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

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

Figure 1

Representative Feynman diagrams at LO for the production of three massive $W$ bosons, including diagrams sensitive to triple and quartic gauge couplings.

Figure 2

Postfit BDT output distribution in the $e^{\pm }{e}^{\pm }$ (top left), $e^{\pm }{\mu }^{\pm }$ (top right), ${\mu }^{\pm }{\mu }^{\pm }$ (bottom left), and $3\ell$ (bottom right) channels. The bottom panel of each plot shows the ratio of the data to the total prediction. The uncertainty bands include both the statistical and systematic uncertainties as obtained by the fit. The signal is scaled to the fitted signal strength of 1.61.