Higgs coupling constants as a probe of new physics

We study new physics effects on the couplings of weak gauge bosons with the lightest CP-even Higgs boson (h), $hZZ$, and the trilinear coupling of the lightest Higgs boson, $hhh$, at the one-loop order, as predicted by the two Higgs doublet model. Those renormalized coupling constants can deviate from the standard model (SM) predictions due to two distinct origins: the tree level mixing effect of Higgs bosons and the quantum effect of additional particles in loop diagrams. The latter can be enhanced in the renormalized $hhh$ coupling constant when the additional particles show the nondecoupling property. Therefore, even in the case where the $hZZ$ coupling is close to the SM value, deviation in the $hhh$ coupling from the SM value can become as large as plus 100%, while that in the $hZZ$ coupling is at most minus 1% level. Such large quantum effect on the Higgs trilinear coupling is distinguishable from the tree level mixing effect, and is expected to be detectable at a future linear collider.