Sub-GeV dark matter and MeV gamma-ray detection with COSI

A bstract Sub-GeV dark matter (DM) has been attracting more attention due to the negative results searching for traditional weakly interacting massive particles (WIMPs). Such particles are expected to produce gamma rays from annihilation in the MeV energy region. Advancements in technology have opened up possibilities to precisely detect MeV gamma rays, leading to the upcoming space-based mission of the Compton Spectrometer and Imager (COSI). We comprehensively and quantitatively study the phenomenology of sub-GeV DM to determine if the COSI observations will probe their viable model parameter regions. We first construct models to describe sub-GeV DM based on the minimality and renormalizability of quantum field theory. Next, we impose various constraints on the models obtained from cosmological observations (CMB, BBN) and DM searches (accelerator, underground, astrophysical experiments, etc.). Finally, we identify viable parameter regions in each model and discuss whether or not COSI will be sensitive to the parameter regions. We find that a velocity-dependent annihilation cross-section is predicted in some regions, enabling COSI to detect the DM signal while avoiding severe constraints from cosmological observations.