Observation of $WWW$ Production in $pp$ Collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS Detector

This Letter reports the observation of $WWW$ production and a measurement of its cross section using $139\text{}\text{}{\mathrm{fb}}^{-1}$ 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 $820±100\text{}\left(\mathrm{stat}\right)±80\text{}\left(\mathrm{syst}\right)\text{}\text{}\mathrm{fb}$, approximately 2.6 standard deviations from the predicted cross section of $511±18\text{}\text{}\mathrm{fb}$ calculated at next-to-leading-order QCD and leading-order electroweak accuracy.