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 f₀ = 0.684 ± 0.005(stat.) ± 0.014(syst.), f_L = 0.318 ± 0.003(stat.) ± 0.008(syst.) and f_R = −0.002 ± 0.002(stat.) ± 0.014(syst.), in agreement with the Standard Model prediction.