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 wakfield accelerator provided a broadband 𝛾 spectrum extending to above 𝐸𝛾 = 200 MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at 𝐸𝑥 ≈ 1.4 keV, corresponding to an invariant mass of √𝑠 = 1.22 ± 0.22 MeV. 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 1.5 × 1015 μb. Although far from the QED prediction, this represents the lowest upper limit obtained so far for √𝑠 ≲ 1 MeV.