Absolutely Stable Proton and Lowering the Gauge Unification Scale
  A unified model is constructed, based on flipped $SU(5)$ in which the proton is absolutely stable. The model requires the existence of new leptons with masses of order the weak scale. The possibility that the unification scale could be extremely low is discussed. 
S. M. Barr (University of Delaware) and X. Calmet (University of   Sussex)
hep-ph