The inelastic dark matter model is one kind of popular models for the sub-GeV dark matter (DM). If the mass splitting between DM excited and ground states is small enough, the co-annihilation becomes the dominant channel for thermal relic density
and the DM excited state can be long-lived at the collider scale. We study scalar and fermion inelastic dark matter models for sub-GeV DM at Belle II with $U(1)_D$ dark gauge symmetry broken into its $Z_2$ subgroup. We focus on dilepton displaced
vertex signatures from decays of the DM excited state. With the help of precise displaced vertex detection ability at Belle II, we can explore the DM spin, mass and mass splitting between DM excited and ground states. Especially, we show scalar and
fermion DM candidates can be discriminated and the mass and mass splitting of DM sector can be determined within the percentage of deviation for some benchmark points.
https://yonsei.zoom.us/j/89500845677