  We study the mechanism of single top production at the LHC in the framework
of an effective electroweak Lagrangian, analyzing the sensitivity of different
observables to the magnitude of the effective couplings that parametrize new
physics beyond the Standard Model. The observables relevant to the distinction
between left and right effective couplings involve in practice the measurement
of the spin of the top and this can be achieved only indirectly by measuring
the angular distribution of its decay products. We show that the presence of
effective right-handed couplings implies that the top is not in a pure spin
state. A unique spin basis is singled out which allows one to connect top decay
products angular distribution with the polarized top differential cross
section. We present a complete analytical expression of the differential
polarized cross section of the relevant perturbative subprocess including
general effective couplings. The mass of the bottom quark, which actually turns
out to be more relevant than naively expected, is retained. Finally we analyze
different aspects the total cross section relevant to the measurement of new
physics through the effective couplings. The above analysis also applies to
anti-top production in a straightforward way.
