Fermi/LAT observations of Dwarf Galaxies highly constrain a Dark Matter Interpretation of Excess Positrons seen in AMS-02, HEAT, and PAMELA

TL;DR

Fermi/LAT observations of dwarf galaxies impose strong constraints on dark matter explanations for the positron excess observed in AMS-02, HEAT, and PAMELA, significantly limiting viable annihilation channels.

Contribution

This study demonstrates that most dark matter annihilation channels cannot explain the positron excess due to Fermi/LAT constraints, leaving only the 4-muon channel as a viable option.

Findings

Most annihilation channels are ruled out by Fermi/LAT data.

Only the 4-muon channel remains compatible with observations.

Multichannel annihilation into quarks and leptons is largely excluded.

Abstract

It is shown that a Weakly Interacting Massive dark matter Particle (WIMP) interpretation for the positron excess observed in a variety of experiments, HEAT, PAMELA, and AMS-02, is highly constrained by the Fermi/LAT observations of dwarf galaxies. In particular, this paper has focused on the annihilation channels that best fit the current AMS-02 data (Boudaud et al., 2014). The Fermi satellite has surveyed the $\gamma$-ray sky, and its observations of dwarf satellites are used to place strong bounds on the annihilation of WIMPs into a variety of channels. For the single channel case, we find that dark matter annihilation into {$b\bar{b}$, $e^+e^-$, $\mu^+\mu^-$, $\tau^+\tau^-$, 4-$e$, or 4-$\tau$} is ruled out as an explanation of the AMS positron excess (here $b$ quarks are a proxy for all quarks, gauge and Higgs bosons). In addition, we find that the Fermi/LAT 2$\sigma$ upper limits, assuming the best-fit AMS-02 branching ratios, exclude multichannel combinations into $b\bar{b}$ and leptons. The tension between the results might relax if the branching ratios are allowed to deviate from their best-fit values, though a substantial change would be required. Of all the channels we considered, the only viable channel that survives the Fermi/LAT constraint and produces a good fit to the AMS-02 data is annihilation (via a mediator) to 4-$\mu$, or mainly to 4-$\mu$ in the case of multichannel combinations.