Bounds on Particle Masses in Grand Unified Theories with Four Fermion Generations

Abstract

A requirement, in which coupling constants of all interactions cannot be large and the vacuum should be stable up to the unification mass scale in the grand unified theories, leads to bounds on masses of fermions and a Higgs scalar. We calculate these bounds for a model with a heavy lepton and a heavy quark doublet in the framework of the effective 5U(3)c x 5U( 2) x UO) gauge theory with a single Higgs doublet and four fermion generations. These are applied to grand unified models with 5U(5) or 50(0) in the minimal scheme.