Observation of photon-induced $W^{+}{W}^{-}$ production in $pp$ collisions at $\sqrt{s}=13$ TeV using the ATLAS detector

This letter reports the observation of photon-induced production of $W$-boson pairs, $\gamma \gamma \to WW$. The analysis uses 139 fb${}^{-1}$ of LHC proton-proton collision data taken at $\sqrt{s}=13$ TeV recorded by the ATLAS experiment during the years 2015-2018. The measurement is performed selecting one electron and one muon, corresponding to the decay of the diboson system as $WW\to e^{\pm }\nu {\mu }^{\mp }\nu$ final state. The background-only hypothesis is rejected with a significance of well above 5 standard deviations consistent with the expectation from Monte Carlo simulation. A cross section for the $\gamma \gamma \to WW$ process of 3.13$\pm$0.31(stat.)$\pm$0.28(syst.) fb is measured in a fiducial volume close to the acceptance of the detector, by requiring an electron and a muon of opposite signs with large dilepton transverse momentum and exactly zero additional charged particles. This is found to be in agreement with the Standard Model prediction.