Observation of $WZγ$ Production in $pp$ Collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

Abstract

This Letter reports the observation of $WZγ$ production and a measurement of its cross-section using 140.1 $\pm$ 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 $WZγ$ production cross-section, with both the $W$ and $Z$ bosons decaying leptonically, $pp \rightarrow WZγ\rightarrow {\ell'}^{\pm}ν\ell^{+}\ell^{-}γ$ ($\ell^{(')} = e, μ$), 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 $\pm$ 0.30 (stat.) $\pm$ 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 $\pm$ 0.06 fb. The observed significance of the $WZγ$ signal is 6.3$σ$, compared with an expected significance of 5.0$σ$.