Observation of Triboson Production in Proton-proton Collisions at TeV with the ATLAS detector

This letter reports the observation of $W\left(\ell \nu \right)\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\left(e\nu \right)\gamma \gamma $ and $W\left(\mu \nu \right)\gamma \gamma $ events is measured to be ${\sigma }_{\mathrm{fid}}=13.8±1.1\left(\mathrm{stat}\right)\begin{array}{c}+2.1\\ -2.0\end{array}\left(\mathrm{syst}\right)±0.1\left(\mathrm{lumi}\right)$ fb, in agreement with the Standard Model prediction.