Measurement of \[W^{\pm }Z\] production cross sections and gauge boson polarisation in pp collisions at \[\sqrt{s} = 13~\text {TeV}\] with the ATLAS detector

This paper presents measurements of \[W^{\pm }Z\] production cross sections in pp collisions at a centre-of-mass energy of 13 \[\text {TeV}\]. The data were collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of \[36.1~\hbox {fb}^{-1}\]. The \[W^{\pm }Z\] candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is \[\sigma _{W^\pm Z \rightarrow \ell ^{'} \nu \ell \ell }^{\text {fid.}} = 63.7 \, \pm ~1.0~\text {(stat.)} \, \pm ~2.3~\text {(syst.)} \, \pm ~1.4~\text {(lumi.)}\] fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of \[61.5^{+1.4}_{-1.3}\] fb. Cross sections for \[W^+Z\] and \[W^-Z\] production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.