Electroweak symmetry breaking by vectorlike fermion condensation with smallSandTparameters

In order to build realistic models in which electroweak symmetry is dynamically broken we should explain the smallness of the Peskin-Takeuchi S and T parameters. In accordance with the decoupling theorem, these parameters must be suppressed by SU(2${)}_{\mathit{L}}$×U(1${)}_{\mathit{Y}}$ invariant masses. From this fact we can expect that if fermions with large SU(2${)}_{\mathit{L}}$×U(1${)}_{\mathit{Y}}$ invariant masses undergo condensation and break electroweak symmetry, the S and T parameters can be small. It is intersting that not only the S but also the T parameter can become small even if there exists a large isospin violation in fermion condensation. In this paper we examine the possibility that, by the strong four-Fermi interaction, massive vectorlike fields undergo condensation and break electroweak symmetry. The model becomes almost the same as the standard model at a low energy scale and predicts a heavy Higgs boson. Moreover, we discuss a model in which the four-Fermi interaction can be induced by massive gauge boson exchange. In this model, the masses of ordinary matter fermions (quark and lepton) are enhanced.