Crossing two-component dark matter models and implications for 511 keV $γ$-ray and XENON1T excesses

We study scalar and fermionic crossing two-component dark matter (C2CDM) models with $ U(1)_X $ dark gauge symmetry. This $ U(1)_X $ gauge symmetry is broken via the dark Higgs mechanism of a dark Higgs field $Φ$ and the dark photon becomes massive. On the other hand, the same dark Higgs field $Φ$ also serves as a bridge between each component of two DM sectors such that the dark flavor-changing neutral current (DFCNC) interaction between them is induced. Moreover, the mass splitting between these two DM sectors is generated after $ U(1)_X $ gauge symmetry breaking by nonzero $\langle Φ\rangle$. We discuss the stability of DM candidates and allowed parameter space from the relic density and other constraints in C2CDM models. Some novel signatures with displaced vertices at Belle II are studied. Finally, the possible explanations of 511 keV $γ$-ray line and XENON1T excess in C2CDM models are discussed.