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

A measurement of the WZγ 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 s=13 TeV recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb−1. The analysis focuses on the final state with three charged leptons, ℓ±νℓ+ℓ−, where ℓ=e or μ, accompanied by an additional photon. The observed (expected) significance of the WZγ 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±1.11 fb, which is compatible with the prediction of 3.69±0.24 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.