Nonsupersymmetric SU(5) Resuscitated by New Quarks and Leptons

Abstract

The issue of gauge unification in the (non-supersymmetric) Standard Model is\nreinvestigated. It is found that with just an additional fourth generation of\nnon-sequential and long-lived quarks and leptons, SU(3)xSU(2)xU(1) gauge\ncouplings converge to a common point of approximately 3.5x10^{15} GeV\n(corresponding to a proton lifetime of approximately 10^{34 plus/minus 1}\nyears). This result is due to the non-negligible- but still perturbative-\ncontributions of the top and fourth generation Yukawa couplings to the gauge\ntwo-loop beta functions, in contrast with the three generation case where such\na contribution is too small to play an important role in unification.\n