Astrophysicists are still trying to make sense out of an excess of positrons recently revealed in cosmic-ray data. One possible explanation is that these antielectrons result from dark matter decays or annihilations. A new analysis shows that certain dark matter models are constrained by the smoothness of the positron energy spectrum. In Physical Review Letters, the authors place the strictest limits yet on potential annihilation scenarios for dark matter particles.
Many cosmologists assume that the dark matter is a weakly interacting particle with a mass between and GeV/c (where the proton mass is about GeV/c). These particles may decay or self-annihilate through collisions, producing positrons and other secondary particles. Such a mechanism could explain the observed rise in the fraction of cosmic-ray positrons above GeV. However, it’s also possible that the detected positron excess comes from nearby pulsars.
Even if dark matter is not the main source of the positron excess, it may be contributing on a small level. That is the assumption of Lars Bergström from Stockholm University, Sweden, and his colleagues, who have looked for a possibly hidden dark matter signal in the positron spectrum from the Alpha Magnetic Spectrometer (AMS) onboard the International Space Station (see 3 April 2013 Viewpoint) This predicted signal would be a slight bump in the spectrum near the energy corresponding to the mass of the dark matter particle. Not finding any bumps in the AMS data, the researchers determine upper limits for dark matter annihilation parameters. These constraints are tightest at the low-energy range, where the AMS error bars are smaller. The results call into question the viability of low mass (around GeV/c) dark matter candidates. – Michael Schirber