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 $f_{X0}=0.32\pm 0.09$, 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 $\ddot{I}{ƒ}_{WZjj-\mathrm{EW}\to {\ell }^{{}^{\prime }}\hat{I}½\ell \ell jj}=0.374\pm 0.077(stat.)\pm 0.053(syst.)\pm 0.009(lumi.)\mathrm{fb}$, where $\ell$ and ${\ell }^{{}^{\prime }}$ are either an electron or a muon. This measurement is in agreement with the Standard Model expectation.

489pages in total, author list starting page 32, 6 figures, 5 tables, submitted to JHEP. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2024-03/