  In preparation for upcoming nucleon decay searches at Hyper-Kamiokande, it is
important to derive a theoretical upper bound on the proton lifetime in a
general class of grand unified theory (GUT) models. In this paper, we make an
attempt along this direction for non-SUSY SU(5) models, under the mild
restrictions that only one or two SM-decomposed multiplets are singularly
light, and that the SU(5) gauge theory is asymptotically free and thus there
are no too large representations in the model. We derive criteria for
SM-decomposed multiplets that potentially enhance the proton lifetime when they
are singularly light. We perform a numerical analysis on the proton lifetime
and show that some choices of singularly light multiplets can provide a
testable upper bound on the proton lifetime.
