激光驱动的高强度 $γ$-光子束的出现，为设计先进的光子-光子对撞机开辟了新的机遇。此类对撞机有望在单次发射中产生大量线性 Breit-Wheeler（LBW）正负电子对，从而为研究极化 LBW 过程提供了独特平台。在我们近期的工作中 [Phys. Rev. D 105, L071902(2022)]，我们研究了 CP $γ$-光子碰撞中 LBW 正负电子对产生的极化特性。为了全面澄清同时涉及 CP 和 LP $γ$-光子的极化效应，本文进一步利用具有显式方位角依赖性的极化截面来研究 LBW 过程，该方位角依赖性来源于光子极化矢量的基旋转。我们通过为正电子和电子定义新的自旋基来实现这一目标，该自旋基使我们能够解耦 $e^\pm$ 的横向和纵向自旋分量。通过解析计算和 Monte Carlo 模拟，我们发现光子的线极化能够诱导出高度依赖角度的正负电子对产额和极化分布。对极化 LBW 过程的全面认识也将为研究高阶光子-光子散射、激光驱动的量子电动力学等离子体以及高能天体物理开辟途径。
英文摘要：
The advent of laser-driven high-intensity $γ$-photon beams has opened up new opportunities for designing advanced photon-photon colliders. Such colliders have the potential to produce a large yield of linear Breit-Wheeler (LBW) pairs in a single shot, which offers a unique platform for studying the polarized LBW process. In our recent work [Phys. Rev. D 105, L071902(2022)], we investigated the polarization characteristics of LBW pair production in CP $γ$-photon collisions. To fully clarify the polarization effects involving both CP and LP $γ$-photons, here we further investigate the LBW process using the polarized cross section with explicit azimuthal-angle dependence due to the base rotation of photon polarization vectors. We accomplished this by defining a new spin basis for positrons and electrons, which enables us to decouple the transverse and longitudinal spin components of $e^\pm$. By means of analytical calculations and Monte Carlo simulations, we find that the linear polarization of photon can induce the highly angle-dependent pair yield and polarization distributions. The comprehensive knowledge of the polarized LBW process will also open up avenues for investigating the higher-order photon-photon scattering, the laser-driven quantum electrodynamic plasmas and the high-energy astrophysics.
Quantum electrodynamics predicts that the quantum vacuum is birefringent, but due to the very small cross-section this is yet to be confirmed by experiment. Vacuum birefringence arises as the elastic part of photon-photon scattering; the inelastic part is Breit-Wheeler pair-production. We outline how measurements of the photon-polarised nonlinear Breit-Wheeler process can be used to infer a measurement of nonlinear vacuum birefringence. As an example scenario, we calculate the accuracy of such a measurement for parameters anticipated at upcoming laser-particle experiments.
A possible setup for the experimental verification of linear Breit-Wheeler pair creation of electrons and positrons in photon-photon collisions is studied theoretically. It combines highly energetic bremsstrahlung photons, which are assumed to be generated by an incident beam of GeV electrons penetrating through a high-$Z$ target, with keV photons from an X-ray laser field, which is described as a focused Gaussian pulse. We discuss the dependencies of the pair yields on the incident electron energy, target thickness, laser parameters, and collision geometry. It is shown that, for suitable cond…
We investigate the optimization of the photon polarization to increase the yield of the Breit-Wheeler pair production in arbitrarily polarized plane wave backgrounds. We show that the optimized photon polarization can improve the positron yield by more than $20\%$ compared to the unpolarized case, in the intensity regime of current laser-particle experiments. The seed photon's optimal polarization is resulting from the polarization coupling with the polarization of the laser pulse. The compact expressions of the coupling coefficients in both the perturbative and nonperturbative regime are give…
We study the non-linear Breit-Wheeler process $\vec γ' + \vec L \to e^+ + e^-$ in the interaction of linearly polarized probe photons ($\vec γ'$) with a linearly polarized laser beam ($\vec L$). In particular, we consider the asymmetry of the total cross section and the azimuthal electron distributions when the polarizations of the photon and laser beams in the initial state are mutually perpendicular or parallel. Considering intense laser beams and the strong field asymptotic we explore essentially the multi-photon dynamics. The asymmetry exhibits some non-monotonic behavior depending on init…
Fully polarized nonlinear Breit-Wheeler pair production from a beam of polarized seed photons is investigated in pulsed plane-wave backgrounds. The particle (electron and positron) spin and photon polarization are comprehensively described with the theory of the density matrix. The compact expressions for the energy spectrum and spin polarization of the produced particles, depending on the initial polarization of the seed photon beam, are derived and discussed in both linearly and circularly polarized laser backgrounds. The numerical results suggest an appreciable improvement of the positron y…
The Breit-Wheeler pair production process in $2+1$ dimensional spacetime is investigated. In the perturbative regime, non-vanishing rates at the energy threshold are found when odd numbers of photons take part on the reaction. This behaviour is understood as a direct consequence of the reduced dimensionality and resembles a corresponding prediction made in gapped graphene monolayers. In the non-perturbative strong field regime the effect of the dimensionality manifests itself in a different rate dependence on the quantum non-linearity parameter. The consequence of this deviation is discussed b…
