Proton Stability in $SU(5)\times U(1)$ and $SU(6)\times SU(2)$ GUTs
  We consider explicit unified models based on the flipped ${SU(5)\times U(1)}$ and $SU(6)\times SU(2)$ gauge groups in which gauge mediated proton decay operators are suppressed at leading order due to the special placement of matter fields in unified multiplets. We discuss both the theoretical structure and phenomenological implications of these models. For the latter, we examine the viability of the physical spectrum in each scenario and focus on the possible presence of other operators that could also contribute significantly to the proton decay rate. 
A.E. Faraggi (Liverpool U., Dept. Math.), M. Paraskevas, J. Rizos and   K. Tamvakis (Ioannina U.)
hep-ph