Measurement of light-by-light scattering in ultraperipheral PbPb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

A measurement of light-by-light scattering, $\gamma\gamma\to\gamma\gamma$, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of $390~\mu\mathrm{b}^{-1}$ recorded by the CMS experiment at the LHC. Light-by-light scattering processes are selected in events with two photons exclusively produced, each with transverse energy $\mathrm{E}_{\mathrm{T}}^{\gamma}>2~\mathrm{GeV}$, pseudorapidity $|\eta^{\gamma}|<2.4$, diphoton invariant mass $\mathrm{m}^{\gamma\gamma}>5~\mathrm{GeV}$, diphoton transverse momentum $\mathrm{p}_{\mathrm{T}}^{\gamma\gamma}<1~\mathrm{GeV}$, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of $11.1 \pm 1.1$ (th) events for the signal and $3.8 \pm 1.3$ (stat) for the background processes. The significance of the light-by-light signal against the background-only hypothesis is 4.1 standard deviations. The measured fiducial light-by-light scattering cross section, $\sigma_{\mathrm{fid}} (\gamma\gamma\to\gamma\gamma)=122\pm46$ (stat) $\pm29$ (syst) $\pm4$ (th) is consistent with the standard model prediction.