Evidence for light-by-light scattering in ultraperipheral PbPb collisions at s_NN=5.02TeV

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 E_T ^γ>2GeV, pseudorapidity |η^γ|<2.4; and diphoton invariant mass m^γγ>5GeV, transverse momentum p_T ^γγ<1GeV, and acoplanarity (1−Δϕ^γγ/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, σ_fid(γγ→γγ)=122±46(stat)±29(syst)±4(theo) nb is consistent with the standard model prediction.