Measurement of the polarisation of $W$ bosons produced in top-quark decays using dilepton events at $\sqrt{s}=13$ TeV with the ATLAS experiment

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 $\sqrt{s}=13$ 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 $t\bar{t}$ 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 $\cos {\theta }^{\ast }$ variable, where ${\theta }^{\ast }$ 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 $\cos {\theta }^{\ast }$ angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be $f_0=0.684\pm 0.005(stat.)\pm 0.014(syst.)$, $f_{\mathrm{L}}=0.318\pm 0.003(stat.)\pm 0.008(syst.)$ and $f_{\mathrm{R}}=-0.002\pm 0.002(stat.)\pm 0.014(syst.)$, in agreement with the Standard Model prediction.

arXiv comment41 pages in total, author list starting page 23, 3 figures, 3 tables, published in PLB. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-28/

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• Top-Quark Decay at Next-to-Next-to-Next-to-Leading Order in QCD
• W-boson helicity fractions in top decay as probes of dimension-6 and dimension-8 SMEFT operators
• $CP$-odd form factor in the $HWW$ vertex