E6 inspired SUSY models with exact custodial symmetry
  The breakdown of E_6 gauge symmetry at high energies may lead to supersymmetric (SUSY) models based on the Standard Model (SM) gauge group together with extra U(1)_{\psi} and U(1)_{\chi} gauge symmetries. To ensure anomaly cancellation the particle content of these E_6 inspired models involves extra exotic states that generically give rise to non-diagonal flavour transitions and rapid proton decay. We argue that a single discrete \tilde{Z}^{H}_2 symmetry can be used to forbid tree-level flavor-changing transitions, as well as the most dangerous baryon and lepton number violating operators. We present 5D and 6D orbifold GUT constructions that lead to the E_6 inspired SUSY models of this type. The breakdown of U(1)_{\psi} and U(1)_{\chi} gauge symmetries that preserves E_6 matter parity assignment guarantees that ordinary quarks and leptons and their superpartners, as well as the exotic states which originate from 27 representations of E_6 survive to low energies. These E_6 inspired models contain two dark-matter candidates and must also include additional TeV scale vectorlike lepton or vectorlike down type quark states to render the lightest exotic quark unstable. We examine gauge coupling unification in these models and discuss their implications for collider phenomenology and cosmology. 
R. Nevzorov
hep-ph