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Photon Polarization in Nonlinear Breit-Wheeler Pair Production and $\Gamma$ Polarimetry

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Photon Polarization in Nonlinear Breit-Wheeler Pair Production and $\Gamma$ Polarimetry PHYSICAL REVIEW RESEARCH 2, 032049(R) (2020) Rapid Communications High-energy γ-photon polarization in nonlinear Breit-Wheeler pair production and γ polarimetry Feng Wan ,1 Yu Wang,1 Ren-Tong Guo,1 Yue-Yue Chen,2 Rashid Shaisultanov,3 Zhong-Feng Xu,1 Karen Z. Hatsagortsyan ,3,* Christoph H. Keitel,3 and Jian-Xing Li 1,† 1MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, Xi’an 710049, China 2Department of Physics, Shanghai Normal University, Shanghai 200234, China 3Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany (Received 24 February 2020; accepted 12 August 2020; published 27 August 2020) The interaction of an unpolarized electron beam with a counterpropagating ultraintense linearly polarized laser pulse is investigated in the quantum radiation-dominated regime. We employ a semiclassical Monte Carlo method to describe spin-resolved electron dynamics, photon emissions and polarization, and pair production. Abundant high-energy linearly polarized γ photons are generated intermediately during this interaction via nonlinear Compton scattering, with an average polarization degree of more than 50%, further interacting with the laser fields to produce electron-positron pairs due to the nonlinear Breit-Wheeler process. The photon polarization is shown to significantly affect the pair yield by a factor of more than 10%. The considered signature of the photon polarization in the pair’s yield can be experimentally identified in a prospective two-stage setup. Moreover, with currently achievable laser facilities the signature can serve also for the polarimetry of high-energy high-flux γ photons. DOI: 10.1103/PhysRevResearch.2.032049 Rapid advancement of strong laser techniques [1–4] en- that an electron beam cannot be significantly polarized by a ables experimental investigation of quantum electrodynamics monochromatic laser wave [46–48]. Polarization properties (QED) processes during laser-plasma and laser-electron beam of electrons, positrons, and γ photons in ultrastrong laser- interactions. Nowadays, ultrashort ultrastrong laser pulses can electron beam interaction have been investigated comprehen- achieve peak intensities of about 1022 W/cm2, with a duration sively in Refs. [41,44,49]. It is known that an efficient way of about tens of femtoseconds and an energy fluctuation of the polarization transfer from electrons to high-energy ≈ 0.01 [5–7]. In such laser fields, QED processes become photons can be realized via linear Compton scattering; see, nonlinear, involving multiphoton processes [8]: A γ photon e.g., Refs. [50,51]. The extension of this technique into non- can be generated via nonlinear Compton scattering [9–11], or linear regime is possible [47,52–54]. In the extreme non- similarly a γ photon can create an electron-positron pair in linear regime, it will allow us to obtain circularly polar- the interaction with a strong laser wave in the nonlinear Breit- ized brilliant γ rays via nonlinear Compton scattering from Wheeler (BW) process [12]. These processes have been exper- longitudinally spin-polarized electrons [49], highly sought imentally observed in Refs. [13–15] and recently were con- in detecting schemes of vacuum birefringence in ultrastrong sidered in all-optical experimental setups [16–20]. Presently, laser fields [55–57]. there are many theoretical proposals aiming at γ -ray and Significant efforts have been devoted to the investigation pair production with ultrastrong laser fields of achievable or of pair production channels in ultrastrong laser-electron beam almost achievable intensities [21–28] and even avalanche-like interaction [21,22,24–32]. General theory for the pair produc- electromagnetic cascades in future extreme laser intensities tion by polarized photons in a monochromatic plane wave 1024 W/cm2 [29–36]. is given in Ref. [58]. A particular case of the multiphoton Recently, it has been realized that the radiation reaction BW process with linearly polarized (LP) γ photons of MeV due to γ photon emissions in laser fields can be harnessed to energy and moderately strong x-ray laser field is considered substantially polarize electrons [37–43] or to create polarized in Ref. [59]. The role of the γ -photon polarization within positrons [44,45], while it has been known for a long time the BW process in a constant crossed field is considered in Ref. [60], applying a spin-averaged treatment for the photon emission by an electron. While the latter gives a hint about *[email protected] the photon polarization effect, it is not straightforwardly †[email protected] extendible to the realistic setups with tightly focused laser beams. Most simple expressions for probabilities of the polar- Published by the American Physical Society under the terms of the ization effects in pair production process have been obtained Creative Commons Attribution 4.0 International license. Further in local constant