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

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 [Figure presented]. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at [Figure presented], corresponding to an invariant mass of [Figure presented]. 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 [Figure presented]. Although far from the QED prediction, this represents the lowest upper limit obtained so far for [Figure presented].