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 $\nu'$) can still fit, surviving all present experimental constraints, provided $\lambda_b(M_U)=\lambda_\tau(M_U)$ 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 $b',t'$ must have masses within the reach of the Tevatron. For example, for $\mt>150\gev$ we find $m_{\nu'},m_{\tau'}< 86\gev$, $m_{t'}<178$, and $m_{b'}<156\gev$. Experiments at Fermilab are already sensitive to the latter mass regions; we comment on direct $b'$ searches and on the $m_{t'}\simeq \mt$ 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.