  We study possible theoretical deviations from SM in top quark physics which
alter the decay properties of the top quark. Using effective filed theory
techniques we parametrize the effects of potential NP of scales well above the
electroweak scale in terms of effective operators.
  On one side we investigate NP manifestation in the form of FCNC decays of the
top quark, which are highly suppressed in SM and potential observation of which
would undoubtedly signal the presence of NP. We examine the two-body "t -> q
Z,\gamma" decays at NLO in QCD and three-body "t -> q l^+ l^-" decays where we
exploit the increased phase space of the final state by defining different
types of observables which could help to discriminate between structures of the
vertices governing the FCNC transition of the top quark.
  On the other side we examine possible deviations from SM predictions in top
quark's main decay channel, which is governed by the charged quark current
interactions. Introduction of higher dimensional operators that modify tWb
interactions, has additional consequences in low energy observables of rare
processes in B physics, where virtual top quarks and their charged current
interactions play a dominant role. We perform a detailed study of indirect
constraints on the NP operator basis as well as the effects in the decays of
on-shell produced top quarks. We conduct the analysis of NP effects on W
helicity fractions at NLO in QCD. We confront our predictions with the
experimental measurements to obtain the direct constraints on NP further
comparing them with the indirect constraints from low energy processes
revealing an interesting interplay of top and bottom physics.
