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

This Letter reports the observation of $WZ\gamma$ production and a measurement of its cross section using $140.1\pm 1.2{\mathrm{fb}}^{-1}$ 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 $WZ\gamma$ production cross section, with both the $W$ and $Z$ bosons decaying leptonically, $pp\to WZ\gamma \to {{\ell }^{\prime }}^{\pm }\nu {\ell }^{+}{\ell }^{-}\gamma$ (${\ell }^{{(}^{\prime })}=e$, $\mu$), 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 $2.01\pm 0.30\left(\mathrm{stat}\right)\pm 0.16\left(\mathrm{syst}\right)\mathrm{fb}$. 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 $1.50\pm 0.06\mathrm{fb}$. The observed significance of the $WZ\gamma$ signal is $6.3\sigma$, compared with an expected significance of $5.0\sigma$.