Unified universal seesaw models.

A set of Grand Unified Theories based upon the gauge groups SU(5)L × SU(5)R, SO(10)L×SO(10)R and SU(4)C×SU(4)L×SU(4)R is explored. Several novel features distinguish these theories from the well-known SU(5), SO(10) and SU(4)C×SU(2)L×SU(2)R models which they generalize. Firstly, Standard Model quarks and leptons are accompanied by and mix with heavy SU(2)L × SU(2)R singlet partners. The resulting fermion mass matrices are seesaw in form. Discrete parity symmetries render the determinants of these mass matrices real and eliminate CP violating gauge terms. The unified seesaw models consequently provide a possible resolution to the strong CP problem. Secondly, sin θW at the unification scale is numerically smaller than the experimentally measured Z scale value. The weak angle must therefore increase as it evolves down in energy. Finally, proton decay is suppressed by small seesaw mixing factors in all these theories. 4/93 ∗ Work supported in part by the U.S. Dept. of Energy under Contract no. DEAC-03-81ER40050.