SU(5) × SU(5) unification revisited

The idea of grand unification in a minimal supersymmetric SU(5) × SU(5) framework is revisited. It is shown that the unification of gauge couplings into a unique coupling constant can be achieved at a high-energy scale compatible with proton decay constraints. This requires the addition of a minimal particle content at intermediate energy scales. In particular,the introduction of the SU(2) L triplets belonging to the representations, as well as of the scalar triplet Σ 3 and octet Σ 8 in the (24, 1) representation, turns out to be crucial for unification. The masses of these intermediate particles can vary over a wide range, and even lie in the TeV region. In contrast, the exotic vector-like fermions must be heavy enough and have masses above 10 10 GeV. We also show that, if the SU(5) × SU(5) theory is embedded into a heterotic string scenario, it is not possible to achieve gauge coupling unification with gravity at the perturbative string scale.