Bounding elastic photon-photon scattering at s≈1 MeV using a laser-plasma platform

Abstract

We report on a direct search for elastic photon-photon scattering using x-ray and 𝛾 photons from a laser-plasma based experiment. A 𝛾 photon beam produced by a laser wak­efield accelerator provided a broadband 𝛾 spectrum extending to above 𝐸𝛾=200MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at 𝐸𝑥≈1.4keV, corresponding to an invariant mass of √𝑠=1.22±0.22MeV. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy 𝛾 photon and outputs two lower energy 𝛾 photons. No changes in the 𝛾 photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of 1.5e15 μb. Although far from the QED prediction, this represents the lowest upper limit obtained so far for √𝑠≲1MeV.