Observation of $W\gamma \gamma$ triboson production in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This letter reports the observation of $W(\mathrm{\ell }\nu )\gamma \gamma$ production in proton-proton collisions. This measurement uses the full Run 2 sample of events recorded at a center-of-mass energy of $\sqrt{s}=13$ TeV by the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 fb${}^{-1}$. Events with a leptonically-decaying $W$ boson and at least two photons are considered. The background-only hypothesis is rejected with an observed and expected significance of $5.6$ standard deviations. The inclusive fiducial production cross section of $W(e\nu )\gamma \gamma$ and $W(\mu \nu )\gamma \gamma$ events is measured to be ${\sigma }_{\mathrm{f}\mathrm{i}\mathrm{d}}=13.8\pm 1.1(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\begin{array}{c}+2.1\\ -2.0\end{array}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\pm 0.1(\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{i})$ fb, in agreement with the Standard Model prediction.