Anders, John KennethJohn KennethAndersBeck, Hans PeterHans PeterBeck0000-0001-7212-1096Castro, N FN FCastroChatterjee, MeghranjanaMeghranjanaChatterjeeEreditato, AntonioAntonioEreditatoFranconi, LauraLauraFranconiHalser, LeaLeaHalserHaug, SigveSigveHaug0000-0003-0442-3361Ilg, ArminArminIlgMüller, RomanRomanMüller0000-0002-5835-0690O'Neill, Aaron PaulAaron PaulO'NeillOnofre, AAOnofreWeber, MichaelMichaelWeber0000-0002-2770-90312024-10-112024-10-112022https://boris-portal.unibe.ch/handle/20.500.12422/87134This 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.en500 - Science::530 - Physicsarticle10.48350/1725163601863810.1103/PhysRevLett.129.061803