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

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 Image 1 process, in the mass range Image 2.

Physics Letters B, Volume 797, article id. 134826, 2019

 

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Authors: 
A.Al-bataineh, P. Baringer, A. Bean, S. Boren, J. Bowen, A. Bylinkin, J. Castle, S. Khalil, A. Kropivnitskaya, D. Majumder, W. Mcbrayer, M. Murray, C. Rogan, S. Sanders, E. Schmitz, J.D. Tapia Takaki, Q. Wang & CMS Collaboration

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