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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/Z₂ orbifold, a discrete subgroup of the six-dimensional Lorentz group continues to forbid dangerous operators.
Appelquist et al. (Mon,) studied this question.
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