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Constraints on new physics from top quark decays at high precision

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,γ" 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.