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, γγ→γγ, 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 EγT>2GeV, pseudorapidity |ηγ|<2.4; and diphoton invariant mass mγγ>5 GeV, transverse momentum pγγT<1 GeV, and acoplanarity (1−Δφγγ/π)<0.01. After all selection criteria, 14 events are observed, compared to 11.1±1.1 (theo) and 3.8±1.3 (stat) 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, σfid(γγ→γγ)=122±46 (stat)±29 (syst)±4 (theo) nb is consistent with the standard model prediction.