Coupling unification and dark matter in a standard model extension with adjoint Majorana fermions

We revisit an extension of the Standard Model with Majorana fermions in adjoint representations, where SU(2)L triplet and SU(3)c octet fermions are introduced. The model provides not only a precise coupling unification but also a dark matter candidate [an SU(2)L triplet fermion]. In particular, we show that an SU(3)c octet fermion, which is required for successful unification, can be a good nonthermal source of the triplet fermion dark matter. We also show that the scenario predicts a rather short lifetime of the proton compared with the minimal supersymmetric grand unified theory, and the most parameter space can be explored by future experiments such as the Hyper-Kamiokande experiment.