Measurement of the cross-section of the electroweak production of a $Z \gamma$ pair in association with two jets in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

This note presents the measurement of the cross-section of the electroweak production of a $Z \gamma$ pair in association with two jets. The analysis uses 139 fb$^{-1}$ of LHC proton-proton collision data taken at $\sqrt{s}$=13 TeV recorded by the ATLAS experiment during the years 2015-2018. Events with a $Z$ boson candidate decaying into either an $e^+e^-$ or $\mu^+ \mu^-$ pair, a photon and two jets are selected. The electroweak component is extracted by requiring a large dijet mass and a large rapidity gap between the two jets and is measured with an observed and expected significance well above five standard deviations. The fiducial $pp \rightarrow Z \gamma jj$ cross-section for the electroweak production is measured to be 4.49 $\pm$ 0.58 fb. The total fiducial cross-section that also includes contributions where the jets arise from strong interactions is measured to be 20.6$^{+1.4}_{-1.2}$ fb. The results are consistent with the Standard Model predictions.