Coupling constant unification in extended SUSY models

The unification of gauge coupling constants in the minimal supersymmetric model (MSSM) is unaffected at the one-loop level by the inclusion of additional mass-degenerate SU(5) multiplets. Perturbativity puts an upper limit on the number of additional fields. We analyse the evolution of the gauge coupling constants in all models satisfying these criteria using two-loop β functions and including low energy threshold effects. We find that similarly to the minimal supersymmetric model (MSSM) unification takes place within the theoretical and experimental errors. The dominant proton decay mode is more suppressed in all extended models as opposed to the MSSM due to renormalization group effects. However, the prediction for the bottom to τ mass ratio becomes worse in all models under consideration.