  This paper complements the study of single top production at the LHC aiming
to estimate the sensitivity of different observables to the magnitude of the
effective couplings. In a previous paper the dominant $W$-gluon fusion
mechanism was considered, while here we extend the analysis to the subdominant
(10% with our set of experimental cuts) s-channel process. In order to
distinguish left from right effective couplings it is required to consider
polarized cross-sections and/or include $m_b$ effects. The spin of the top is
accessible only indirectly by measuring the angular distribution of its decay
products. We show that the presence of effective right-handed couplings implies
necessarily that the top is not in a pure spin state. We discuss to what extent
quantum interference terms can be neglected in the measurement and therefore
simply multiply production and decay probabilities clasically. The coarsening
involved in the measurement process makes this possible. We determine for each
process the optimal spin basis where theoretical errors are minimized and,
finally, discuss the sensitivity in the s-channel to the effective right-handed
coupling. The results presented here are all analytical and include $m_b$
corrections. They are derived within the narrow width approximation for the
top.
