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

Abstract

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 $ν&#39;$) can still fit, surviving all present experimental constraints, provided $λ_b(M_U)=λ_τ(M_U)$ Yukawa unification is relaxed. For the theory to remain perturbative up to $M_U$, the new leptonic generation must lie within reach of LEP-II and the new $b&#39;,t&#39;$ must have masses within the reach of the Tevatron. For example, for $\mt>150\gev$ we find $m_{ν&#39;},m_{τ&#39;}< 86\gev$, $m_{t&#39;}<178$, and $m_{b&#39;}<156\gev$. Experiments at Fermilab are already sensitive to the latter mass regions; we comment on direct $b&#39;$ searches and on the $m_{t&#39;}\simeq \mt$ 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.