Approximating higher-order nonlinear QED processes with first-order building blocks

TL;DR

This paper introduces a new method to approximate complex higher-order QED processes in moderate laser intensities by combining first-order processes, accounting for spin effects, thus extending the applicability beyond the locally constant field approximation.

Contribution

The paper presents a novel 'gluing' approach that estimates higher-order QED processes using sequences of first-order processes with spin considerations, applicable at moderate intensities.

Findings

The 'gluing' method accurately estimates higher-order processes at moderate intensities.

Spin and polarization effects are crucial in the new approximation.

The approach extends the regime where first-order process sequences are valid.

Abstract

Higher-order tree-level processes in strong laser fields, i.e. cascades, are in general extremely difficult to calculate, but in some regimes the dominant contribution comes from a sequence of first-order processes, i.e. nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. At high intensity the field can be treated as locally constant, which is the basis for standard particle-in-cell codes. However, the locally-constant-field (LCF) approximation and these particle-in-cell codes cannot be used when the intensity is only moderately high, which is a regime that is experimentally relevant. We have shown that one can still use a sequence of first-order processes to estimate higher orders at moderate intensities provided the field is sufficiently long. An important aspect of our new "gluing" approach is the role of the spin/polarization of intermediate particles, which is more nontrivial compared to the LCF regime.