1 research outputs found
W polarisation beyond helicity fractions in top quark decays
We calculate the density matrix for the decay of a polarised top quark into a
polarised W boson and a massive b quark, for the most general Wtb vertex
arising from dimension-six gauge-invariant effective operators. We show that,
in addition to the well-known W helicity fractions, for polarised top decays it
is worth to define and study the transverse and normal W polarisation
fractions, that is, the W polarisation along two directions orthogonal to its
momentum. In particular, a rather simple forward-backward asymmetry in the
normal direction is found to be very sensitive to complex phases in one of the
Wtb anomalous couplings. This asymmetry, which indicates a normal W
polarisation, can be generated for example by a P-odd, T-odd transition
electric dipole moment. We also investigate the angular distribution of decay
products in the top quark rest frame, calculating the spin analysing powers for
a general Wtb vertex. Finally we show that, using a combined fit to top decay
observables and the tW cross section, at LHC it will be possible to obtain
model-independent measurements of all the (complex) Wtb couplings as well as
the single top polarisation. Implications for spin correlations in top pair
production are also discussed.Comment: LaTeX 36 pages, 51 EPS figures. Typos corrected. To appear in NP