Diurnal and annual modulation of cold dark matter signals

Abstract

We calculate the diurnal and annual modulation of the signals in axion and weakly interacting massive particle (WIMP) dark matter detectors on Earth caused by a cold flow of dark matter in the solar neighborhood. The effects of the Sun's and the Earth's gravity, and of the orbital and rotational motions of the Earth are included. A cold flow on Earth produces a peak in the spectrum of microwave photons in cavity detectors of dark matter axions, and a plateau in the nuclear recoil energy spectrum in WIMP detectors. Formulas are given for the positions and heights of these peaks and plateaux as a function of time in the course of day and year, including all corrections down to the 0.1% level of precision. The results can be applied to an arbitrary dark matter velocity distribution $f(\stackrel{\ensuremath{\rightarrow}}{v})$ by integrating the one-flow results over velocities. We apply them to the set of flows predicted by the caustic ring model of the Galactic halo. The caustic ring model predicts the dark matter flux on Earth to be largest in December $\ifmmode\pm\else\textpm\fi{}$ one month. Nonetheless, because of the role of energy thresholds, the model is consistent with the annual modulation results published by the DAMA Collaboration provided the WIMP mass is larger than approximately 100 GeV.