Unitarity Constraints for the Mass of the Higgs in the $SU(2)_L\otimes   U(1)\otimes U(1)'$ Gauge Model
  The critical values for the mass of the Higgs bosons, at which the theory becomes strongly interacting, are calculated using the equivalence theorem, at high energies this allows us to replace the longitudinally polarized gauge bosons in the $S$ matrix for the corresponding Goldstone bosons. An appropriate ansatz for defining the would-be Goldstone bosons in the case of an additional neutral current, beyond the minimal standard model, is also presented. 
R. Mart\'inez and N. Vanegas