A Selection Rule for Enhanced Dark Matter Annihilation

Self-interacting dark matter is a promising candidate for solving the small-cale problems in structure formation. A natural consequence of having self-interactions among dark matter particles is the Sommerfeld effect which can affect the annihilation rate of dark matter. And thereby changing the indirect detection signal in the present Universe by a few orders of magnitude. Usually the s-wave annihilation process is considered to calculate the expected signal from various astro-physical sources. I will show that in a class of dark matter models, it is possible to have the p wave process dominate over the s-wave part due to a selective Sommerfeld enhancement in one case and suppression in the other. Finally I will demonstrate this mechanism in a two-level dark matter model.