Extended locally monochromatic approximations of Strong-Field QED processes

TL;DR

This paper revisits and refines the locally monochromatic approximation in strong-field QED, introducing LMA$^+$ to better account for finite bandwidth effects and improve agreement with full predictions.

Contribution

It explicitly derives LMA rates for arbitrary polarizations and introduces LMA$^+$, restoring bandwidth effects and enhancing accuracy over standard LMA.

Findings

LMA$^+$ better matches full SFQED predictions than LMA.

Analytical formulas for angular-integrated and differential probabilities.

New limitations on the applicability of locally monochromatic approximations.

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 time scales, neglection 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. The bandwidth-restored result we refer to 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 a new 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.