In this chapter we discuss the prospects for detecting dark matter (DM) by means of the model-independent annual modulation signature, using large-mass highly radiopure NaI(Tl) detectors at the Gran Sasso National Laboratory of the INFN.
The annual modulation signature and the target material
A model-independent approach is necessary in order to find the presence of DM particles in the Galactic halo. In principle, two main possibilities exist; they are based on the correlation between the distribution of the events, detected in a suitable underground set-up, and the Galactic motion of the Earth.
The first one (which mainly applies just to WIMP or WIMP-like DM candidates) correlates the direction of WIMP-induced nuclear recoils with that of the Earth's velocity. This directionality signature is, however, difficult to exploit in practice, mainly because of technical difficulties in reliably and efficiently detecting the short recoil track and in realizing suitably large mass detectors; this will be discussed elsewhere in this volume.
Another possibility is the DM annual modulation signature, which is the only feasible one at present; it is sensitive to wide ranges both of DM candidates and of interactions, and it is also able to test a large interval of cross-sections and of halo densities. This was originally suggested in the 1980s in refs. Such a signature exploits the effect of the Earth's revolution around the Sun on the number of events induced by the DM particles in a suitable low-background set-up placed deep underground.