Observation of WZγ Production in pp Collisions at √s = 13 TeV with the ATLAS Detector

This Letter reports the observation of 𝑊⁢𝑍⁢𝛾 production and a measurement of its cross section using 140.1 ±1.2  fb−1 of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The 𝑊⁢𝑍⁢𝛾 production cross section, with both the 𝑊 and 𝑍 bosons decaying leptonically, 𝑝⁢𝑝 →𝑊⁢𝑍⁢𝛾 →ℓ′±⁢𝜈⁢ℓ+⁢ℓ−⁢𝛾 (ℓ(′) =𝑒, 𝜇), is measured in a fiducial phase-space region defined such that the leptons and the photon have high transverse momentum and the photon is isolated. The cross section is found to be 2.01 ±0.30⁢(stat) ±0.16⁢(syst)  fb. The corresponding standard model predicted cross section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in the electroweak coupling constant is 1.50±0.06  fb. The observed significance of the 𝑊⁢𝑍⁢𝛾 signal is 6.3⁢𝜎, compared with an expected significance of 5.0⁢𝜎.