Unification of standard and exotic matter through a $Z_2$ symmetry
  We consider a scenario in which the discrete weak symmetry between quarks and leptons is extended to the weak force by introducing exotic partners. We conjecture that there exists a hidden discrete symmetry $\tilde{P}$, defining a $Z_2$ group, between standard and exotic quarks and leptons. The unified model $SU(3)_{q} \times SU(3)_{\tilde{q}} \times SU(2)_{q \tilde{\ell}} \times SU(2)_{\tilde{q} \ell} \times U(1)_{Y} \times \tilde{P}$ is discussed, where the unifying discrete symmetry extends over particles and forces. It is shown that the lighter neutral and charged weak bosons generated upon spontaneous symmetry breaking have the same properties as those of the standard model. Cabbibo-Kobayashi-Maskawa unitarity is used to set a mass of order 2.8 TeV for the nonstandard weak bosons, which do not exhibit quark-lepton universality. A grand unified theory of $GUT_{q \tilde{\ell}} \times GUT_{\tilde{q} \ell} \times \tilde{P}$ type, with decay of exotic matter into standard matter and no decay of the ordinary nucleon, is put forward. 
Ernesto A. Matute