Observation of WZ$\gamma$ production in pp collisions $\sqrt(s)=13$ TeV with the ATLAS detector

This letter reports the observation of $WZ\gamma$ production and a measurement of its cross-section using $140 \pm 1.2~\textrm{fb}^{-1}$ 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 $W$ and $Z$ bosons decaying leptonically, $pp \rightarrow WZ\gamma \rightarrow {\ell'}^\pm\nu\ell^{+}\ell^{-}\gamma$ ($\ell^{(')} = e, \mu$), in a fiducial phase space region defined such that the leptons and the photon have high transverse momentum and the photon is isolated, is measured to be $2.01 \pm 0.30~(\textrm{stat.}) \pm 0.16~(\textrm{syst.}) ~\textrm{fb}$. The corresponding Standard Model predicted cross-section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in electroweak coupling constant is $1.50 \pm 0.06~\textrm{fb}$. The observed significance of the \WZy\ signal is $6.3\sigma$, compared with an expected significance of $5.0\sigma$.