# Absorption cross section in an intense plane wave background

## TL;DR

This paper investigates photon absorption by electrons in intense laser fields, analyzing gauge invariance, developing a local constant field approximation, and examining effects of pulse shape and infrared limits.

## Contribution

It introduces a new LCFA for absorption in intense fields and explores its validity, along with gauge invariance and pulse shape effects.

## Key findings

- LCFA accurately approximates absorption in strong fields
- Pulse shape significantly affects absorption rates
- Infra-red and collinear limits are characterized

## Abstract

We consider the absorption of probe photons by electrons in the presence of an intense, pulsed, background field. Our analysis reveals an interplay between regularisation and gauge invariance which distinguishes absorption from its crossing-symmetric processes, as well as a physical interpretation of absorption in terms of degenerate processes in the weak field limit. In the strong field limit we develop a locally constant field approximation (LCFA) for absorption which also exhibits new features. We benchmark the LCFA against exact analytical calculations and explore its regime of validity. Pulse shape effects are also investigated, as well as infra-red and collinear limits of the absorption process.