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

Understand

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.