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→πν¯ν, B→π(K)ν¯ν, and Bs,d→μ+μ−. After demonstrating in detail the cancellation between the leading nondecoupling terms, we show that two residual forms ∼s2lnm2T and ∼s4m2T act in a complementary way, so that the maximal allowed values of the decay rates are practically independent of mT. While they correspond to ∼20% or ∼30% corrections to the SM rates for K→πν¯ν and B→π(K)ν¯ν, an increase by ∼50% for Bs,d→μ+μ− decays offers a possibility to reveal an additional isosinglet state by measurements of these decays at the Large Hadron Collider