Photoproduction of electron-positron pairs by bremsstrahlung gamma-rays propagating through a high-intensity optical laser wave is studied. By calculating total pair production yields as well as energy and angular distributions of the produced particles for a range of laser intensities, we analyze the influence that the broad frequency spectrum of bremsstrahlung exerts on the properties of the process in the transition region between the perturbative and non-perturbative interaction regimes. In particular, we demonstrate a continuous shifting of the mainly contributing interval of bremsstrahlu…
Electron-positron pairs are produced through the Breit-Wheeler process when energetic photons traverse electromagnetic fields of sufficient strength. Here we consider a possible experimental geometry for observation of pair creation in the highly nonlinear regime, in which bremsstrahlung of an ultrarelativistic electron beam in a high-$Z$ target is used to produce $γ$ rays that collide with a counterpropagating laser pulse. We show how the target thickness may be chosen to optimize the yield of Breit-Wheeler positrons, and verify our analytical predictions with simulations of the cascade in th…
Laser-driven brilliant controllable polarized $γ$-photon sources open the way for designing compact $γγ$ collider, which enable the large yield of linear Breit-Wheeler (LBW) pairs in a single shot and thus provide an opportunity for the investigation of polarized LBW process. In this work we investigate the polarization characteristics of LBW pair production via our developed spin-resolved binary collision simulation method. Polarization of $γ$-photons modifies the kinematics of scattering particles and induces the correlated energy-angle shift of LBW pairs, and the latter's polarization chara…
The non-linear (strong-field) Breit-Wheeler $e^+e^-$ pair production by a probe photon traversing two consecutive short and ultra short (sub-cycle) laser pulses is considered within a QED framework. The temporal shape of the pulses and the distance between them are essential for the differential cross section as a function of the azimuthal angle distribution of the outgoing electron (positron). The found effect of a pronounced azimuthal anisotropy is important for sub-cycle pulses and decreases rapidly with increasing width of the individual pulses.
Ultra-relativistic heavy-ion collisions are expected to produce the strongest electromagnetic fields in the known Universe. These highly-Lorentz contracted fields can manifest themselves as linearly polarized quasi-real photons that can interact via the Breit-Wheeler process to produce lepton anti-lepton pairs. The energy and momentum distribution of the produced dileptons carry information about the strength and spatial distribution of the colliding fields. Recently it has been demonstrated that photons from these fields can interact even in heavy-ion collisions with hadronic overlap, providi…
In this paper we derive and discuss the completely spin- and photon-polarisation dependent probability rates for nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. The locally constant field approximation, which is essential for applications in plasma-QED simulation codes, is rigorously derived from the strong-field QED matrix elements in the Furry picture for a general plane-wave background field. We discuss important polarisation correlation effects in the spectra of both processes. Asymptotic limits for both small and large values of $χ$ are derived and their spin/pol…
Worldline instantons have previously been used to study the probability of Schwinger pair production (both the exponential and pre-exponential parts) and photon-stimulated pair production (the exponential part). Previous studies obtained the pair-production probability on the probability level by using unitarity, i.e. the imaginary part of the effective action for Schwinger pair production or the imaginary part of the polarization tensor for photon-stimulated pair production. The corresponding instantons are closed loops in the complex plane. Here we show how to use instantons on the amplitude…
The effect of the pulse envelope on electron-positron pair creation in a circularly-polarised laser pulse is investigated. Interference on the length scale of the pulse envelope, and smoothness of the pulse edges are found to influence the pair spectrum. A toy model of a flat-top pulse is used to identify pulse envelope effects inaccessible to local approaches. Broadening of channel openings and a widening of the energy and transverse momentum distribution of the pair are found to receive contributions that are below the local harmonic threshold. By comparing pair yields in a flat-top, sine-sq…
Experimental efforts toward the detection of the nonperturbative strong-field regime of the Breit-Wheeler pair creation process plan to combine incoherent sources of GeV $γ$ quanta and the coherent fields of tightly focussed optical laser pulses. This endeavour calls for a theoretical understanding of how the pair yields depend on the applied laser field profile. We provide estimates for the number of produced pairs in a setup where the high-energy radiation is generated via bremsstrahlung. Attention is paid to the role of the transversal and longitudinal focussing of the laser field, along wi…
