High Energy Positrons From Annihilating Dark Matter

Abstract

Recent preliminary results from the PAMELA experiment indicate the presence of an excess of cosmic ray positrons above 10 GeV. In this letter, we consider possibility that this signal is the result of dark matter annihilations taking place in the halo of the Milky Way. Rather than focusing on a specific particle physics model, we take a phenomenological approach and consider a variety of masses and two-body annihilation modes, including W+W-, ZZ, b bbar, tau+ tau-, mu+ mu-, and e+e. We also consider a range of diffusion parameters consistent with current cosmic ray data. We find that a significant upturn in the positron fraction above 10 GeV is compatible with a wide range of dark matter annihilation modes, although very large annihilation cross sections and/or boost factors arising from inhomogeneities in the local dark matter distribution are required to produce the observed intensity of the signal. We comment on constraints from gamma rays, synchrotron emission, and cosmic ray antiproton measurements.

Figures (1)

• Figure 1: The positron fraction as a function of energy for various dark matter masses, annihilation modes and diffusion parameters, compared to the background from secondary production alone (bottom line). In each frame, the annihilation rate was chosen to produce the best fit to the PAMELA data above 10 GeV. The required boost factor was calculated using our default values for the annihilation cross section ($\sigma v = 3 \times 10^{-26}$ cm$^3$/s) and the local dark matter density (0.35 GeV/cm$^3$).