Observation of electroweak $W^{\pm}Z$ boson pair production in association with two jets in pp collisions at $\sqrt{s}$ = 13TeV with the ATLAS Detector

An observation of electroweak $W^{\pm}Z$ production in association with two jets in proton--proton collisions is presented. The data collected by the ATLAS detector at the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of $\sqrt{s}$ = 13TeV are used, corresponding to an integrated luminosity of 36.1 fb$^{-1}$. Events containing three identified leptons, either electrons or muons, and two jets are selected. The electroweak production of $W^{\pm}Z$ bosons in association with two jets is measured with an observed significance of 5.6 standard deviations. A fiducial cross section for electroweak production including interference effects is measured to be $\sigma_{\mathrm{EW}} ( W^{\pm}Zjj \to \ell' \nu \ell\ell jj ) = 0.57 \; ^{+ 0.14} _{- 0.13} \,(\mathrm{stat.}) \; ^{+ 0.05} _{- 0.04} \,(\mathrm{syst.}) \; ^{+ 0.04} _{- 0.03} \,(\mathrm{th.}) \; \mathrm{fb}$. Differential cross sections of $W^{\pm}Zjj$ production for several kinematic observables are also measured.