Search for Relic Neutralinos with Milagro

The neutralino, the lightest stable supersymmetric particle, is a strong theoretical candidate for the missing astronomical ”dark matter”. Depending on their annihilation cross section, relic neutralinos from early formation of the Universe trapped in orbits around massive objects may currently be annihilating at measurable rates. The Minimal Supersymmetric extension of the Standard Model predicts that the gamma rays emerging from one of the annihilation modes will give a distinct monochromatic signal with energy between 100GeV and 10TeV, depending on the neutralino mass. An additional ”continuum” spectrum signal of photons will be produced by the decay of secondaries produced in the non-photonic annihilation modes. Milagro is an air shower array which uses the water Cherenkov technique and is capable of detecting TeV gamma rays from the direction of the Sun with an angular resolution of less than a degree. It is the first instrument capable of establishing a limit on the gamma-ray flux from neutralino annihilations near the Sun. In this report results of a search for neutralino to photon annihilation with the Milagro gamma-ray observatory are presented. Results of a Monte Carlo computer simulation of the neutralino annihilation density in the Solar System suggest that a large portion of neutralino annihilations (40− 50%) happens outside the Sun which may give rise to a detectable gamma-ray signal from the solar region. No significant gamma-ray signal was observed from the Sun resulting in an upper limit on the sought for photon flux. The upper limit can be translated to a neutralino-mass dependent limit on the product of the neutralino-proton scattering crossection σpχ, the integrated photon yield per neutralino in neutralino-neutralino annihilation bγ and the local galactic halo dark matter density ρ0. For example, assuming a 1TeV neutralino and ignoring the continuum contribution to the signal gives an upper limit of ρ0 0.3 (GeV cm−3) σpχ 10−41 (cm2) bγ < 2.3.