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

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