Abstract
An observation of electroweak W±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-energy of 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±Z bosons in association with two jets is measured with an observed significance of 5.3 standard deviations. A fiducial cross section for electroweak production including interference effects is measured to be σWZjj−EW = 0.57^{+0.14}_{-0.13} (stat.) ^{+0.07}_{-0.06} (syst.) fb. Differential cross section of W±Zjj production for several kinematic observables are also measured. Fiducial phase space definition for the measurement: - pT of electrons and muons from Z0 decays > 15 GeV - pT of electrons and muons from the W± decays > 20 GeV - |η| of muons and electrons < 2.5 - Leptons from the Z0 boson are separated by ΔR(ℓ,ℓ)>0.2 from each other - Leptons from the Z0 and W bosons are separated by ΔR(ℓ,ℓ)>0.3 from each other - |dilepton mass - Z0 mass| < 10 GeV - mT of W± > 30 GeV. - at least 2 jets with opposite |η|. Jets are particle level jets with anti-kt R=0.4. - pT of the two jets > 40 GeV - |η| of the two jets < 4.5 - the first jet is chosen as the leading pT jet - the second jet is chosen among the remaining jets as the jet of highest pT with |η| opposite to the first jets - invariant mass of the two jets > 500 GeV - leptons and jets separated by ΔR(ℓ,jet)>0.3 - processes with a b-quark in the initial state are excluded At particle level, the kinematics of final-state prompt electrons and muons is computed including the contributions from final-state radiated photons within a distance in the (η,ϕ) plane of ΔR=(Δη)2+(Δϕ)2−−−−−−−−−−−−√=0.1 around the direction of the charged lepton. These dressed leptons and the final-state neutrinos that do not originate from hadron or τ decays are associated with the W and Z boson decay products with an algorithmic approach, called ``resonant shape''. This algorithm is based on the value of an estimator expressing the product of the nominal line shapes of the W and Z resonances P=∣∣∣∣1m2(ℓ+,ℓ−)−(mPDGZ)2+iΓPDGZmPDGZ∣∣∣∣2×∣∣∣∣1m2(ℓ′,νℓ′)−(mPDGW)2+iΓPDGWmPDGW∣∣∣∣2 where mPDGZ (mPDGW) and ΓPDGZ (ΓPDGW) are the world average mass and total width of the Z (W) boson, respectively, as reported by the Particle Data Group \cite{Agashe:2014kda}. The input to the estimator is the invariant mass m of all possible pairs (ℓ+,ℓ−) and (ℓ′,νℓ′) satisfying the fiducial selection requirements defined in the next paragraph. The final choice of which leptons are assigned to the W or Z bosons corresponds to the configuration exhibiting the highest value of the estimator.
