Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV

Abstract

Evidence for the light-by-light scattering process, $\gamma \gamma$ $\to$ $\gamma \gamma$, 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 $\mu$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 E${}_{\mathrm{T}}^{\gamma }$ $>$ 2 GeV, pseudorapidity $|{\eta }^{\gamma }|$ $<$ 2.4, diphoton invariant mass $m^{\gamma \gamma }$ $>$ 5 GeV, diphoton transverse momentum $p_{\mathrm{T}}^{\gamma \gamma }$ $<$ 1 GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 9.0 $\pm$ 0.9 (theo) events for the signal and 4.0 $\pm$ 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, ${\sigma }_{\mathrm{fid}}(\gamma \gamma$ $\to$ $\gamma \gamma )=$ 120 $\pm$ 46 (stat) $\pm$ 28 (syst) $\pm$ 12 (theo) nb, is consistent with the standard model prediction. The $m^{\gamma \gamma }$ distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles, via the $\gamma \gamma$ $\to$ a $\to$ $\gamma \gamma$ process, in the mass range $m_{\mathrm{a}}=$ 5-90 GeV.

Replaced with the published version. Added the journal reference. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/FSQ-16-012 (CMS Public Pages)

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