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Extended locally monochromatic approximations of strong-field QED processes

Nikita Larin* and Daniel Seipt

  • *Contact author: nikita.larin@uni-jena.de
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Phys. Rev. A 112, 032819 – Published 29 September, 2025

DOI: https://doi.org/10.1103/cmmp-rt2g

Abstract

Strong-field QED (SFQED) probability rates in the locally monochromatic approximation (LMA) have become an indispensable tool for simulations of processes like gamma-ray emission or electron-positron pair production in laser-particle collisions. We revisit the LMA derivation and explicitly demonstrate that it is based on the separation of timescales, neglect of the long-range interference effects, and subsequent averaging over the cycle scale. Doing so, we obtain unambiguously LMA rates for arbitrary polarizations of the plane-wave background. Additionally, we partially restore the finite bandwidth effects that are lost in the LMA derivation. We refer to the bandwidth-restored result as the LMA+ and show that it agrees with the full SFQED predictions better than the standard LMA. We use LMA+ to address previously inaccessible observables and formulate an additional limitation on the applicability of locally monochromatic approximations in general. We provide analytical results for the angular-integrated LMA+ probability rate and the fully differential probability that account for the finite bandwidth effects.

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Phys. Rev. A 112, 032819– Published 29 September, 2025
  • Received 5 June 2025
  • Accepted 9 September 2025
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