The light Higgs window in the 2HDM at GigaZ

Abstract.

The sensitivity to a light Higgs boson in the general 2HDM (II), with a mass below 40 GeV, is estimated for a future $e^+e^-$ Linear Collider operating with a very high luminosity at the Z peak (GigaZ). We consider possible Higgs boson production via the Bjorken process, the (hA) pair production, the Yukawa process $Z\rightarrow b{\bar b}h(A), \rightarrow \tau{\bar \tau} h(A)$, and the decay $Z{\rightarrow}h(A)+\gamma$. Although the discovery potential is considerably extended compared to the current sensitivities, mainly from LEP, the existence of a h or A even with a mass of a few GeV cannot be excluded with two billion Z decays. The need to study the very light Higgs scenario at a $e^+e^-$ Linear Collider running at several hundred GeV and the LHC is emphasized.