Measurement of the polarisation of W bosons produced in top-quark decays using dilepton events at √s = 13 TeV with the ATLAS experiment

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 ƒ₀ = 0.684 ± 0.005 (stat.) ± (syst.), ƒ _L = 0.318 ± 0.003 (stat.) ± 0.008 (syst.) and ƒ_R -0.002 ± 0.002 (stat.) ± 0.014 (syst.), in agreement the Standard Model Prediction