Observation of heavy triboson production in leptonic final states in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$

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