ar X iv : h ep - p h / 06 02 13 2 v 1 1 5 Fe b 20 06 Fermion Mass Hierarchy in the Nu MSGUT I : The Real Core Charanjit

The recent failure of the scenario which uses only 10, 126 Higgs multiplets to yield large enough neutrino masses in the SO(10) MSGUT motivates an alternative scenario[1] where the 120 -plet collaborates with the 10 -plet to fit the dominant charged fermion masses. The small 126 -plet couplings give appreciable contributions only to light charged fermion masses and enhance the Type I seesaw masses to viable values. We analyze the 2-3 generation core of the complete hierarchical fermion mass system in the CP conserving approximation. Ansatz consistency requires mb −ms = mτ −mμ at the GUT scale MX and predicts near maximal (PMNS) mixing in the leptonic sector for central values of charged fermion parameters and for wide ranges of the other relevant parameters : righthanded neutrino masses and relative strength of contributions of the two doublet pairs from the 120-plet to the effective MSSM Higgs pair. These features are preserved in the CP preserving 3 generation system whose results are previewed : an additional consistency requirement θ 13 = θ 12 θ 23 on the CKM angles at MX arises for 3 generations. Right handed neutrino masses in such scenarios are all less than about 10GeV .