initial polarization state of the electrons. Numerical calculations show a radial polarization of the scattered electron beam in a circularly polarized laser, and we find azimuthal asymmetries in the polarization patterns for ultrashort laser pulses. A degree of polarization approaching 9% is achieved after emitting just a single photon. We develop the theory by deriving a local constant crossed-field approximation (LCFA) for the polarization density matrix, 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. In particular, these expressions include the spin-flip rates for arbitrary initial polarization of the electrons. The validity of the LCFA is confirmed by explicit comparison with an exact QED calculation of electron polarization in an ultrashort laser pulse.

Spin-dependent radiative deflection in the quantum radiation-reaction regime

“…However, in a linearly-polarized (LP) laser pulse the electron spin projection onto the magnetic field axis in its rest frame oscillates with the laser field and the spin-induced net contribution is averaged to zero. Considering the SG effect being too weak as compared to the RR effect [2], present scenarios thus predict no sign of significant spin-dependent dynamics for LP laser fields with symmetric field distribution, except that asymmetry is introduced to the field itself in the few-cycle regime [12,13]. The new mechanism results from the coupling between the strong radiation reaction and spin effect, which is not active in the parameters investigated in [12] In this work, we show a new mechanism that leads to significant deflection of electrons by spin-induced effect in a symmetric LP laser field.…”

“…· the photon energy fraction. The theory for arbitrarily polarized electrons is further developed by D Seipt et al in [12]. For electron polarization vector of s in its rest frame, the radiation probability rate is [12] d d y…”

“…and non-flip one d y P d d d nonflip [12], for convenience of numerical modeling, where the electron undergoes s s during photon emission in the spin-flip case. The QED-MC algorithm is implemented to calculate photon emission events.…”

“…In the QED perspective, when the spin state of an electron is considered it has been found out that the spinanti-paralleled electron tends to radiate more energy than the spin-paralleled electron and that the electron may undergo a spin-flip process during photon emission [11,12]. Spin-flip effect along with the so-called 'quantumjump' process were also proposed to polarize an electron beam in collision with an elliptically polarized laser pulse [13].…”

Identifying the electron–positron cascade regimes in high-intensity laser-matter interactions

“…Recent work has shown that photon polarisation [51] and electron spin [52][53][54] could affect the development of the cascade. These effects are not considered here.…”