Quark mixings in SU(6)×SU(2)R and suppression of Vub

The quark mixing matrix VCKM is studied in depth on the basis of superstring inspired SU(6)×SU(2)R model with global flavor symmetries. The sizable mixings between right-handed down-type quark Dc and colored Higgs field gc potentially occur but no such mixings in up-type quark sector. In the model the hierarchical pattern of VCKM is understood systematically. It is shown that due to large Dc–gc mixings Vub is naturally suppressed compared to Vtd. It is pointed out that the observed suppression of Vub is in favor of the presence of SU(2)R gauge symmetry but not in accord with generic SU(5) GUT.