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

Aad, G., Abbott, B., Abbott, D.C., Abeling, K., Abidi, S.H., Aboulhormae, A., et al. (2023). Measurement of the polarisation of Wbosons produced in top-quark decays using dilepton events at √s=13TeV with the ATLAS experiment. PHYSICS LETTERS. SECTION B, 843, 137829-1-137829-22 [10.1016/j.physletb.2023.137829].

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

Bindi, M;Boscherini, D;Carratta, G;Cavalli, N;De Castro, S;Fabbri, F;Fabbri, L;Franchini, M;Gabrielli, A;Massa, L;