Observation of Wγγ triboson production in proton-proton collisions at sqrt(s) = 13 TeV with the ATLAS detector

This letter reports the observation of W(lnu) 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(enu) gamma gamma and W(munu) gamma gamma events is measured to be sigma_fid = 13.8 +- 1.1(stat) +2.1-2.0(syst) +- 0.1(lumi) fb, in agreement with the Standard Model prediction.