Indirect searches for heavy quarks in rare decays

Investigation is made of the effects of ultra-heavy quarks (top and fourth generation) via one-loop diagrams on the rare decays of K+→π+νν-bar, Z0→qq-bar'+q'q-bar(q≠q'), B0-B-bar0 mixing and CP violation in B decays. This is one of the powerful methods to discover and study the properties of new quarks, as its validity was historically shown in the indirect discovery of the charm quark via KL→μμ-bar. The present study is particularly focused on the effects of heavy top quark in the range of 40 - 160 GeV and/or fourth generation up quark of around 200 GeV. As for the quark mixing matrix, the study adopts the Wolfenstein parametrization with the expansion of its matrix elements by the Cabbibo angle, since in this parametrization, various hierarchical schemes can be assumed for the mixing of the fourth generation with first three generations. The rare decays are studied for the following two cases (Case A and Case B): |V(b't', bt', st', dt'| ∼ (1, λ, λ2, λ3) and ∼ (1, λ, λ2, λ3). First, the branching ratio for K+→π+νν-bar and Z0→qq-bar'+q'q-bar(q≠q') is discussed in relation to the 3- and 4-generation models. Then the mass splitting of mass eigenstates due to B0-B-bar0 mixing is examined. Discussion is also made of CP asymmetries in the non-leptonic B0 decays. (Nogami, K.)