Ultrarelativistic Electron-Beam Polarization in Single-Shot Interaction with an Ultraintense Laser Pulse

Abstract

Spin-polarization of an ultrarelativistic electron beam head-on colliding with an ultraintense laser pulse is investigated in the quantum radiation-reaction regime. We develop a Monte-Carlo method to model electron radiative spin effects in arbitrary electromagnetic fields by employing spin-resolved radiation probabilities in the local constant field approximation. Due to spin-dependent radiation reaction, the applied elliptically polarized laser pulse polarizes the initially unpolarized electron beam and splits it along the propagation direction into two oppositely transversely polarized parts with a splitting angle of about tens of milliradians. Thus, a dense electron beam with above 70% polarization can be generated in tens of femtoseconds. The proposed method demonstrates a way for relativistic electron beam polarization with currently achievable laser facilities.Introduction. Spin-polarized electron beams have been extensively employed to investigate matter properties, atomic and molecular structures [1][2][3]. In high-energy physics, relativistic polarized electron beams can be used to probe the nuclear structure [4,5], generate polarized photons [6,7] and positrons [6,8], study parity violation in Møller scattering [9] and new physics beyond the Standard Model [10]. There are many methods to generate polarized electron beams at low energies [1]. However, for relativistic electron beams, there are mainly two methods [11]. In the first method mostly used in the Stanford Linear Accelerator, the polarized electrons are first extracted from a photocathode (illuminated by a circularly polarized light) [12,13] and then, accelerated by the linear accelerator (alternatively one may use polarized electrons from spin filters [14] or beam splitters [15], with subsequent laser wakefield acceleration [16]). The second method is a direct way of polarization of a relativistic electron beam in a storage ring via radiative polarization (Sokolov-Ternov effect) [17][18][19][20][21][22][23][24]. The polarization time of the latter due to the synchrotron radiation is rather slow (typically from minutes to hours), since the magnetic fields of a synchrotron are too weak (in the order of 1 Tesla). The electrons are polarized transversely due to Sokolov-Ternov effect. As mostly longitudinal polarization is interesting in high-energy physics, spin rotation systems are applied [25]. Moreover, for creating polarized positron beams (also applicable for electrons) Compton scattering or Bremsstrahlung of circularly polarized lasers and successive pair creation are commonly used [26][27][28][29][30]. The polarization of relativistic electrons can be detected by Compton scattering [31], Møller scattering [32], or other methods.

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.…”

“…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]. 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.…”

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

“…[23], i.e. a generalization of the Sokolov-Ternov effect [24] to fields other than that of a permanent magnetic field [25,26]. We briefly demonstrate this in the case of positrons channeling in a bent germanium crystal where one has two kinds of motion superimposed, the oscillatory channeling motion between the bent planes, which in the unbent case would not lead to polarization, along with the motion along the bending arc which leads to transverse polarization of the positrons.…”

Polarized Ultrashort Brilliant Multi-GeV γ Rays via Single-Shot Laser-Electron Interaction

“…Here, W f i ≡ W R F 0 is the electron-spin-resolved radiation probability averaged by the photon polarization, cf. [50]. Averaging over the electron spin in Eq.…”

“…The electron initial kinetic energy is ε 0 = 10 GeV, the angular divergence 0.2 mrad, and the energy spread ∆ε 0 /ε 0 = 0.06. Such electron bunches are achievable via laser wakefield acceleration [61,62] with further radiative polarization [50,63] or alternatively, via directly wakefield acceleration of LSP electrons [64].…”