Speaker
Description
Precise measurements of the properties of the top quark allow for
testing the Standard Model (SM) and can be used to constrain new
physics models. The top quark is predicted in the SM to decay almost
exclusively to a W boson and b quark. Thus, studying the Wtb vertex
structure in high precision and details is motivated. A measurement
of the W boson helicity fractions in top quark decays with ttbar
events in the lepton+jets final state using protonproton collisions
at a centerofmass energy of 8 TeV recorded in 2012 with the ATLAS
detector at the LHC is presented. The data sample corresponds to an
integrated luminosity of 20.3 fb1. The angular distributions of two
different analysers, the charged lepton and the downtype quark are
used to measure the helicity fractions. The results are obtained by
performing a combined fit of both analysers to data. The measured
helicity fractions are compatible with the Standard Model. As the
polarization state of the W bosons in top quark decays is sensitive
to the Wtb vertex structure, limits on anomalous Wtb couplings are
set.
