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.
