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 ${\nu }^{\prime }$) 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 $b^{\prime },t^{\prime }$ must have masses within the reach of the Tevatron. For example, for $\text{mt}>150\text{gev}$ we find $m_{{\nu }^{\prime }},m_{{\tau }^{\prime }}<86\text{gev}$, $m_{t^{\prime }}<178$, and $m_{b^{\prime }}<156\text{gev}$. Experiments at Fermilab are already sensitive to the latter mass regions; we comment on direct $b^{\prime }$ searches and on the $m_{t^{\prime }}\simeq \text{mt}$ 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.

Comments: 13 pages, requires phyzzx.tex, tables.tex, full postscript file including embedded figures available via anonymous ftp at ucdhep.ucdavis.edu as [anonymous.gunion]4th.ps