Sub-MeV Bosonic Dark Matter, Misalignment Mechanism and Galactic Dark Matter Halo Luminosities

摘要

We explore a scenario that the dark matter is a boson condensate created bythe misalignment mechanism, in which a spin 0 boson (an axion-like particle)and a spin 1 boson (the dark photon) are considered, respectively. We find thatalthough the sub-MeV dark matter boson is extremely stable, the huge number ofdark matter particles in a galaxy halo makes the decaying signal detectable. Agalaxy halo is a large structure bounded by gravity with a typical$\sim10^{12}$ solar mass, and the majority of its components are made of darkmatter. For the axion-like particle case, it decays via $\phi\to \gamma\gamma$,therefore the photon spectrum is monochromatic. For the dark photon case, it isa three body decay $A'\to\gamma\gamma\gamma$. However, we find that the photonspectrum is heavily peaked at $M/2$ and thus can facilitate observation. Wealso suggest a physical explanation for the three body decay spectrum bycomparing the physics in the decay of orthopositronium. In addition, for bothcases, the decaying photon flux can be measured for some regions of parameterspace using current technologies.