Proton Stability in Six Dimensions

We show that Lorentz and gauge invariance explain the long proton lifetime within the standard model in six dimensions. The baryon-number violating operators have mass dimension 15 or higher. Upon TeV-scale compactification of the two universal extra dimensions on a square $T^2\mathrm{/}{Z}_2$ orbifold, a discrete subgroup of the 6-dimensional Lorentz group continues to forbid dangerous operators.