Higgs effect in SU(15) grand unified theory

We analyze proton decay and the choice of Higgs fields in grand unified theories where the baryon number is a gauge symmetry [e.g., SU(15)]. Although Higgs effects forbid low-energy unification in the SU(15) model (within the extended survival hypothesis) as claimed by Frampton and Lee, other breaking patterns exist which allow unification at ${10}^9$ GeV, as well as chiral color symmetry, quark-lepton ununified symmetry, and baryon-number symmetry breaking at the TeV scale, without any observable proton decay.