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

This note reports the observation of photon-induced production of $W$-boson pairs, $\gamma\gamma\rightarrow WW$. The analysis uses 139~fb$^{-1}$ of proton--proton collision data taken at $\sqrt{s}=13$~TeV recorded by the ATLAS experiment during the years 2015-2018. The measurement is performed in the $e^\pm\nu\mu^\mp\nu$ final state. The background-only hypothesis is rejected with a significance of 8.4 standard deviations as was expected from MC simulation. A cross-section for the \yyWW\ process of \ensuremath{3.13\pm0.31\textrm{(stat.)}\pm0.28\textrm{(syst.)}}~fb is measured in a fiducial volume close to the acceptance of the detector, requiring an electron and a muon of opposite sign with large dilepton transverse momentum and exactly zero additional charged particles.