Observation of photon-induced W + W − production in pp collisions at 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 $3.13\pm 0.31\text{(stat.)}\pm 0.28\text{(syst.)}$ 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.