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

Abstract

A measurement of the polarisation of Wbosons produced in top-quark decays is presented, using proton–proton collision data at a centre-of-mass energy of √s=13TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139fb⁻¹. The measurement is performed selecting t¯tevents 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 cosθ∗variable, where θ∗is the angle between the momentum direction of the charged lepton from the Wboson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the Wboson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the cosθ∗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.