The Search for Neutralino Dark Matter with the AMANDA Neutrino Telescope

Abstract

There is convincing indirect evidence based on cosmological data that approximately one quarter of the universe is made of dark matter. However, to this date there is no direct detection of the dark matter and its nature is unknown. Most theories suggest that this dark matter is made of Weakly Interacting Massive Particles (WIMPs), or more specifically: supersymmetric particles. The most promising candidate out of the supersymmetric particles is the lightest neutralino. These neutralinos can get trapped in the gravitational field of the Earth, where they accumulate and annihilate. The annihilation products decay and produce neutrinos (among other particles). These neutrinos (the focus is on muon-neutrinos here) can be detected with the AMANDA neutrino telescope located between one and two kilometers deep in the ice of the glacier near the South Pole.

Neutrinos cannot be detected directly. However, there is a small possibility that they interact with nuclei of the ice and create charged leptons. These charged leptons continue to travel in the same direction as the neutrinos (accompanied by electromagnetic/hadronic cascades, and  electrons). As long as their speed is higher than the speed of light of the ice, they emit Cherenkov radiation which can be captured by photomultipliers installed inside the ice. The signals collected by the photomultipliers can be used to reconstruct the track of the lepton.

AMANDA - the Antarctic Muon and Neutrino Detector Array - makes use of the unique properties of the neutrino: Since neutrinos interact only weakly, they can travel through the Earth without being stopped. Therefore all detected particles which have been identified as upward going (i.e. through the Earth coming) must have been produced by charged leptons originating from neutrinos after they reacted with the nuclei of the ice. All other particles which do not come from below are rejected.

If the neutrino flux coming from the neutralino annihilation inside Earth is strong enough to be detected with AMANDA, it should show up as an excess over the expected neutrino flux, which comes from the atmospheric neutrinos produced in the northern hemisphere. This analysis which used data from 2001 and 2002 showed that there is no significant excess, yielding an upper limit on the neutrino flux that could have come from WIMP annihilation.