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.