Numerical approach to the semiclassical method of radiation emission for arbitrary electron spin and photon polarization

Abstract

We show how the semiclassical formulas for radiation emission of Baier, Katkov and Strakhovenko for arbitrary initial and final spins of the electron and arbitrary polarization of the emitted photon can be rewritten in a form which numerically converges quickly. We directly compare the method in the case of a background plane wave with the result obtained by using the Volkov state solution of the Dirac equation, and confirm that we obtain the same result. We then investigate the interaction of a circularly polarized short laser pulse scattering with GeV electrons and see that the finite duration of the pulse leads to a lower transfer of circular polarization than that predicted by the known formulas in the monochromatic case. We also see how the transfer of circular polarization from the laser beam to the gamma ray beam is gradually deteriorated as the laser intensity increases, entering the nonlinear regime. However, this is shown to be recovered if the scattered photon beam is collimated to only allow for passage of photons emitted with angles smaller than 1/γ with respect to the initial electron direction, where γ is the approximately constant Lorentz factor of the electron. The obtained formulas also allow us to answer questions regarding radiative polarization of the emitting particles. In this respect we briefly discuss an application of the present approach to the case of a bent crystal and high-energy positrons.

Loop spin effects in intense background fields

“…To this end, consider the behavior of the probabilities for small η, beginning with the nonradiative result (14), which is exact to order α 2 . The main contribution to the integrand in (15) originates in the region where the argument of S is small, see Fig. 2.…”

“…2), the sign of the integrand in (15) is determined by the sign of a μ . It follows that there can be no nonradiative spin flip if the potential is an even function, because then the integral over φ in (15) gives zero. This difference in dependence on the driving laser field appears because we look at flips between lightfront helicity states.…”

“…In the above plane wave calculations, a unipolar pulse structure is required to counter the cancellation of oppositesign contributions in (15), coming from oscillations of the field. What we will now show is that it is possible to realize an effective unipolar structure in the collision of electrons with focused laser fields, which in principle allows experimental access to the loop; we will however see that loop effects are more subtle in focused pulses than in plane waves.…”

“…Radiative spin flip in photon emission from an electron in a laser background through nonlinear Compton scattering [6,7] has been studied in a constant crossed field [8][9][10] and in pulses [11][12][13][14][15], for which a comprehensive density matrix formalism has also been developed [16]. Using the locally constant field approximation (LCFA) [8], this radiative spin flip has been included in numerical schemes [17][18][19].…”

Helicity transfer in strong laser fields via the electron anomalous magnetic moment

“…The polarization effects in strong laser fields have a capability of detecting the quantum stochastic nature of electron dynamics [19], diagnosing magnetic fields of plasma [20], and providing ultra-short, high-brilliance, low-emittance polarized beam sources for fundamental studies in high-energy physics [21][22][23] and material science [24,25]. The completely spin-and photon-polarization-resolved probability rates for nonlinear Compton scattering have been derived from strong-field QED theory in the Furry picture for a plane-wave laser field [26], and for the locally constant fields [27], as well as via the quantum operator method [28], and employed for a deep analysis of all polarization channels, helicity transfer in the perturbative regime, and investigation of the polarization dependent energy and angle distributions [29]. Furthermore, the study of polarization effects has been extended to higher-order QED processes [30][31][32][33][34][35], and QED cascades [36].…”