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It is commonly thought that small couplings in a low-energy theory, such as needed for the fermion mass hierarchy or proton stability, must originate symmetries in a high-energy theory. We show that this expectation is in theories where the Standard Model fields are confined to a thick in extra dimensions, with the fermions "stuck" at different points in the. Couplings between them are then suppressed due to the exponentially small of their wave functions. This provides a framework for understanding the fermion mass hierarchy and proton stability without imposing, but rather in terms of higher dimensional geography. A model prediction of this scenario is non-universal couplings of the Model fermions to the ``Kaluza-Klein'' excitations of the gauge. This allows a measurement of the fermion locations in the extra at the LHC or NLC if the wall thickness is close to the TeV scale.
Arkani–Hamed et al. (Tue,) studied this question.