Observation of W Z gamma production and constraints on new physics scenarios in proton-proton collisions at sqrt(s) = 13 TeV

A measurement of the 𝑊⁢𝑍⁢𝛾 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±⁢𝜈⁢ℓ+⁢ℓ−, where ℓ =𝑒 or 𝜇, accompanied by an additional photon. The observed (expected) significance of the 𝑊⁢𝑍⁢𝛾 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.