Observation of $ WZ\gamma $ production and constraints on new physics scenarios in proton-proton collisions at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$

A measurement of the $ WZ\gamma $ triboson production cross section is presented. The analysis is based on a data sample of proton-proton collisions at a center-of-mass energy of $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of $ 138\text{}\text{}{\mathrm{fb}}^{-1}$. The analysis focuses on the final state with three charged leptons, $ {\ell }^{\pm }\nu {\ell }^{+}{\ell }^{-}$, where $ \ell =e$ or $ \mu $, accompanied by an additional photon. The observed (expected) significance of the $ WZ\gamma $ signal is 5.4 (3.8) standard deviations. The cross section is measured in a fiducial region, where events with an ℓ originating from a tau lepton decay are excluded, to be $ 5.48\pm 1.11\text{}\text{}\mathrm{fb}$, which is compatible with the prediction of $ 3.69\pm 0.24\text{}\text{}\mathrm{fb}$ at next-to-leading order in quantum chromodynamics. Exclusion limits are set on anomalous quartic gauge couplings and on the production cross sections of massive axionlike particles.