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 ${T}^{2}{/Z}_{2}$ orbifold, a discrete subgroup of the six-dimensional Lorentz group continues to forbid dangerous operators.