How accurately can we estimate the proton lifetime in an SU(5) grand unified model

We reduce the theoretical uncertainties in the prediction of the proton lifetime in the simplest SU(5) model to an uncertainty reflecting our lack of precise knowledge of the QCD coupling constant, of the mass of color Higgs' bosons, and of the proton wave function at the origin. This enables us to put a confident upper bound on the lifetime in this model. Our analysis includes all renormalization prescription dependence effects and phase-space effects. Branching ratios to experimentally significant decay modes are also discussed. (orig.)