Three Types of Fermion Mixing and Possible Manifestations of a Pati–salam Leptoquark in the Low–energy Processes

I report the recent studies1,2) on the low–energy manifestations of a minimal extension of the Standard Model based on the quarklepton symmetry SU(4)V ⊗ SU(2)L ⊗ GR of the Pati-Salam type. Given this symmetry the third type of mixing in the interactions ot the SU(4)V leptoquarks with quarks and leptons is shown to be required. An additional arbitrariness of the mixing parameters could allow, in principle, to decrease noticeably the lower bound on the vector leptoquark mass originated from the low-energy rare processes. The decays μ → eγ, μ → eγγ, and μ → eeē via the vector leptoquark are analysed and the specific hierarchy of the decay probabilities Γ(μ → eeē) ≫ Γ(μ → eγγ) ≫ Γ(μ → eγ) is shown to exist. The upper limits on the branching ratios at a level of 10−18 for the μ → eγγ decay and at a level of 10−15 for the μ → eeē decay are established.