WIMP direct detection and halo structure

Weakly Interacting Massive Particle (WIMP) direct detection experiments are just reaching the sensitivity required to detect Galactic dark matter in the form of neutralinos (or indeed any stable weakly interacting particle). Detection strategies and data analyses are often based on the simplifying assumption of a standard spherical, isothermal halo model, but observations and numerical simulations indicate that galaxy halos are in fact triaxial and anisotropic, and contain substructure. The annual modulation and direction dependence of the event rate (due to the motion of the Earth) provide the best prospects of distinguishing WIMP scattering from background events, however these signals depend sensitively on the local WIMP velocity distribution. I briefly review the status of WIMP direct detection experiments before discussing the dependence of the annual modulation signal on astrophysical input, in particular the structure of the Milky Way halo, and the possibility that the local WIMP distribution is not smooth. 1. WIMPs a (very) brief introduction Any stable weakly interacting massive particle (WIMP) in thermal equilibrium in the early universe will generically have an interesting present day density, ΩWIMP ∼ O(ΩCDM) ≈ 0.3. Furthermore supersymmetry provides a natural WIMP candidate, the lightest supersymmetric particle, the neutralino. There are basically two methods of detecting WIMPs: indirect detection, which involves detecting the products of WIMP annihilation (γ, ν, p̄, e), and direct detection, which involves detecting the energy deposited in a detector due to elastic scattering of WIMPs on the detector nuclei. I will focus on WIMP direct detection. For a review of particle dark matter see e.g. Bergström (2000). 2. Direct detection signals Direct detection experiments are just reaching the sensitivity required to detect WIMPs. The expected event rates are very small ( O(10 − 10) counts kgday) and distinguishing a putative Weakly Interacting Massive Particle (WIMP) signal from backgrounds, such as neutrons from cosmic-ray induced muons or natural radioactivity, is crucial. The event rate depends on the velocity of the detector relative to the Galactic rest frame and the Earth’s motion (as shown in Fig. 1) provides two potential WIMP smoking guns. Firstly the event