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arXiv · 1904.03536

Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector

ATLAS Collaboration
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Abstract

This letter describes the observation of the light-by-light scattering process, $γγ\rightarrowγγ$, in Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 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 $E_{\textrm{T}}^γ > 3$ GeV and pseudorapidity $|η_γ| < 2.37$, 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 $\pm$ 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 $\pm$ 13 (stat.) $\pm$ 7 (syst.) $\pm$ 3 (lumi.) nb.

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ATLAS Collaboration. 2019-08-02. Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector. https://doi.org/10.1103/physrevlett.123.052001

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