Gauge-Coupling Unification and the Minimal SUSY Model: A Fourth Generation Below the Top?

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We explore the possibility of a fourth generation in the gauge-coupling-unified, minimal supersymmetric (MSSM) framework. We find that a sequential fourth generation (with a heavy neutrino ν') can still fit, surviving all present experimental constraints, provided λb(Mᵤ)=λτ(Mᵤ) Yukawa unification is relaxed. For the theory to remain perturbative up to Mᵤ, the new leptonic generation must lie within reach of LEP-II and the new b',t' must have masses within the reach of the Tevatron. For example, for >150 we find mν',mτ'< 86, mₜ'<178, and mb'<156. Experiments at Fermilab are already sensitive to the latter mass regions; we comment on direct b' searches and on the mₜ'≃ case in light of new CDF data. Discovery may involve novel decay signatures; however, CDF and LEP-II will confirm or exclude an MSSM fourth generation in the near future.