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Nondecoupling of a terascale isosinglet quark and rare K and B decays

We examine recent extensions of the standard model with an up-type vectorlike isosinglet T quark that mixes dominantly with the top quark. We take under scrutiny the nondecoupling effects which may reveal such a new heavy fermion through loop diagrams relevant for rare decays such as K-->pi nu nu-bar, B-->pi(K) nu nu-bar, and B_{; ; ; ; s, d}; ; ; ; -->mu^+ mu^-. After demonstrating in detail the cancellation between the leading nondecoupling terms, we show that two residual forms ~ s^2 ln{; ; ; ; m_T^2}; ; ; ; and ~ s^4 m_T^2 act in complementary way, so that the maximal allowed values of the decay rates are practically independent of m_T. While they correspond to ~ 20% or ~ 30% corrections to the SM rates for K-->pi nu nu-bar and B-->pi(K) nu nu-bar, an increase by ~ 50% for B_{; ; ; ; s, d}; ; ; ; -->mu^+ mu^- decays offers a possibility to reveal an additional isosinglet state by measurements of these decays at the Large Hadronic Collider.

models beyond the SM; electroweak transitions; decays of K and B mesons

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