ATLAS evidence for light-by-light scattering in 2017 (2015 collision data)

The ATLAS experiment used lead ion collisions at the Large Hadron Collider to study the vacuum QED process. In an ultra-peripheral collision, the nuclei pass without an ordinary nuclear collision; their strong electromagnetic fields supply nearly real photons. Selecting two nearly back-to-back photons with little other detector activity isolated the rare channel. The 2017 result used $480\mu {\mathrm{b}}^{-1}$ at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ and found 13 candidates against an expected background of $2.6\pm 0.7$. This was evidence, with a reported significance of $4.4$ standard deviations.^[ref][ref]