Gamma Rays Carry No Trace of Dark Matter
Dark matter is thought to make up 25% of the mass of the universe, but clear evidence of what constitutes it has not yet been obtained. A number of major experimental efforts are currently devoted to unfolding this mystery. An indirect approach is to look for gamma rays that could result from the annihilation of dark matter particles colliding with their antiparticles. The dwarf spheroidal galaxies, small galaxies near the Milky Way, are ideal regions in which to look for this radiation: they contain a high ratio of dark matter to normal matter and the background gamma radiation from baryonic processes is expected to be low.
In 2010, the Fermi Large Area Telescope (LAT) collaboration reported limits on dark matter particles obtained from their observations of the dwarf galaxies. Now the same collaboration reports in Physical Review Letters a new study based on an updated data set, which includes data from more galaxies, involves a joint likelihood analysis of the data from all the galaxies at once (rather than an analysis that studies each galaxy separately), and takes account of uncertainties in the dark matter distribution. The team has not observed any gamma-ray signal indicative of dark matter, and as a result, is able to place strong limits on the cross section for dark matter particle annihilation. Specifically, these limits strongly disfavor the existence of weakly interacting massive particles (WIMPs) with a purely s-wave annihilation cross section and masses less than around 30GeV.
An independent analysis, also reported in Physical Review Letters, by Alex Geringer-Sameth and Savvas Koushiappas of Brown University, Rhode Island, comes to generally similar conclusions. Geringer-Sameth and Koushiappas focused on a different selection of the Fermi-LAT data and used a different statistical method and treatment of the background.
The limits presented in these papers are among the strongest dark matter limits obtained to date. – Stanley Brown