Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at sqrt(s[NN]) = 5.02 TeVстатья
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Дата последнего поиска статьи во внешних источниках: 11 ноября 2019 г.
Аннотация:Evidence for the light-by-light scattering process, 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 EγT> 2 GeV, pseudorapidity |ηγ|< 2.4, diphoton invariant mass mγγ> 5 GeV, diphoton transverse momentum pγγT< 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 (theo) events for the signal and 4.0 ± 1.2 (stat) for the background processes. The excess observed in data relative to the background-only expectation corresponds to a significance of 4.1 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) ± 4 (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.