Next Contents Previous


The experiments to reveal the nature and abundance of particle dark matter can be divided in two conceptually different approaches, direct and indirect detection. The physics underlying the direct detection technique is the elastic scattering of a WIMP with a nucleus of the detector. Therefore, the main observable is the deposited energy by the recoiling nucleus. For indirect detection the WIMPs first have to be accumulated in a gravitational potential to increase their density and therefore their annihilation rate. The annihilation products, high energy neutrinos, are then detected via their conversion to muons. Hence the signatures are muons originating from the centre of the earth or the sun, so called upward-going muons from these well defined sources.

Additional observables for the direct detection technique which can serve to increase the signal to noise ratio are either recoil-specific or WIMP-specific. The recoil-specific observables are e.g. pulse-shapes or the partitioning of energy release into ionization and phonon signals or scintillation and phonon signals (see Fig. 2 for a summary of applied techniques to reduce background).

Figure 2

Figure 2. Summary of existing and planned direct detection experiments, classified according to their ability to discriminate nuclear recoils. The numbers to the left indicate the applied background reduction technique which is given in the legend below. Note the variety of methods which gives a hint on the diverse experimental techniques and detector concepts involved in this fast evolving field of research.

WIMP-specific observables result from the assumption of the existence of a WIMP halo around our galaxy as motivated from galaxy rotation curves [8]. Such a halo would yield kinematic signatures for WIMP-detection. The movement of the sun through the halo (excluding a conspiracy of a co-rotating halo) induces an annual modulation of WIMP-rates in the detector because the earth velocity would add to the mean kinetic energy of WIMPs impinging on the detector in summer and subtract in winter [9]. The asymmetry of the WIMP-wind itself would also induce a diurnal modulation for a recoil-direction sensitive detector [10].

In order to rank the various background suppression mechanisms for direct detection one has to keep in mind that the powerful background discrimination via recoil-specific observables (factors of 100 and more have been reported, see e.g. [11, 12]) is systematically limited by neutron elastic scattering processes since these also produce nuclear recoils. The WIMP-specific observables are limited as well. First of all, the distribution function of WIMPs in the halo, see below, is unknown. Second, the annual modulation effect is small, of the order of a few percent. A recoil-direction sensitive detector would exploit the much larger diurnal modulations of the order of tens of percent modulations but detecting the tiny tracks from nuclear recoils is a formidable experimental task, see the DRIFT proposal below.

A comparison of indirect and direct detection methods has been worked out either model-independent or for a specific candidate particle [13, 14, 15]. A general feature of such a comparison is that indirect searches are more sensitive for large WIMP-masses and spin-dependent interactions (see below) than direct searches (see also [16]). Therefore these two approaches are complementary. Additionally, in case both techniques would give a consistent signal it would be possible to obtain in principle not only the approximate mass and elastic scattering cross section but also the annihilation cross section [17]. For more details about the indirect detection method, I refer to [16] and references therein. Note that in case of the neutralino as the dark matter particle candidate another complementarity between direct detection and accelerator experiments has been shown [18] (for a discussion, see [19]).

Now for the direct detection technique, it is worth summarising the main experimental requirements.

The exact dependencies of the direct detection technique on physical parameters can be extracted from eq. 1:

Equation 1 (1)

where dR/dQ is the measured quantity, the energy spectrum in rates over unit energy and unit detector mass. The other parameters can be classified as either completely unknown, like properties of the unknown WIMP, mass mW and elastic scattering cross section sigma0, or estimated input from astrophysics, like the local halo density rho0 (0.3-0.7 GeV/cc), escape velocity vmax (approx 600 km/s) from the galactic potential and the WIMP-halo distribution function f(v), often approximated by a Maxwellian distribution (see [20] and references therein). The last set of values represents the number of targets NT, target nucleus mass mN, reduced mass µ = (mWmN) / (mW + mN) and detector threshold Ethr. The form factor F2(Q) parametrises the loss of coherence for the WIMP-nucleus interaction as the WIMP-energy increases. It represents an input from nuclear physics and depends on the type of WIMP interaction considered, since the elastic scattering cross section has two distinct interaction channels, a scalar or spin-independent and an axial or spin-dependent channel (for details, see [5] and the discussion in [20]). Hence, depending on the spin-properties of the target nuclei, the appropriate form factor has to be used.

Next Contents Previous