Yukawa couplings and quark and lepton masses in an SO(10) model with a unified Higgs sector

Fermion mass generation for the third generation is considered within a unified Higgs model with a single pair of vector-spinor 144 + (144) over bar of Higgs multiplets. Extending a previous work it is shown that much larger masses can arise for all the third generation fermions from mixing with 45 and 120 matter multiplets via the cubic couplings 16 . 45 . (144) over bar and 16 . 120 . 144 in addition to the quartic couplings. Further, it is found that values of tan beta as low as 10 can allow for a b - tau - t unification consistent with the current data. The quartic and cubic couplings naturally lead to Dirac as well as Majorana neutrino masses necessary for the generation of a seesaw neutrino mass of the right size for the tau neutrino.