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Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at √sNN = 5.02 TeV
Sirunyan, A.M. Sanders, Stephen J. The CMS Collaboration
Sirunyan, A.M.
Sanders, Stephen J.
The CMS Collaboration
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Abstract
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 Eγ T > 2 GeV, pseudorapidity |ηγ | < 2.4, diphoton invariant mass mγγ > 5 GeV, diphoton transverse momentum pγγ T < 1 GeV, 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 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 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.
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This work is licensed under a Creative Commons Attribution 4.0 International License.
Date
2019-10-10
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Elsevier
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Keywords
Light-by-light, CMS, UPC, Photoproduction, PbPb
Citation
A.M. Sirunyan et al., "Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at sNN=5.02TeV", Physics Letters B, Volume 797, 2019, 134826, ISSN 0370-2693, https://doi.org/10.1016/j.physletb.2019.134826.
(http://www.sciencedirect.com/science/article/pii/S0370269319305404)