New directions for top quark polarization in the $t$-channel process
  We define three orthogonal axes to investigate the top quark polarization in the $t$-channel single top process. We provide expressions for the polarization in these axes in terms of anomalous $Wtb$ couplings. It is found that the polarizations in the two axes orthogonal to the spectator quark axis are very sensitive to an anomalous coupling involving a $\bar b_L \sigma^{\mu \nu} t_R$ dipole term. In particular, an asymmetry based on the polarization normal to the production plane is more sensitive to the imaginary part of this coupling than previously studied observables. 
J.A. Aguilar-Saavedra, S. Amor dos Santos
hep-ph hep-ex