The 130 GeV gamma-ray line and Sommerfeld enhancements

TL;DR

This paper proposes that Sommerfeld enhancement with scalar force-carriers can explain the large gamma-ray line signals from dark matter annihilation, while maintaining the correct relic density and reducing coupling requirements.

Contribution

It introduces a model where Sommerfeld enhancement explains gamma-ray lines and relic density simultaneously, addressing previous cross-section and coupling issues.

Findings

Sommerfeld enhancement can increase annihilation cross-section significantly.

Scalar force-carriers enable p-wave annihilation to dominate at freeze-out.

Couplings can be reduced by an order of magnitude under relic density constraints.

Abstract

Recently, possible indications of line spectral features in the Fermi-LAT photon spectrum towards the galactic center have been reported. If the distinct line features arise from dark matter (DM) annihilation into $\gamma X (X=\gamma, Z^{0} or h^{0})$, the corresponding annihilation cross-section is unnaturally large for typical loop-induced radiative processes. On the other hand, it is still too small to be responsible for the observed DM relic density. We show that the mechanism of Sommerfeld enhancement with scalar force-carrier can provide a simple solution to these puzzles. The possibly large Sommerfeld enhancement of the cross-section for s-wave DM annihilation can significantly reduce the required effective couplings between DM and charged particles in typical loop diagrams. The DM particles necessarily annihilate into scalar force-carriers through tree-level p-wave process, which can dominate the total DM annihilation cross-section at freeze out, resulting in the correct thermal relic density, but has subdominant contributions to the DM annihilation today due to velocity suppression. We perform detailed analysis on the effects of p-wave Sommerfeld enhancement on freeze out. The results show that with the constraints from the thermal relic density, the required effective couplings can be reduced by an order of magnitude.