Observation of the Production of Three Massive Gauge Bosons at $\sqrt{s}=13\mathrm{TeV}$

The first observation is reported of the combined production of three massive gauge bosons ($VVV$ with $V=W$, $Z$) in proton-proton collisions at a center-of-mass energy of 13 TeV. The analysis is based on a data sample recorded by the CMS experiment at the CERN LHC corresponding to an integrated luminosity of $137{\mathrm{fb}}^{-1}$. The searches for individual $WWW$, $WWZ$, $WZZ$, and $ZZZ$ production are performed in final states with three, four, five, and six leptons (electrons or muons), or with two same-sign leptons plus one or two jets. The observed (expected) significance of the combined $VVV$ production signal is 5.7 (5.9) standard deviations and the corresponding measured cross section relative to the standard model prediction is ${1.02}_{-0.23}^{+0.26}$. The significances of the individual $WWW$ and $WWZ$ production are 3.3 and 3.4 standard deviations, respectively. Measured production cross sections for the individual triboson processes are also reported.

Figure 1

Comparison of the observed numbers of events to the predicted yields after fitting. For the $WWW$ and $WWZ$ channels, the results from the BDT-based selections are used. The $VVV$ signal is shown stacked on top of the total background. The points represent the data and the error bars show the statistical uncertainties. The expected significance $L$ in the middle panel represents the number of standard deviations (s.d.) with which the null hypothesis (no signal) is rejected; it is calculated for the fit for ${\mu }_{\mathrm{comb}}$. The lower panel shows the pulls for the fit result.

Figure 2

Best fit values of the signal strengths for the BDT-based analyses (blue solid circles) and the sequential-cut analyses (black solid diamonds). The error bars represent the total uncertainty. For $ZZZ$ production, a 95% confidence level upper limit is shown. The stated numerical values correspond to the BDT-based analysis.