SUSY dark matter in the universe—theoretical direct detection rates

Exotic dark matter, together with the vacuum energy (associated with the cosmological constant), seems to dominate in the Universe. An even higher density of such matter seems to be gravitationally trapped in the Universe. Thus, its direct detection is central to particle physics and cosmology. Currently fashionable supersymmetric models provide a natural dark matter candidate that is the lightest supersymmetric particle (LSP). Such models, combined with fairly well understood physics like the quark substructure of the nucleon and the nuclear structure (form factor and/or spin response function) permit the evaluation of the event rate for LSP-nucleus elastic scattering. The thus obtained event rates are, however, very low or even undetectable. Therefore, it is imperative to exploit the modulation effect, i.e., the dependence of the event rate on the Earth's annual motion. Also, it is useful to consider the directional rate, i.e., its dependence on the direction of the recoiling nucleus. In this paper, we study such a modulation effect in both nondirectional and directional experiments. We calculate both the differential and the total rates using both isothermal, symmetric as well as only axially asymmetric, and nonisothermal, due to caustic rings, velocity distributions. We find that, in the symmetric case, the modulation amplitude is small. The same is true for the case of caustic rings. The inclusion of asymmetry, with a realistic enhanced velocity dispersion in the galactocentric direction, yields an enhanced modulation effect, especially in directional experiments.