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

Abstract

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 (Pb) ions. Using 480 µb −1 of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, the ATLAS Collaboration reports evidence for the γγ → γγ reaction. A total of 13 candidate events are 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 > 3 GeV, photon absolute pseudorapidity |η| < 2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with Standard Model predictions