ATLAS Collabration2022-09-052022-09-0520220031-9007http://doi.org/10.1103/PhysRevLett.129.061803https://hdl.handle.net/20.500.12713/3114This Letter reports the observation of WWW production and a measurement of its cross section using 139 fb^{-1} of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Events with two same-sign leptons (electrons or muons) and at least two jets, as well as events with three charged leptons, are selected. A multivariate technique is then used to discriminate between signal and background events. Events from WWW production are observed with a significance of 8.0 standard deviations, where the expectation is 5.4 standard deviations. The inclusive WWW production cross section is measured to be 820±100 (stat)±80 (syst) fb, approximately 2.6 standard deviations from the predicted cross section of 511±18 fb calculated at next-to-leading-order QCD and leading-order electroweak accuracy.eninfo:eu-repo/semantics/closedAccessArticle36018638WOS:000842367600009Q110.1103/PhysRevLett.129.061803Q1