A Minimal Four-Family Supergravity Model

Abstract

We investigate the phenomenology of minimal four-family MSSM supergravity theories containing an additional generation of heavy fermions along with their superpartners. We demand: gauge coupling constant unification at high energy scales; perturbative values for all Yukawa couplings for energy scales $\leq \mgut$; radiative electroweak symmetry breaking via renormalization group evolution; and a neutral LSP. The perturbative constraints imply a light fourth-family quark and lepton spectrum, and $\tanb\lsim 3$. The lightest CP-even Higgs mass is increased. Fourth-family Yukawa coupling contributions to the evolution of scalar masses lead to unexpected mass hierarchies; \eg\ the $\staupone$ is generally the lightest slepton and the lightest squark is the $\wt\bpr_1$. A significant lower bound is placed on the gluino mass by the requirement that the $\staupone$ not be the LSP. Sleptons of the first two families are much more massive compared to the LSP and other neutralinos and charginos than in the three-family models, and could easily lie beyond the reach of a $\sqrt s=500\gev$ $\epem$ collider. Relations between slepton masses and gaugino masses are shown to be very sensitive to the presence of a fourth generation. The most important near-future experimental probes of the four-family models are reviewed. A scenario with $\mt\sim\mw$ and $t\rta {\wt t_1}\cnone$ is shown to be inconsistent with universal soft-SUSY-breaking boundary conditions. Full four-family evolution of ${\alpha }_s$ is shown to lead to a significant enhancement in inclusive jet and di-jet spectra at Tevatron energies when all sparticle masses are near their lower bounds.

56 pages, requires phyzzx.tex, tables.tex, figures not included, full postscript files including figures via anonymous ftp at ftp://ucdhep.ucdavis.edu/gunion/ , get 4gen.ps, 4gen_figs1to12.ps, and 4gen_figs13to19.ps

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