Violations of universality in a vectorlike extension of the standard   model
  Violations of universality of couplings in a vectorlike extension of the standard model with three heavy mirror fermion families are considered. The recently observed discrepancies between experiments and the standard model in the hadronic branching fractions $R_b$ and $R_c$ of the Z-boson are explained by the mixing of fermions with their mirror fermion partners. 
Istv\'an Montvay