Observation of $WWW$ production in $pp$ collisions at $\sqrt{s}$= 13 TeV with the ATLAS detector

This note presents the observation and the cross-section measurement of $WWW$ production 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 electrons or muons in association with at least two jets as well as events with three charged electrons or muons, without any same-flavor opposite-sign lepton pairs, are selected. $WWW$ production is observed with a significance of 8.2 standard deviations, where the expectation is 5.4 standard deviations. The inclusive $WWW$ production cross section is measured to be 850 $\pm$ 100 (stat.) $\pm$ 80 (syst.) fb.