Measurement of ${W}^{±}Z$ production cross sections and gauge boson polarisation in pp collisions at $\sqrt{s}=13\phantom{\rule{3.33333pt}{0ex}}\text{TeV}$ with the ATLAS detector

This paper presents measurements of ${W}^{±}Z$ 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 $36.1\phantom{\rule{3.33333pt}{0ex}}{\text{fb}}^{-1}$ . The ${W}^{±}Z$ 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 ${\sigma }_{{W}^{±}Z\to {\ell }^{{}^{\prime }}\nu \ell \ell }^{\text{fid.}}=63.7\phantom{\rule{0.166667em}{0ex}}±\phantom{\rule{3.33333pt}{0ex}}1.0\phantom{\rule{3.33333pt}{0ex}}\text{(stat.)}\phantom{\rule{0.166667em}{0ex}}±\phantom{\rule{3.33333pt}{0ex}}2.3\phantom{\rule{3.33333pt}{0ex}}\text{(syst.)}\phantom{\rule{0.166667em}{0ex}}±\phantom{\rule{3.33333pt}{0ex}}1.4\phantom{\rule{3.33333pt}{0ex}}\text{(lumi.)}$ fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of $61.{5}_{-1.3}^{+1.4}$ 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.