Observation of Light-by-Light Scattering in Ultraperipheral Pb plus Pb Collisions with the ATLAS Detector
PHYSICAL REVIEW LETTERS
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This Letter describes the observation of the light-by-light scattering process, γ γ → γ γ , in Pb + Pb collisions at √ s NN = 5.02 TeV . The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb − 1 , collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E γ T > 3 GeV and pseudorapidity | η γ | < 2.4 , diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 ± 3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 ± 13 ( stat ) ± 7 ( syst ) ± 3 ( lumi ) nb .
AuthorsAad, G; Abbott, B; Abbott, DC; Abdinov, O; Abud, AA; Abeling, K; Abhayasinghe, DK; Abidi, SH; AbouZeid, OS; Abraham, NL
- Physics and Astronomy 
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