Uniqueness of a recently proposed U(1) gauge extension

Consider the addition of a right-handed SU(2) fermion multiplet (with neither color nor hypercharge) to each family of quarks and leptons. The resultant theory admits a new U(1) gauge symmetry only if the additional multiplet is a singlet $N_R$, which corresponds to the well-known case of $U(1{)}_{B-L}$, or a triplet $({\mathrm{\Sigma }}^{+},{\mathrm{\Sigma }}^0,{\mathrm{\Sigma }}^{-})$, which corresponds to the proposal of hep-ph/0112232. This disproves the assertion that the latter is somehow a trivial'' or expected’’ discovery.