An Improved Supersymmetric SU(5)

By supplementing minimal supersymmetric SU(5) (MSSU(5)) with a flavor ${\cal U}(1)$ symmetry and two pairs of $\bar{15}+15$ `matter' supermultiplets, we present an improved model which explains the charged fermion mass hierarchies and the magnitudes of the CKM matrix elements, while avoiding the undesirable asymptotic mass relations $m_s=m_μ, m_d/m_s = m_e/m_μ$. The strong coupling $α_s (M_Z)$ is predicted to be approximately 0.115, and the proton lifetime is estimated to be about five times larger than the MSSU(5) value. The atmospheric and solar neutrino puzzles are respectively resolved via maximal $ν_μ-ν_τ$ and small mixing angle $ν_e-ν_s$ MSW oscillations, where $ν_s$ denotes a sterile neutrino. The ${\cal U}(1)$ symmetry ensures not only a light $ν_s$ but also automatic `matter' parity.

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