Measurement of W±Z production cross sections and gauge boson polarisation in pp collisions at √s= 13 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 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 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 σfid.W±Z→ℓ′νℓℓ=63.7± 1.0 (stat.)± 2.3 (syst.)± 1.4 (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.