Nonsupersymmetric SU(5) Resuscitated by New Quarks and Leptons

The issue of gauge unification in the (nonsupersymmetric) standard model is reinvestigated. It is found that, with just an additional fourth generation of quarks and leptons, $\mathrm{SU}\left(3\right)\otimes \mathrm{SU}\left(2\right)\otimes \mathrm{U}\left(1\right)$ gauge couplings converge to a common point $\sim 3.5\times {10}^{15}\mathrm{GeV}$ ( ${\tau }_p\sim 3.3\times {10}^{34\pm 2}\mathrm{yr}$). This result is due to the non-negligible, but still perturbative, contributions of the top and fourth generation Yukawa couplings to the gauge two-loop $\beta$ functions, in contrast with the three generation case where such a contribution is too small to play an important role in unification.