Measurement of the cross-section for electroweak production of dijets in association with a $Z$ boson in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The cross-section for the production of two jets in association with a leptonically decaying Z boson ($Zjj$) is measured in proton-proton collisions at a centre-of-mass energy of 13 TeV, using data recorded with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb${}^{-1}$. The electroweak $Zjj$ cross-section is extracted in a fiducial region chosen to enhance the electroweak contribution relative to the dominant Drell-Yan $Zjj$ process, which is constrained using a data-driven approach. The measured fiducial electroweak cross-section is $\sigma^{Zjj}_{EW}= 119\pm 16 ($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}$) \pm 20 ($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}$)\pm 2 ($\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{.}$)$fordijetinvariantmassgreaterthan250GeV,and$34.2\pm 5.8 ($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}$)\pm 5.5 ($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}$)\pm 0.7 ($\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{.}$)$ for dijet invariant mass greater than 1 TeV. Standard Model predictions are in agreement with the measurements. The inclusive $Zjj$ cross-section is also measured in six different fiducial regions with varying contributions from electroweak and Drell-Yan $Zjj$ production.