Speaker
Description
A measurement of light-by-light scattering, γγ → γγ, 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 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 ET > 2 GeV, pseudorapidity |eta| < 2.4, diphoton invariant mass m(gamgam) > 5 GeV, diphoton transverse momentum p T < 1 GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 11.1 ± 1.1 (th) events for the signal and 3.8 ± 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, σ fid ( γγ → γγ ) = 122 ± 46 (stat) ± 29 (syst) ± 4 (th) is consistent with the standard model prediction. The present results allow also to place new competitive limits, reported for the first time, on the production of pseudoscalar axion-like particles, produced in the process γγ → a → γγ, over the mass range m a = 5–50 GeV.