When a photon collides with a laser pulse, an electron-positron pair can be produced via the nonlinear Breit-Wheeler process. A simulation framework has been developed to calculate this process, which is based on a ponderomotive approach that includes strong-field quantum electrodynamical effects via the locally monochromatic approximation (LMA). Here we compare simulation predictions for a variety of observables, in different physical regimes, with numerical evaluation of exact analytical results from theory. For the case of a focussed laser background, we also compare simulation with a high-…
Particles in quantum vortex states (QVS) carrying definite orbital angular momenta (OAM) brings new perspectives in various fundamental interaction processes. When unique properties arise in the QVS, understanding how OAM manifest itself between initial particles and the outcome in vortex particle collisions becomes essential. This is made possible by applying the complete vortex description for all involved particles such that angular momenta (AM) are represented by explicit quantum numbers and their connections are naturally retrieved. We demonstrate the full-vortex quantum-electrodynamics (…
Nonlinear Breit-Wheeler pair production using polarized photons from inverse Compton scattering
Helmholtz Institute Jena（耶拿亥姆霍兹研究所）；GSI Helmholtzzentrum für Schwerionenforschung GmbH（GSI重离子研究中心有限公司）；Institute of Optics and Quantum Electronics, Friedrich-Schiller-Universität（弗里德里希·席勒大学光学与量子电子学研究所）；Department of Physics, University of Gothenburg（哥德堡大学物理系）
Generation of arbitrarily spin-polarized lepton (here refer in particular to electron and positron) beams has been investigated in the single-shot interaction of high-energy polarized $γ$ photons with an ultraintense asymmetric laser pulse via nonlinear Breit-Wheeler (BW) pair production. We develop a fully spin-resolved semi-classical Monte Carlo method to describe the pair creation and polarization in the local constant field approximation. In nonlinear BW process the polarization of created pairs is simultaneously determined by the polarization of parent $γ$ photons, the polarization and as…
Entering the overcritical regime of nonlinear Breit-Wheeler pair production in collisions of bremsstrahlung $γ$-rays and superintense, tightly focused laser pulses
Comparative study of nonperturbative electron-positron pair production by intense laser fields colliding with either bremsstrahlung $γ$-rays or relativistic ions
Generation of quasi-particle--hole pairs in gapped graphene monolayers in the combined field of two counterpropagating light waves is studied. The process represents an analogue of electron-positron pair production from the QED vacuum by the Breit-Wheeler effect. We show, however, that the two-dimensional structure of graphene causes some striking differences between both scenarios. In particular, contrary to the QED case, it allows for non-zero pair production rates at the energy threshold when the Breit-Wheeler reaction proceeds nonlinearly with absorption of three photons.
We estimate the energy distribution of positrons produced in the interaction of ultra-relativistic electrons with a high-intensity laser beam. The underlying trident process is factorized on the probabilistic level. That is, we deploy a two-step mechanism for the formation of electron-positron pairs. In the first step, a high-energy photon is produced as a result of nonlinear Compton scattering. In the second step, an electron-positron pair is created by the nonlinear (multi-photon) Breit-Wheeler process.
Volkov states are exact solutions of the Dirac equation in the presence of an arbitrary plane wave. Volkov states, as well as free photon states, are not stable in the presence of the background plane-wave field but "decay" as electrons/positrons can emit photons and photons can transform into electron-positron pairs. By using the solutions of the corresponding Schwinger-Dyson equations within the locally-constant field approximation, we compute the probabilities of nonlinear single Compton scattering and nonlinear Breit-Wheeler pair production by including the effects of the decay of electron…
High energy photons can decay to electron-positron pairs via the nonlinear Breit-Wheeler process when colliding with an intense laser pulse. The energy spectrum of the produced particles is broadened because of the variation of their effective mass in the course of the laser pulse. Applying a suitable chirp to the laser pulse can narrow the energy distribution of the generated electrons and positrons. We present a scenario where a high-energy electron beam is collided with a chirped laser pulse to generate a beam of quasi-monoenergetic $γ$-photons, which then decay in a second chirped, UV puls…
In the centre of mass frame, we have studied theoretically the $Z$-boson resonant production in the presence of an intense laser field via the weak process $e^+e^- \to μ^+μ^-$. Dressing the incident particles by a Circularly Polarized laser field (CP-laser field), at the first step, shows that for a given laser field's parameters, the $Z$- boson cross section decreases by several orders of magnitude. We have compared the the Total Cross Section (TCS) obtained by using the scattering matrix method with that given by the Breit-Wigner approach in the presence of a CP-laser field and the results a…
