Light Dark Matter
  We show how light spin-1/2 or spin-0 particles may be acceptable Dark Matter candidates, provided they annihilate sufficiently strongly through new interactions, such as those induced by a new light neutral spin-1 boson U. The corresponding interaction is stronger than weak interactions at lower energies, but weaker at higher energies.   Annihilation cross sections of (axially coupled) spin-1/2 Dark Matter particles, induced by a U vectorially coupled to matter, are the same as for spin-0 particles. In both cases, the cross sections (sigma v_rel/c) into e+e- automatically include a v_dm^2 suppression factor, as desirable to avoid an excessive production of gamma rays from residual Dark Matter annihilations. We also relate Dark Matter annihilations with production cross sections in e+e- reactions. As for spin-0, light spin-1/2 Dark Matter particles annihilating into e+e- could be responsible for the bright 511 keV gamma ray line observed by INTEGRAL from the galactic bulge. 
P. Fayet