Substructure boosts to dark matter annihilation from Sommerfeld enhancement

Abstract

The recently introduced Sommerfeld enhancement of the dark matter annihilation cross section has important implications for the detection of dark matter annihilation in subhalos in the Galactic halo. In addition to the boost to the dark matter annihilation cross section from the high densities of these subhalos with respect to the main halo, an additional boost caused by the Sommerfeld enhancement results from the fact that they are kinematically colder than the Galactic halo. If we further believe the generic prediction of CDM that in each subhalo there is an abundance of substructure which is approximately self-similar to that of the Galactic halo, then I show that additional boosts coming from the density enhancements of these small substructures and their small velocity dispersions enhance the dark matter annihilation cross section even further. I find that very large boost factors (10 5 to 10 9 ) are obtained in a large class of models. The implications of these boost factors for the detection of dark matter annihilation from dwarf Spheroidal galaxies in the Galactic halo are such that, generically, they outshine the background gamma-ray flux and are detectable by the Fermi Gamma-ray Space Telescope.

VERITAS deep observations of the dwarf spheroidal galaxy Segue 1

“…Such astrophysical enhancements, combined with particle physics enhancements, have been proposed as an explanation for the cosmic ray lepton anomalies [43,52,53]. Finally, decaying dark matter models have also been suggested to explain the lepton excesses and are good alternatives to annihilating dark matter models [54][55][56][57][58].…”

Cosmic X-ray and gamma-ray background from dark matter annihilation

“…[12,41]). For the purposes of this work, we follow the description of [17] and mention that the enhancement S(σ vel ) to the s-wave contribution to the annihilation rate is given by:…”

“…[10][11][12]40]), restricting it to the case where the interaction between WIMPs prior to annihilation is mediated by a scalar boson of mass m φ through a Yukawa potential with coupling constant α c (e.g. [41]). This case encompasses the large majority of the models that are typically used in the literature to account for the Sommerfeld enhancement, including the most common of these where S ∝ 1/σ vel (the so-called "1/v" boost).…”

Indirect searches for dark matter

“…C) Thanks to the Sommerfeld enhancement (see [25,159,160], and then [161,162,166]), a non-perturbative effect which modifies the annihilation cross section in the regime of small relative velocity of the annihilating particles and in presence of an effectively long-range force between them. Indeed this well known quantum mechanical effect, first discussed by Sommerfeld in the context of positronium…”