Galactic Positron Excess from Selectively Enhanced Dark Matter Annihilation

TL;DR

This paper proposes a dark matter annihilation model with velocity-dependent enhancement to explain the positron excess observed in cosmic rays, fitting the data without conflicting with existing astrophysical constraints.

Contribution

It introduces a novel velocity-dependent annihilation mechanism using p-wave Sommerfeld enhancement to account for the positron excess.

Findings

Dark matter annihilation with specific parameters fits the positron excess data.

Velocity-dependent enhancement allows compatibility with CMB and dwarf galaxy constraints.

Potential signatures in future experiments are outlined.

Abstract

Precision measurements of the positron flux in cosmic ray have revealed an unexplained bump in the spectrum around $E\simeq 300\,\mathrm{GeV}$, not clearly attributable to known astrophysical processes. We propose annihilation of dark matter of mass $m_\chi = 780\,\mathrm{GeV}$ with a late-time cross section $\sigma v = 4.63\times 10^{-24}\,\mathrm{cm^3\,s^{-1}}$ as a possible source. The nonmonotonic dependence of the annihilation rate on dark matter velocity, owing to a selective $p$-wave Sommerfeld enhancement, allows such a large signal from the Milky Way without violating corresponding constraints from CMB and dwarf galaxy observations. We briefly explore other signatures of this scenario, and outline avenues to test it in future experiments.