Observation of Light-by-Light Scattering in Ultraperipheral $\mathrm{Pb}+\mathrm{Pb}$ Collisions with the ATLAS Detector

This Letter describes the observation of the light-by-light scattering process, $\gamma \gamma \to \gamma \gamma$, in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. The analysis is conducted using a data sample corresponding to an integrated luminosity of $1.73{\mathrm{nb}}^{-1}$, 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 $E_T^{\gamma }>3\mathrm{GeV}$ and pseudorapidity $|{\eta }_{\gamma }|<2.4$, 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 $12\pm 3$ events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is $78\pm 13(\text{stat})\pm 7(\text{syst})\pm 3(\text{lumi})\mathrm{nb}$.

Figure 1

(a) Measured level-1 trigger efficiency as a function of the reconstructed transverse energy in $\gamma \gamma \to e^{+}{e}^{-}$ events, (b) photon reconstruction efficiency as a function of the photon $E_T$ (approximated with $E_{T,1}^e-p_T^{\mathrm{trk}2}$, where trk2 denotes the track of the second leading electron), and (c) photon particle-identification efficiency as a function of the photon $E_T$.

Figure 2

(a) The diphoton $A_{\varphi }$ distribution for events satisfying the signal selection, but before the $A_{\varphi }<0.01$ requirement. (b) Diphoton invariant mass and (c) diphoton transverse momentum for events satisfying the signal selection. Data (points) are compared with the sum of signal and background expectations (histograms). Systematic uncertainties of the signal and background processes, excluding that of the luminosity, are shown as shaded bands.