A measurement of the polarisation of W bosons produced in top-quark decays is presented, using proton–proton collision data at a centre-of-mass energy of √s = 13 TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139 fb−1. The measurement is performed selecting tt¯ events decaying into final states with two charged leptons (electrons or muons) and at least two b-tagged jets. The polarisation is extracted from the differential cross-section distribution of the cos θ ∗ variable, where θ ∗ is the angle between the momentum direction of the charged lepton from the W boson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the W boson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the cos θ ∗ angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be f0 = 0.684 ± 0.005 (stat.) ± 0.014 (syst.), fL = 0.318 ± 0.003 (stat.) ± 0.008 (syst.) and fR = −0.002 ± 0.002 (stat.) ± 0.014 (syst.), in agreement with the Standard Model prediction.

41 pages in total, author list starting page 23, 3 figures, 3 tables, published in PLB. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-28/

The data for this manuscript are not available. The values in the plots and tables associated to this article are stored in HEPDATA (https://hepdata.cedar.ac.uk

Peer reviewed