Würzburg 2018 – scientific programme

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T: Fachverband Teilchenphysik

T 58: Top-Quarks: Eigenschaften II

T 58.5: Talk

Wednesday, March 21, 2018, 17:35–17:50, Z6 - SR 1.002

Precision measurement of W helicity fractions from top-quark decays in √s = 13 TeV with the ATLAS detector — Thomas Peiffer, •Ishan Pokharel, Arnulf Quadt, Elizaveta Shabalina, and Royer Ticse Torres — II. Physikalisches Institut, Georg-August-Universität Göttingen

The top quark, the heaviest quark in the Standard Model (SM), was discovered at the Tevatron in 1995. Due to its large mass, the top quark decays before hadronisation. This provides a unique opportunity to study the properties of a bare quark. The top quark decays almost exclusively into a bottom quark and a W boson (Wtb vertex), with the W boson decaying leptonically or hadronically. Due to the large mass difference between the top and the bottom quark and the V-A structure of the interaction, the W boson from top quark decays are highly polarized in the SM.
This analysis focuses on the leptonic decay of the W boson from top quark decays. To this end, the single lepton and di-lepton channels of tt decays are considered. The neutrino and b-jet reconstruction plays a very important role in the reconstruction of the top quark. The observable cosθ^* defined as the angle between the momentum direction of the charged lepton and the reverse direction of the b-quark, is used to distinguish the different helicity fractions. To extract helicity fractions of the W boson from top decays, a template fit to the cosθ^* distribution is performed.