Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at √sNN=5.02 TeV

Abstract

Evidence for the light-by-light scattering process,γγ→γγ, 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 μb−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 EγT>2GeV, pseudorapidit yηγ|<2.4 , diphoton invariant mass mγγ>5GeV , diphoton transverse momentum pγγT<1GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 9.0±0.9 (theo) events for the signal and4.0±1.2 (stat) for the background processes. The excess observed in data relative to the background-only expectation corresponds to a significance of 3.7 standard deviations, and has properties consistent with those expected for the light-by-light scattering signal. The measured fiducial light-by-light scattering cross section, σfid(γγ→γγ)=120±46 (stat)±28 (syst)±12 (theo) nb, is consistent with the standard model prediction. The mγγ distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles, via the γγ→a→γγ process, in the mass range ma=5–90 GeV.