Radiative polarization of electrons in a strong laser wave

Abstract

We reanalyze the problem of radiative polarization of electrons brought into collision with a circularly polarized strong plane wave. We present an independent analytical verification of formulae for the cross section given by D.\,Yu. Ivanov et al [Eur.\ Phys.\ J. C \textbf{36}, 127 (2004)]. By choosing the exact electron's helicity as the spin quantum number we show that the self-polarization effect exists only for the moderately relativistic electrons with energy $\gamma = E/mc^2 \lesssim 10$ and only for a non-head-on collision geometry. In these conditions polarization degree may achieve the values up to 65%, but the effective polarization time is found to be larger than 1\,s even for a high power optical or infrared laser with intensity parameter $\xi = |{\bf E}| m c^2/E_c \hbar \omega \sim 0.1$ ($E_c = m^2 c^3/e \hbar$). This makes such a polarization practically unrealizable. We also compare these results with the ones of some papers where the high degree of polarization was predicted for ultrarelativistic case. We argue that this apparent contradiction arises due to the different choice of the spin quantum numbers. In particular, the quantum numbers which provide the high polarization degree represent neither helicity nor transverse polarization, that makes the use of them inconvenient in practice.Comment: minor changes compared to v3; to appear in PR

Theory of radiative electron polarization in strong laser fields

“…In fact, there was some discussion in the literature whether or not spin-flip transitions are relevant and the electrons do spin-polarize [24,28,78]. These discrepancies were attributed to different choices of the quantization direction [28]. We stress here that no such ambiguities arise when working in the density matrix formalism, where the final electron polarization vector unambiguously describes the polarization after the scattering.…”

“…If one does not employ the density matrix formalism the initial polarization direction coincides with the chosen basis for the quantization of the spin. In fact, there was some discussion in the literature whether or not spin-flip transitions are relevant and the electrons do spin-polarize [24,28,78]. These discrepancies were attributed to different choices of the quantization direction [28].…”

“…Most studies of spinor QED refer to unpolarized electrons where the electron spin is averaged over. More detailed calculations of the non-linear Compton scattering probabilities with regard to the spin and spin polarization have been performed for monochromatic laser fields [24][25][26][27][28], short laser pulses [22,29,30], or constant crossed fields [31,32]. Moreover, in extremely strong fields electrons may polarize without radiating via electromagnetic self-interaction [33].…”

Ultrarelativistic polarized positron jets via collision of electron and ultraintense laser beams

“…There are many proposals to generate unpolarized electronpositron beams in the nonlinear QED regime, see [32][33][34][35][36][37][38] and references therein, and even avalanche-like electromagnetic cascades in the case of future extreme laser intensities 10 24 W/cm 2 , see [39][40][41][42] and references therein. Although the laser magnetic field can be much stronger (of the order of 10 5 T) than the synchrotron magnetic field (of order 1 T), the radiative polarization with laser fields is suppressed due to the symmetric character of the field [43][44][45][46], i.e., the particles in adjacent half-cycles are polarized oppositely. Attractiveness of a strong laser fields for particle polarization has been recently demonstrated in the case of a model laser field in the form of a strong rotating electric field [47,48].…”

Spin-polarization effects of an ultrarelativistic electron beam in an ultraintense two-color laser pulse

“…Recently, the rapid development of ultrashort (duration ∼ tens of femtoseconds) ultraintense (peak intensity ∼ 10 22 W cm −2 , and the corresponding magnetic field ∼ 4 × 10 5 Tesla) laser techniques [17,18] is providing opportunities to investigate electron polarization effects in such strong laser fields, analogous to the Sokolov-Ternov effect. A plenty of theoretical works have been performed in nonlinear Compton scattering, e.g., see [19][20][21][22][23] and the references therein. However, only a small polarization can be obtained in a monochromatic laser field [24] or a laser pulse [25].…”