  The Effective Field Theory (EFT) approach is widely used in the search for
possible deviations from the predictions of the Standard Model. Such an
approximation of possible BSM physics is valid up to a certain levels of energy
scale and accuracy. In this article, we investigate potential limitation of the
EFT approach related to unitarity to describe possible contributions of flavor
changing neutral currents (FCNC) involving the top quark. The numerical and
analytical calculations of the FCNC processes used in the EFT approach
demonstrate the constant asymptotic behavior of the total cross section with
increasing energy. It is shown that the EFT approach for studying the possible
contribution of FCNC does not violate the restrictions following from
perturbative unitarity, the asymptotic behavior of the cross section does not
exceed the Froissart bound, and the approach itself can be used to set the
corresponding experimental limits for FCNC couplings or Wilson coefficients at
present and future colliders.
