Testing Higgs portal dark matter via Z fusion at a linear collider

We investigate the possibility of detecting dark matter at TeV scale linear colliders in the scenario where the dark matter is a massive particle weakly interacting only with the Higgs boson h in the low energy effective theory (the Higgs portal dark matter scenario). The dark matter in this scenario would be difficult to be tested at the CERN Large Hadron Collider when the decay of the Higgs boson into a dark matter pair is kinematically forbidden. We study whether even in such a case the dark matter D can be explored or not via the Z boson fusion process at the International Linear Collider and also at a multi-TeV lepton collider. It is found that for the collision energy s>1TeV with the integrated luminosity 1ab−1, the signal (e±e−→e±e−h⁎→e±e−DD) can be seen after appropriate kinematical cuts. In particular, when the dark matter is a fermion, which is supposed to be singlet under the standard gauge symmetries, the signal with the mass up to about 100 GeV can be tested for the Higgs boson mass to be 120 GeV.