Evidence for light-by-light scattering in ultraperipheral PbPb collisions at √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 E_T^γ > 2 GeV, pseudorapidity |η^γ| < 2.4; and diphoton invariant mass m^(γγ) > 5 GeV, transverse momentum p_T^(γγ) < 1 GeV, 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.

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