Observation of 𝑊𝑍𝛾 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 $\sqrt{s}$ =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.