Dortmund 2021 – scientific programme
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T: Fachverband Teilchenphysik
T 29: Top quark decay and top properties I
T 29.8: Talk
Tuesday, March 16, 2021, 17:50–18:05, Td
Measurement of helicity fractions of W bosons decaying from top quarks in dileptonic tt events at √s = 13 TeV with the ATLAS detector. — Tomas Dado1,2, Thomas Peiffer1, •Ishan Pokharel1, Arnulf Quadt1, Elizaveta Shabalina1, and Knut Zoch1 — 1II. Physikalisches Institut, Georg-August-Universität Göttingen — 2now at: Technische Universität Dortmund
Due to the large difference between the top and b-quark masses, the W boson in the Wtb vertex is heavily polarised. A measurement of three helicity fractions of W bosons from top-quark decays is presented. They are extracted from the differential distribution of the observable cosθ*, defined as the angle between the charged lepton and the top quark in the W boson rest frame. The reconstructed cosθ*, distribution in the dilepton channel of tt events is unfolded to parton level. Event reconstruction is performed using the neutrino weighting algorithm, owing to the fact that two neutrinos need to be reconstructed from one measurement of missing transverse energy. The helicity fractions are extracted from the fit of the unfolded cosθ* differential distribution to an analytical function that relates the observable to the helicity fractions of the W boson. The fractions are constrained to unity in the fit via Lagrange multiplier.