Proton stability in low-scale extra-dimensional grand unified theories

We propose an extra-dimensional mechanism that adequately suppresses the rate of proton decay mediated by the X-bosons in grand unified theories. The X-bosons and their Kaluza-Klein modes are localized in the extra spatial dimensions due to the kink configuration of the adjoint Higgs multiplet that is responsible for the spontaneous breaking of the unified gauge group while the standard model gauge bosons freely propagate in the bulk. By localizing fermion multiplets apart from the X-bosons in the extra dimensions, the resulting four-dimensional couplings that give rise to proton decay are exponentially suppressed, leading to the longevity of proton even when the mass scale of the X-bosons is much lower than the usual unification scale.