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

Evidence for light-by-light (LbL) scattering, $\gamma\gamma\to\gamma\gamma$, 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 $E_T^{\gamma}>2$ GeV, pseudorapidity $|\eta^{\gamma}|<2.4$; and diphoton invariant mass $m^{\gamma\gamma}>5$ GeV, transverse momentum $p_{T}^{\gamma\gamma}<1$ GeV, and acoplanarity $(1-\Delta \phi^{\gamma\gamma}/\pi)<0.01$. After all selection criteria, 14 events are observed, compared to $11.1 \pm 1.1$ (theo) and $3.8 \pm 1.3$ (stat) events expected for signal and background processes respectively. The significance of the signal excess over the background-only hypothesis is $4.1\sigma$. The measured fiducial LbL scattering cross section, $\sigma_{fid}(\gamma\gamma\to\gamma\gamma) = 122 \pm 46$ (stat) $\pm 29$ (syst) $\pm 4$ (theo) nb is consistent with the standard model prediction.