Abstract
We write down the four-dimensional fully differential decay distribution for the top quark decay t rightarrow Wb rightarrow ell nu b. We discuss how its eight physical parameters can be measured, either with a global fit or with the use of selected one-dimensional distributions and asymmetries. We give expressions for the top decay amplitudes for a general tbW interaction, and show how the untangled measurement of the two components of the fraction of longitudinal W bosons – those with b quark helicities of 1 / 2 and -1/2, respectively – could improve the precision of a global fit to the tbW vertex.
Highlights
The detailed study of the properties of the top quark has a widespread interest as a probe of new physics beyond the Standard Model (SM) [1,2,3]
One-dimensional distributions or asymmetries were already proposed long ago to measure some physical quantities involved in the top decay, such as the W helicity fractions, measured from the θ ∗ distribution, or the top polarisation, measured from the θ and φ distributions [5]
W helicity fractions have been experimentally measured in a number of experiments, with the most precise measurements obtained by the ATLAS [6] and CMS [7] Collaborations, and the top polarisation in certain directions has been measured in single top [8,9] and top pair [10,11] production
Summary
The detailed study of the properties of the top quark has a widespread interest as a probe of new physics beyond the Standard Model (SM) [1,2,3]. One-dimensional distributions or asymmetries were already proposed long ago to measure some physical quantities involved in the top decay, such as the W helicity fractions, measured from the θ ∗ distribution, or the top polarisation, measured from the θ and φ distributions [5]. It has been shown that the full set of eight W boson spin observables can be measured from selected distributions in top quark decays [15], and preliminary measurements have been performed by the ATLAS Collaboration [9]. We write the physical quantities involved in the top decay for a general tbW effective Lagrangian, showing that the untangled measurement of the two components of the fraction of longitudinal W bosons may improve the precision of the global fit
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