Local $Z_2$ scalar dark matter model confronting galactic ${\mathrm GeV}$-scale $γ$-ray

Abstract: We present a scalar dark matter (DM) model where DM ($X_I$) is stabilized by a local $Z_2$ symmetry originating from a spontaneously broken local dark $U(1)_X$. Compared with the usual scalar DM with a global $Z_2$ symmetry, the local $Z_2$ model possesses three new extra fields, dark photon $Z^{'}$, dark Higgs $\phi$ and the excited partner of scalar DM ($X_R$), with the kinetic mixing and Higgs portal interactions dictated by local dark gauge invariance. The resulting model can accommodate thermal relic density of scalar DM without conflict with the invisible Higgs branching ratio and the bounds from DM direct detections, thanks to the newly opened channels, $X_I X_I \rightarrow Z^{'} Z^{'}, \phi\phi$. In particular, due to the new particles, the ${\rm GeV}$ scale $\gamma$-ray excess from the Galactic Center (GC) can be originated from the decay of dark Higgs boson which is produced in DM annihilations.