New physics from W polarisation in top quark decays

Tuesday December 14th, 14.30hs - Seminar room

Speaker: José Bernabeu (IFIC, Valencia)


From the most general Wtb vertex arising from dimension-six gauge invariant effective operators, I discuss the density matrix for the decay of a polarised top quark into a polarised W boson and a b quark.

I show that, in addition to the well-known W helicity fractions, for polarised top decays it is worth to study the transverse and normal components of the W polarisation. A simple forward-backward asymmetry of the lepton from W decay with respect to the normal direction is found to be very sensitive to complex phases which signal novel CP violating effects. This experimental asymmetry, which measures the normal W polarisation, can be generated by a P-odd, T-odd transition weak-electric dipole moment. The angular distribution of decay products in the top quark rest frame is also investigated, presenting the corresponding spin analysing powers. At LHC it will be possible to obtain model-independent results of the complex Wtb couplings as well as the single top polarisation.


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