Measurement of pure electroweak production of a Z boson in association with two forward/backward jets in proton-proton collisions at 8 TeV

The measurement of the electroweak production cross section of a Z-boson with two foward/backward jets (EWK Zjj) in proton-proton collisions at $\sqrt{s} = 8$ TeV is presented based on a data sample recorded by the CMS experiment at the LHC with an integrated luminosity of 19.7 fb$^{-1}$. Different methods are used to extract the signal, including a data-driven approach that models the QCD Zjj background based on $\gamma$jj events. The cross section for this process is measured in di-electron and di-muon final states in the kinematic region $m_{\ell\ell} >50$ GeV, $m_{\rm jj} >120$ GeV, transverse momenta $p_{{\rm T}j} > 25$ GeV and pseudorapidity $\lvert\eta_j\rvert< 5$. The measurement, combining different methods and channels, yields $\sigma=226 \pm 26_{\rm stat} \pm 35_{\rm syst}~~{\rm fb}$, in agreement with the theoretical cross section. The hadronic activity in events with Z-boson production in association with jets is also studied, in particular in the rapidity interval between the jets.