Photon polarisation in light-by-light scattering: finite size effects

TL;DR

This paper derives a simple formula for photon polarization changes in light-by-light scattering within focused laser fields, highlighting the importance of pulse duration in accurately predicting vacuum birefringence signals.

Contribution

It introduces a straightforward expression for photon helicity and polarization-flip probabilities in arbitrary backgrounds, emphasizing the role of pulse shape and collision geometry.

Findings

Models ignoring pulse duration overestimate signals by up to an order of magnitude.

Flip probability is insensitive to collision angle and pulse fine structure when pulse duration is considered.

Pulse duration significantly affects the accuracy of vacuum birefringence predictions.

Abstract

We derive a simple expression for the photon helicity and polarisation-flip probabilities in arbitrary background fields, in the low energy regime. Taking the background to model a focused laser beam, we study the impact of pulse shape and collision geometry on the probabilities and on ellipticity signals of vacuum birefringence. We find that models which do not account for pulse duration can overestimate all signals in near head-on collisions by up to an order of magnitude. Taking pulse duration into account, the flip probability becomes relatively insensitive to both angular incidence and the fine details of the pulse structure.