W polarisation beyond helicity fractions in top quark decays Aguilar Saavedra, Juan Antonio Bernabéu, J. Top quark Density 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 defining and studying 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. 2014-02-27T09:15:56Z 2014-02-27T09:15:56Z 2010 info:eu-repo/semantics/article Aguilar-Saavedra, J.A.; Bernabéu, J. W polarisation beyond helicity fractions in top quark decays. Nuclear Physics B, 840(1-2): 439-378 (2010). [http://hdl.handle.net/10481/30585] 0550-3213 doi: 10.1016/j.nuclphysb.2010.07.012 arXiv:1005.5382v2 http://hdl.handle.net/10481/30585 eng http://creativecommons.org/licenses/by-nc-nd/3.0/ info:eu-repo/semantics/openAccess Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License Elsevier