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

The ATLAS collaboration
Phys.Rev.Lett. 123 (2019) 052001, 2019.

This letter describes the observation of the light-by-light scattering process, $\gamma\gamma\rightarrow\gamma\gamma$, 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 $E_{\textrm{T}}^{\gamma} > 3$ GeV and pseudorapidity $|\eta_{\gamma}| < 2.37$, 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.

  • Acoplanarity

    Data from arXiv:1904.03536 Figure 2a


    The diphoton acoplanarity A$_{\phi}$ distribution for events satisfying the signal selection, but before the A$_{\phi} < 0.01$ requirement. Data points...

  • pt

    Data from arXiv:1904.03536 Figure 2c


    Diphoton transverse momentum for events satisfying the signal selection. Data (points) are compared with the sum of signal and background...

  • crossection

    Example location


    Fiducial cross section for light-by-light scattering

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