Can a millicharged dark matter particle emit an observable gamma-ray line?

Understand

If a gamma-ray line is observed in the near future, it will be important to determine what kind of dark matter (DM) particle could be at its origin.

• We investigate the possibility that the gamma-ray line would be induced by a slow DM particle decay associated to the fact that the DM particle would not be absolutely neutral.
• A "millicharge" for the DM particle can be induced in various ways, in particular from a kinetic mixing interaction or through the Stueckelberg mechanism.
• We show that such a scenario could lead in specific cases to an observable gamma-ray line.