Velocity distributions and annual-modulation signatures of weakly-interacting massive particles

An annual modulation in the event rate of the NaI detector of the DAMA coll aboration has been used to infer the existence of particle dark matter in t he Galactic halo. Bounds on the WIMP mass and WIMP-nucleon cross section ha ve been derived. These analyses have assumed that the local dark-matter vel ocity distribution is either isotropic or has some bulk rotation. Here we c onsider the effects of possible structure in the WIMP velocity distribution on the annual-modulation amplitude. We show that if we allow for a locally anisotropic velocity dispersion tensor, the interpretation of direct detec tion experiments could be altered significantly. We also show that uncertai nties in the velocity distribution function that arise from uncertainties i n the radial density profile are less important if the velocity dispersion is assumed to be isotropic.