Replication of lepton and quark fields and stability of the proton in N=2 SUSY GUTs

We show that in N=2 SUSY GUTs, leptons and quarks tend to combine with their mirror partners and to acquire very large masses proportional to the grand-unification mass. We solve this problem by means of a replication of lepton and quark fields. Grand-unification now associates light leptons and quarks with heavy ones of masses approx.=msub(x) or 2 msub(x). The latter carry a non-vanishing value of the central charge =+-msub(x) or +-2 msub(x) (which may be viewed as the fifth or sixth component of the covariant momentum). The exchanges of the grand-unification gauge bosons do not induce proton decay, and the proton is stable, in minimal N=2 SUSY GUTs. Moreover the grand-unification mass does not get renormalized. (orig.)