Abstract
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.