A Search for Dark Matter Annihilation in the Milky Way Halo

TL;DR

This study searches for dark matter annihilation signals in the Milky Way halo using extensive Fermi data, setting new limits on dark matter properties and challenging some existing explanations for observed gamma-ray excesses.

Contribution

It provides the strongest constraints to date on dark matter annihilation below 70 GeV in the Milky Way halo, using a large dataset and foreground modeling.

Findings

Excluded dark matter masses below 70 GeV for the $b\bar{b}$ channel at 95% confidence.

Set new upper limits on the annihilation cross section in the Milky Way halo.

Challenged the dark matter interpretation of the Galactic Center gamma-ray excess.

Abstract

The Milky Way halo is the brightest source of dark matter annihilation on the sky. Indeed, the potential strength of the Galactic dark matter signal can supersede that expected from dwarf galaxies and galaxy groups even in regions away from the Inner Galaxy. In this paper, we present the results of a search for dark matter annihilation in the smooth Milky Way halo for $|b| > 20^\circ$ and $r < 50^\circ$ using 413 weeks of Fermi Pass 8 data within the energy range of $\sim$0.8-50 GeV. We exclude thermal dark matter with mass below $\sim$70 GeV that annihilates to $b\bar{b}$ at the 95% confidence level using the p6v11 cosmic-ray foreground model, providing the strongest limits on the annihilation cross section in this mass range. These results exclude the region of dark matter parameter space that is consistent with the excess of $\sim$GeV photons observed at the Galactic Center for the $b\bar{b}$ annihilation channel and, for the first time, start probing the $\tau^+\tau^-$ explanation. We explore how these results depend on uncertainties in the foregrounds by varying over a set of reasonable models.