SU(5) grand unification in extra dimensions and proton decay

We analyse proton decay in the context of simple supersymmetric SU(5) grand unified models with an extra compact spatial dimension described by the orbifold S-1/(Z(2) x Z(2)'). Gauge and Higgs degrees of freedom live in the bulk, while matter fields can only live at the fixed point branes. We present an extended discussion of matter interactions on the brane. We show that proton decay is naturally suppressed or even forbidden by suitable implementations of the parity symmetries on the brane, The corresponding mechanism does not affect the SU(5) description of fermion masses also including the neutrino sector, where Majorana mass terms remain allowed. (C) 2001 Published by Elsevier Science B.V.