Bounding elastic photon-photon scattering at $\sqrt s \approx 1\,$MeV using a laser-plasma platform

TL;DR

This paper reports a laser-plasma experiment searching for elastic photon-photon scattering at around 1 MeV energy, setting new upper bounds on the cross section despite not observing the effect.

Contribution

First direct search for elastic photon-photon scattering at MeV energies using laser-plasma technology, establishing the lowest upper limit for the cross section in this energy range.

Findings

No photon-photon scattering observed within experimental sensitivity.

Placed a 95% upper bound on the scattering cross section of 1.5 x 10^{15} μb.

Demonstrated feasibility of using laser-plasma platforms for high-energy photon interaction studies.

Abstract

We report on a direct search for elastic photon-photon scattering using x-ray and $\gamma$ photons from a laser-plasma based experiment. A gamma photon beam produced by a laser wakefield accelerator provided a broadband gamma spectrum extending to above $E_\gamma = 200$ MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at $E_x \approx 1.4$ keV, corresponding to an invariant mass of $\sqrt{s} = 1.22 \pm 0.22$ MeV. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy $\gamma$ photon and outputs two lower energy $\gamma$ photons. No changes in the $\gamma$ photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of $1.5 \times 10^{15}\,\mu$b. Although far from the QED prediction, this represents the lowest upper limit obtained so far for $\sqrt{s} \lesssim 1$ MeV.