Light-by-light scattering (γγ → γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead ions. Using 480 μ1of lead-lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) → Pb(∗)+ Pb(∗)γγ,for photon transverse energy ET> 3GeV, photon absolute pseudorapidity |η| <2.4, diphoton invariant mass greater than 6GeV, diphoton transverse momentum lower than 2GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ±17 (syst.) nb, which is in agreement with the standard model predictions.

Aaboud, M., Aad, G., Abbott, B., Abdallah, J., Abdinov, O., Abeloos, B., et al. (2017). Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC. NATURE PHYSICS, 13(9), 852-858 [10.1038/nphys4208].

Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC

Ceradini, F.;Di Micco, B.;Di Nardo, R.;Orestano, D.;Petrucci, F.;Puddu, D.;Salamanna, G.;