  We investigate several quantities, defined in the decays of top quark pairs,
which can be used to explore non-standard Wtb interactions. Two new angular
asymmetries are introduced in the leptonic decay of top (anti)quarks. Both are
very sensitive to anomalous Wtb couplings, and their measurement allows for a
precise determination of the W helicity fractions. We also examine other
angular and energy asymmetries, the W helicity fractions and their ratios, as
well as spin correlation asymmetries, analysing their dependence on anomalous
Wtb couplings and identifing the quantities which are most sensitive to them.
It is explicitly shown that spin correlation asymmetries are less sensitive to
new interactions in the decay of the top quark; therefore, when combined with
the measurement of other observables, they can be used to determine the t tbar
spin correlation even in the presence of anomalous Wtb couplings. We finally
discuss some asymmetries which can be used to test CP violation in t tbar
production and complex phases in the effective Wtb vertex.
