Evidence for light-by-light scattering in ultraperipheral PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV

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 ETγ>2GeV , pseudorapidity |ηγ|<2.4 ; and diphoton invariant mass mγγ>5GeV , transverse momentum pTγγ<1GeV , 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 4.1 σ . 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.