be so large as compared to the SM result. The reason is simple. Let us look at the effective Yukawa coupling $y_t^{\rm eff}(-q^2,\Lambda)$ given in Eq. (\[eq:yeff\]). While the SM Yukawa interaction is determined by $y_t^{\rm SM} = \sqrt{2} m_t/v \sim {\cal O}(1)$, the magnitude of $C_{t1}$ in Eq. (\[eq:yeff\]) is only constrained by the unitarity bound in Eq. (\[eq:pu-t1\]), and its maximum contribution to $y_t^{\rm eff}(\hat{s},\Lambda)$ can be about $\pm 2.9$ for $\Lambda=1$ TeV. Therefore, $y_t^{\rm eff}(\hat{s}, \Lambda)$ can be enhanced by $C_{t1}$ to be about 2-4 times greater than $y_t^{\rm SM}$. Consequently, the cross section can be about several times 100% enhanced. Such a large enhancement can also occur in the non-standard contribution to the effective self-coupling of the SM-like Higgs boson[@Barger:2003rs; @hhh-thdm]. On the other hand, the contribution of $C_{Dt}$ to $y_t^{\rm eff}(-q^2,\Lambda)$ is strongly limited if the LEP result is taken into account, so that its effect can change the SM Yukawa coupling by at most 10-20 %.

Cross section of the full process {#Sec:Cross section of the full process}
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We here evaluate the cross section of the full process $e^-e^+ \to W^-W^+ \nu \bar \nu \to t \bar t \nu \bar \nu$ in the 