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

Author(s)
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 5.02TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of 390μb−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 ETγ>2GeV, pseudorapidity |ηγ|<2.4, diphoton invariant mass mγγ>5GeV, diphoton transverse momentum pTγγ<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 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 (Figure presented.) process, in the mass range (Figure presented.).
Date issued
2019-08
URI
https://hdl.handle.net/1721.1/125327
Department
Massachusetts Institute of Technology. Department of Physics; Massachusetts Institute of Technology. Department of Nuclear Science and Engineering; Massachusetts Institute of Technology. Laboratory for Nuclear Science
Journal
Physics Letters B
Publisher
Elsevier BV
Citation
CMS Collaboration (Sirunyan, A.M. et al). "Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at √ s NN = 5.02 TeV." Physics Letters B 797 (October 2019): 134826 © 2019 The Author(s)
Version: Final published version
ISSN
0370-2693

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