Temporal distortion of the annual modulation signal of weakly interacting massive particles at low recoil energies

We show that the main features of the annual modulation of the signal expected in a WIMP direct detection experiment, i.e., its sinusoidal dependence with time, the occurrence of its maxima and minima during the year and (under some circumstances) even the one-year period, are modified for anisotropic galactic halos. The most relevant effect is a peculiar distortion of the time-behaviour at low recoil energies for tangential anisotropies. The observation of these effects could provide a direct way of measuring (or setting limits on) the local degree of anisotropy of the velocity distribution of the dark matter particles. (C) 2003 Published by Elsevier B.V.