Characterizing Higgs portal dark matter models at the ILC

We study the Dark Matter (DM) discovery prospect and its spin discrimination in the theoretical framework of gauge invariant and renormalizable Higgs portal DM models at the ILC with $\sqrt{s} = 500$ GeV. In such models, the DM pair is produced in association with a $Z$ boson. In case the singlet scalar DM, the mediator is just the SM Higgs boson, whereas for the fermion or vector DM there is an additional singlet scalar mediator that mixes with the SM Higgs boson, which produces significant observable differences. After careful investigation of the signal and backgrounds both at parton level and at detector level, we find the signal with hadronically decaying $Z$ boson provides a better search sensitivity than the signal with leptonically decaying $Z$ boson. Taking the fermion DM model as a benchmark scenario, when the DM-mediator coupling $g_\chi$ is relatively small, the DM signals are discoverable only for benchmark points with relatively light scalar mediator $H_2$. And the spin discriminating from scalar DM is always promising while it is difficult to discriminate from vector DM. As for $g_\chi$ approaching the perturbative limit, benchmark points with the mediator $H_2$ in the full mass region of interest are discoverable. And the spin discriminating from both the scalar and fermion DM are quite promising.