Precision WZ cross sections and polarisation at 13 TeV

Measurements of W±Z production in pp collisions at a center-of-mass energy of 13 TeV are presented. 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 into electrons and muons of the gauge bosons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is σfid.W±Z→ℓ′νℓℓ=63.7± 1.0 (stat.)± 2.3 (sys.)± 0.3 (mod.)± 1.5 (lumi.) fb, in agreement with 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 lepton decays of W and Z bosons is performed and integrated helicity fractions in the detector fiducial region are measured for each of the W and Z bosons. The longitudinal helicity state of pair-produced vector bosons is measured in hadronic collisions.