= ATLAS evidence for light-by-light scattering in 2017
{c}
{title2=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 $2.6\pm0.7$. This was evidence, with a reported significance of $4.4$ <standard deviations>.<Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC by ATLAS Collaboration>[Abstract: sample size, collision energy, candidate count, background and fiducial photon selection]{ref}https://atlas.cern/updates/press-statement/atlas-sees-first-direct-evidence-light-light-scattering-high-energy[14 August 2017 statement: significance and dedicated two-photon trigger with otherwise little detector activity]{ref}
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