Selection Rule for Enhanced Dark Matter Annihilation
  We point out a selection rule for enhancement (suppression) of odd (even) partial waves of dark matter coannihilation or annihilation using Sommerfeld effect. Using this, the usually velocity-suppressed p-wave annihilation can dominate the annihilation signals in the present Universe. The selection mechanism is a manifestation of the exchange symmetry of identical incoming particles, and generic for multi-state DM with off-diagonal long-range interactions. As a consequence, the relic and late-time annihilation rates are parametrically different and a distinctive phenomenology, with large but strongly velocity-dependent annihilation rates, is predicted. 
Anirban Das (Tata Inst., Mumbai) and Basudeb Dasgupta (Tata Inst.,   Mumbai)
hep-ph astro-ph.CO astro-ph.HE