Bounds on Cross-sections and Lifetimes for Dark Matter Annihilation and Decay into Charged Leptons from Gamma-ray Observations of Dwarf Galaxies

TL;DR

This paper sets conservative gamma-ray flux bounds on dark matter annihilation and decay into charged leptons using dwarf galaxy observations, assessing implications for explaining cosmic-ray excesses and predicting Fermi telescope signals.

Contribution

It provides new constraints on dark matter properties from gamma-ray data and explores the impact of Sommerfeld enhancement on these bounds.

Findings

Constraints are generally unable to exclude dark matter explanations for PAMELA and ATIC/PPB-BETS excesses.

Sommerfeld enhancement could make dark matter annihilation cross-sections consistent with observational bounds.

Fermi has promising prospects to detect gamma-ray signals from Segue 1 if dark matter explains the cosmic-ray excesses.

Abstract

We provide conservative bounds on the dark matter cross-section and lifetime from final state radiation produced by annihilation or decay into charged leptons, either directly or via an intermediate particle $\phi$. Our analysis utilizes the experimental gamma-ray flux upper limits from four Milky Way dwarf satellites: HESS observations of Sagittarius and VERITAS observations of Draco, Ursa Minor, and Willman 1. Using 90% confidence level lower limits on the integrals over the dark matter distributions, we find that these constraints are largely unable to rule out dark matter annihilations or decays as an explanation of the PAMELA and ATIC/PPB-BETS excesses. However, if there is an additional Sommerfeld enhancement in dwarfs, which have a velocity dispersion ~10 to 20 times lower than that of the local Galactic halo, then the cross-sections for dark matter annihilating through $\phi$'s required to explain the excesses are very close to the cross-section upper bounds from Willman 1. Dark matter annihilation directly into $\tau$'s is also marginally ruled out by Willman 1 as an explanation of the excesses, and the required cross-section is only a factor of a few below the upper bound from Draco. Finally, we make predictions for the gamma-ray flux expected from the dwarf galaxy Segue 1 for the Fermi Gamma-ray Space Telescope. We find that for a sizeable fraction of the parameter space in which dark matter annihilation into charged leptons explains the PAMELA excess, Fermi has good prospects for detecting a gamma-ray signal from Segue 1 after one year of observation.