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Theory of radiative electron polarization in strong laser fields

Research output: Contribution to journalJournal article

Article number023417
<mark>Journal publication date</mark>20/08/2018
<mark>Journal</mark>Physical review a
Issue number2
Number of pages19
Publication statusPublished
Original languageEnglish


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. © 2018 authors.

Bibliographic note

Funding details: Engineering and Physical Sciences Research Council, EPSRC, EP/M018156/1 Funding details: W911NF-16-1-0044 Funding details: Science and Technology Facilities Council, STFC, ST/G008248/1 Funding text 1: D.S. acknowledges valuable discussions with J. Gratus, T. Heinzl, A. Ilderton, B. King, and M. Korostelev, and support from the Science and Technology Facilities Council, Grant No. ST/G008248/1. C.P.R. and D.D.S. acknowledge support from Engineering and Physical Sciences Grant No. EP/M018156/1. A.G.R.T. acknowledges support from U. S. DOD under Grant No. W911NF-16-1-0044. APPENDIX A: References: Patrignani, C., The review of particle physics (2016) Chin. Phys. C, 40, p. 100001; David Jackson, J., (1983) Klassische Elektrodynamik, , 2nd ed. (Walter de Gruyter, Berlin); Anselmino, M., Efremov, A., Leader, E., The theory and phenomenology of polarized deep inelastic scattering (1995) Phys. Rep., 261, p. 1; Steven, D., Bass, The Proton Spin Puzzle: Where Are We Today? 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