  We investigate a simple realistic grand unified theory based on the $SU(5)$
gauge symmetry which predicts an upper bound on the proton decay lifetime for
the channels $p \to K^+ \bar{\nu}$ and $p \to \pi^+ \bar{\nu}$, i.e. $\tau (p
\to K^+ \bar{\nu}) \lesssim 3.4 \times 10^{35}$ and $\tau (p \to \pi^+
\bar{\nu}) \lesssim 1.7 \times 10^{34}$ years, respectively. In this context,
the neutrino masses are generated through the type I and type III seesaw
mechanisms, and one predicts that the field responsible for type III seesaw
must be light with a mass below 500 TeV. We discuss the testability of this
theory at current and future proton decay experiments.

High Energy Physics - Phenomenology;High Energy Physics - Experiment;High Energy Physics - Theory