Abstract
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-energy theory approximation. These comparisons are used to quantify the accuracy of the simulation approach in calculating harmonic structure, which appears in the lightfront momentum and angular spectra of outgoing particles, and the transition from multi-photon to all-order pair creation. Calculation of the total yield of pairs over a range of intensity parameters is also used to assess the accuracy of the locally constant field approximation (LCFA).
Highlights
Higher field strengths, the ‘threshold’ number of laser photons required for the photon to decay may not be the most probable channel, and all orders of interaction between the laser pulse and the photon must be taken into account
The particle beams can reach higher energies (13 GeV for E320 and 11.5–16.5 GeV for LUXE), lower emittances, higher repetition rate and energy stability than current laser-wakefield accelerated beams have been achieved. (LUXE will collide the driving electron beam with a solid target to generate a source of bremsstrahlung radiation that will collide with the intense optical laser and form a photon-photon experiment [20,49,58,59,60].) These types of experiments allow
The resulting form, the ‘locally monochromatic approximation’ (LMA), assumes a pulse envelope that varies much slower than the carrier wavelength, and employs a local phase expansion, which includes interference effects between processes taking place within the same laser wavelength. (The slowly varying envelope approximation in the LMA has been used by other authors in first [67,68,69,70], and second-order processes [71].) Unlike the locally constant field approximation’ (LCFA), the LMA is not restricted by its use in a particular intensity or energy regime, but it does assume that the interacting electromagnetic field is well-approximated by a plane electromagnetic wave
Summary
To measure nonlinear Breit–Wheeler pair creation [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] in the ‘all-order’ regime, a sufficiently powerful laser is required: peak intensities > 1022 Wcm−2 are accessible with current laser technology [27,28,29,30]. Typical proposals involve providing a particle beam via laser-wakefield acceleration of electrons, which can produce energies of the order of several GeV [31,32,33,34] This means that, in order for pair creation to be kinematically allowed, many photons are required. Complementary ‘high-energy’ experiments such as E320 at SLAC and LUXE [49] at DESY have been suggested to measure all-order strong-field QED effects such as nonlinear Breit–Wheeler, Compton and the nonlinear trident process [50,51,52,53,54,55,56,57] These experiments will collide particle beams accelerated using conventional radiofrequency cavities, with strong laser pulses. The particle beams can reach higher energies (13 GeV for E320 and 11.5–16.5 GeV for LUXE), lower emittances, higher repetition rate and energy stability than current laser-wakefield accelerated beams have been achieved. (LUXE will collide the driving electron beam with a solid target to generate a source of bremsstrahlung radiation that will collide with the intense optical laser and form a photon-photon experiment [20,49,58,59,60].) These types of experiments allow
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