Abstract
A set of grand unified theories based upon the gauge groups SU(5)[sub [ital L]][times]SU(5)[sub [ital R]], SO(10)[sub [ital L]][times]SO(10)[sub [ital R]] and SU(4)[sub [ital C]][times]SU(4)[sub [ital L]][times]SU(4)[sub [ital R]] is explored. Several novel features distinguish these theories from the well-known SU(5), SO(10), and SU(4)[sub [ital C]][times]SU(2)[sub [ital L]][times]SU(2)[sub [ital R]] models which they generalize. Firstly, standard model quarks and leptons are accompanied by and mix with heavy SU(2)[sub [ital L]][times]SU(2)[sub [ital 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 [ital CP]-violating gauge terms. The unified seesaw models consequently provide a possible resolution to the strong [ital CP] problem. Secondly, sin[sup 2][theta][sub [ital W]] at the unification scale is numerically smaller than the experimentally measured [ital 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.
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