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.

We study the effect of stimulated photon emission from the vacuum in strong space-time dependent electromagnetic fields. We emphasize the viewpoint that the vacuum subjected to macroscopic electromagnetic fields with at least one nonzero electromagnetic field invariant, as, e.g., attainable by superimposing two laser beams, can represent a source term for outgoing photons. We believe that this view is particularly intuitive and allows for a straightforward and intuitive study of optical signatures of quantum vacuum nonlinearity in realistic experiments involving the collision of high-intensity laser pulses, and exemplify this view for the vacuum subjected to a strong standing electromagnetic wave as generated in the focal spot of two counter-propagating, linearly polarized high-intensity laser pulses. Focusing on a comparably simple electromagnetic field profile, which should nevertheless capture the essential features of the electromagnetic fields generated in the focal spots of real high-intensity laser beams, we provide estimates for emission characteristics and the numbers of emitted photons attainable with present and near future high-intensity laser facilities.

Generation of ultrarelativistic polarized positrons during interaction of an ultrarelativistic electron beam with a counterpropagating two-color petawatt laser pulse is investigated theoretically. Our Monte Carlo simulation based on a semi-classical model, incorporates photon emissions and pair productions, using spin-resolved quantum probabilities in the local constant field approximation, and describes the polarization of electrons and positrons for the pair production and photon emission processes, as well as the classical spin precession inbetween. The main reason of the polarization is shown to be the spin-asymmetry of the pair production process in strong external fields, combined with the asymmetry of the two-color laser field. Employing a feasible scenario, we show that highly polarized positron beams, with a polarization degree of ζ ≈ 60%, can be produced in a femtosecond time scale, with a small angular divergence, ∼ 74 mrad, and high density ∼ 10 14 cm −3 . The laser-driven positron source, along with laser wakefield acceleration, may pave the way to small scale facilities for high energy physics studies.

Generation of circularly-polarized (CP) and linearly-polarized (LP) γ-rays via the single-shot interaction of an ultraintense laser pulse with a spin-polarized counterpropagating ultrarelativistic electron beam has been investigated in nonlinear Compton scattering in the quantum radiation-dominated regime. For the process simulation a Monte Carlo method is developed which employs the electron-spin-resolved probabilities for polarized photon emissions. We show efficient ways for the transfer of the electron polarization to the high-energy photon polarization. In particular, multi-GeV CP (LP) γ-rays with polarization of up to about 90% (95%) can be generated by a longitudinally (transversely) spin-polarized electron beam, with a photon flux at a single shot meeting the requirements of recent proposals for the vacuum birefringence measurement in ultrastrong laser fields. Such high-energy, high-brilliance, high-polarization γ-rays are also beneficial for other applications in high-energy physics, nuclear physics, and laboratory astrophysics.

Relativistic spin-polarized positron beams are indispensable for future electron-positron colliders to test modern high-energy physics theory with high precision. However, present techniques require very large scale facilities for those experiments. We put forward a novel efficient method for generating ultrarelativistic polarized positron beams employing currently available laser fields. For this purpose the generation of polarized positrons via multiphoton Breit-Wheeler pair production and the associated spin dynamics in single-shot interaction of an ultraintense laser pulse with an ultrarelativistic electron beam is investigated in the quantum radiation-dominated regime. The pair production spin asymmetry in strong fields, significantly exceeding the asymmetry of the radiative polarization, produces locally highly polarized particles, which are split by a specifically tailored small ellipticity of the laser field into two oppositely polarized beams along the minor axis of laser polarization. In spite of radiative de-polarization, a dense positron beam with up to about 90% polarization can be generated in tens of femtoseconds. The method may eventually usher high-energy physics studies into smaller-scale laser laboratories.