Evidence for light-by-light scattering in ultraperipheral PbPb collisions at root S-NN=5.02 TeV

Abstract

Evidence for light-by-light (LbL) scattering, , in ultraperipheral PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV is reported. LbL scattering processes are selected in events with just two photons produced, with transverse energy , pseudorapidity ; and diphoton invariant mass , transverse momentum , and acoplanarity . After all selection criteria, 14 events are observed, compared to and events expected for signal and background processes respectively. The significance of the signal excess over the background-only hypothesis is 4.1σ. The measured fiducial LbL scattering cross section, nb is consistent with the standard model prediction.