Evidence for electroweak production of two jets in association with a $Z\gamma$ pair in $pp$ collisions at $\sqrt{s} = 13~$TeV with the ATLAS detector

Evidence for electroweak production of two jets in association with a $Z\gamma$ pair in $\sqrt{s} = 13~$TeV proton--proton collisions at the Large Hadron Collider is presented. The analysis uses data collected by the ATLAS detector in $2015$ and $2016$ that corresponds to an integrated luminosity of $36.1~\mathrm{fb}^{-1}$. Events containing a leptonically-decaying $Z$-boson candidate (to either electrons or muons), a photon, and two jets are selected. The electroweak component is measured with an observed significance of $4.1$ standard deviations, and an expected significance of $3.8$ standard deviations. The fiducial cross section for electroweak production is measured to be $\sigma_{Z\gamma jj\mathrm{-EW}} = 7.8 \; ^{+ 2.0} _{- 1.9} \; \mathrm{fb}$, in good agreement with the Standard Model prediction.