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We study possible theoretical deviations from SM in top quark physics whichalter the decay properties of the top quark. Using effective filed theorytechniques we parametrize the effects of potential NP of scales well above theelectroweak scale in terms of effective operators.  On one side we investigate NP manifestation in the form of FCNC decays of thetop quark, which are highly suppressed in SM and potential observation of whichwould undoubtedly signal the presence of NP. We examine the two-body "t -> qZ,\gamma" decays at NLO in QCD and three-body "t -> q l^+ l^-" decays where weexploit the increased phase space of the final state by defining differenttypes of observables which could help to discriminate between structures of thevertices governing the FCNC transition of the top quark.  On the other side we examine possible deviations from SM predictions in topquark's main decay channel, which is governed by the charged quark currentinteractions. Introduction of higher dimensional operators that modify tWbinteractions, has additional consequences in low energy observables of rareprocesses in B physics, where virtual top quarks and their charged currentinteractions play a dominant role. We perform a detailed study of indirectconstraints on the NP operator basis as well as the effects in the decays ofon-shell produced top quarks. We conduct the analysis of NP effects on Whelicity fractions at NLO in QCD. We confront our predictions with theexperimental measurements to obtain the direct constraints on NP furthercomparing them with the indirect constraints from low energy processesrevealing an interesting interplay of top and bottom physics.

Constraints on new physics from top quark decays at high precision