Evidence for longitudinally polarised $Z$ bosons in electroweak $W^{\pm}Z$ production in association with two jets from $pp$ collisions at $\sqrt{s} = $ 13 TeV and 13.6 TeV with the ATLAS detector

Abstract

Measurements of polarisation states of $W$ and $Z$ gauge bosons in electroweak $W^{\pm}Z$ production in association with two jets ($WZjj\mathrm{-EW}$) in proton-proton collisions are presented. The data set used corresponds to integrated luminosities of 140 fb$^{-1}$ and 164 fb$^{-1}$ of proton-proton collisions at centre-of-mass energies of 13 TeV and 13.6 TeV, respectively, recorded by the ATLAS detector at the CERN Large Hadron Collider. The $W^{\pm}Z$ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The observed significance for the presence in data of $WZjj\mathrm{-EW}$ events containing a longitudinally polarised $Z$ boson is of 4.0 standard deviations. The measured fraction of such events, integrated over the fiducial region, is $f_{\mathrm{X0}} = 0.32 \pm 0.09$, in agreement with the Standard Model prediction at next-to-leading-order in electroweak corrections. Upper limits at 95% confidence level of 0.19 and 0.12 are set on the fraction of events with a $W$ boson longitudinally polarised and on the fraction of events with the two bosons longitudinally polarised, respectively. The integrated fiducial cross-section per lepton flavour of $WZjj\mathrm{-EW}$ production at $\sqrt{s} = 13.6$ TeV is also measured to be $σ_{WZjj\mathrm{-EW} \rightarrow \ell^{'} ν\ell \ell jj} = 0.374 \; \pm 0.077 (\mathrm{stat.}) \; \pm 0.053 (\mathrm{syst.}) \; \pm 0.009 (\mathrm{lumi.}) \; \mathrm{fb}$, where $\ell$ and $\ell^{'}$ are either an electron or a muon. This measurement is in agreement with the Standard Model expectation.