field approximation (LCFA) [61,62]. They distribution of this work must maintain attribution to the author(s) show that the pair production probability depends strongly on and the published article’s title, journal citation, and DOI. the photon polarization and that photons emitted via nonlinear 2643-1564/2020/2(3)/032049(7) 032049-1 Published by the American Physical Society FENG WAN et al. PHYSICAL REVIEW RESEARCH 2, 032049(R) (2020) setup, using laser fields of different linear polarizations and different intensities in these stages. Moreover, our results sug- gest an interesting application in high-resolution polarimetry of high-energy and high-flux LP γ rays through the pair yield. Note that the high-resolution polarimetry of high-energy γ rays is an important problem in astrophysics and high-energy physics, which can be employed, e.g., to determine the nature of the emission mechanisms responsible for blazars, γ -ray bursts (GRBs), pulsars, and magnetars and to address prob- lems in fundamental physics [67–70]. Current polarimetries for high-energy γ photons mainly employ the principles of Compton scattering and Bethe-Heitler pair production by the Coulomb fields of atoms, with an accuracy of about several percents [69,70]. The former is not efficient at photon energies larger than 100 MeV because of the kinematic suppression of the Compton rate at large scattering angles; in the lat- ter the photon flux and polarimetry angular resolution are FIG. 1. (a) Scenario of nonlinear BW pair production. A LP restricted by the convertor material (damage threshold and laser pulse, polarized in the x direction and propagating along the multiple scattering), because, in particular, multiple scattering +z direction, collides head on with an electron beam, generating in a converter material decreases significantly the angular + − LP γ photons, and further these pairs “e ”and“e ” indicate resolution of polarimetry [68–72]. Our polarimetry concept positron and electron, respectively. (b) Angle-resolved positron den- via nonlinear BW pair production working in a small duty 2 / ε θ −1 −1 sity log10(d N+ d +d x) (GeV mrad ) vs the deflection angle cycle (determined by the laser pulse) is specifically designed θ = / ε x px pz and the positron energy +, with accounting for the pho- for high-flux GeV γ photons and provides a competitive / ε ε ton polarization. (c) Positron density dN+ d + vs + in the cases of resolution. including (red solid) and excluding (blue dash-dotted) the photon po- = We consider the quantum radiation-dominated regime, larization. The green dashed curve shows the relative deviation + χ ≡ / ε Inc.Pol. − / ε Exc.Pol. / / ε Exc.Pol. which requires a large nonlinear QED parameter e [(dN+ d + ) (dN+ d + ) ] (dN+ d + ) . The laser ν 3 |e| −(Fμν p )2/m 1 (for electrons and positrons) [8] and and electron beam parameters are given in the text [in the paragraph ≡ α χ beginning below Eq. (1)]. R a0 e 1[73]. Significant BW pair production requires ν 2 3 the nonlinear QED parameter χγ ≡|e| −(Fμν kγ ) /m 1 (for γ photons) [8,74]. Here, E0 and ω0 are the laser field Compton scattering are, in general, polarized (the LCFA amplitude and frequency, respectively, p and kγ are the 4- probabilities have been successfully tested in experiments of momenta of electron (positron) and photon, respectively, e pair production in channeling process, where the strong fields and m are the electron charge and mass, respectively, Fμν is are produced within crystal plane and axises [63–65]). From the field tensor, α is the fine structure constant, and a0 = the latter, one may expect that during the ultrastrong laser- |e|E0/mω is the invariant laser field parameter. Relativistic plasma interaction the polarization of intermediate particles units with c = h¯ = 1 are used throughout. will strongly influence the pair production process. There- In our Monte Carlo method, we treat spin-resolved elec- fore, we underline that for the quantitative correct predic- tron dynamics semiclassically and photon emission and pair tions of pair production yields in laser-plasma interaction, production quantum mechanically in LCFA [8,74–76], valid the polarization-resolved treatment of intermediate particles at a0 1 (for the emitted photon energies in the region χ / 3 = ω − = ε − is necessary. k− e a0 p−, with k− kz and p− pz being the In this Rapid Communication, the BW pair production photon and electron light-cone energies, respectively [77]). process in a realistic laser-electron beam interaction setup is At each simulation step, the photon emission is calculated investigated in the quantum radiation-dominated regime. An following the common algorithms [78–80] and the photon unpolarized ultrarelativistic electron beam is considered to polarization following the Monte Carlo algorithm [49]. The head-on collide with an ultrastrong LP tightly focused laser photon Stokes parameters (ξ1, ξ2, ξ3) are defined with respect pulse, which results in radiation of highly LP high-energy γ to the axes eˆ1 = aˆ − vˆ(vˆaˆ ) and eˆ2 = vˆ × aˆ [81], with the photons via nonlinear Compton scattering.
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