In the search for dark matter, among the most interesting candidates is the neutralino, a neutral particle, predicted in supersymmetric extensions of the standard model, which interacts only weakly with other matter. Since the neutralino is expected to be stable, it may be possible to find particles that are relics of the early universe.
Theorists have predicted that the sun’s gravity can trap neutralinos, which could collect in its center and then annihilate each other. The standard-model particles created by these annihilations could subsequently decay, producing high-energy neutrinos that could escape from the sun and be detected on earth. Based on searches for these neutrinos, the IceCube Collaboration has now reported in Physical Review Letters new limits on neutralino annihilations in the sun.
The IceCube neutrino detector is located between and beneath the Antarctic ice, to reduce background events from cosmic rays. When muon neutrinos from the sun interact with the ice, they create relativistic charged particles (muons and showers of hadrons) that produce Cherenkov light, which is picked up by the detector. In an experiment lasting more than three months, no excess of neutrinos from the direction of the sun was detected. The experimentalists have therefore placed stringent limits on neutralino annihilations in the sun—a factor of improvement over some previous limits—and from these, limits on the cross section for neutralino-proton interactions for neutralinos with masses above . These results narrow the possibilities for dark matter. – Stanley Brown