While WIMPs interact weakly, they are potentially
detectable [31,73,37].
The flux of WIMPs through an
experiment is quite large:
10
(m/GeV)
cm
s
. The
difficulty lies in detecting the rare WIMP
interactions with ordinary matter.
The challenge for dark matter experimenters
is to design an experiment that is simultaneously
sensitive to few keV energy depositions and has
a large mass (many kilograms) of detector material.
The experiment must also have
superb background rejection as the expected
event rate, less than an event/kilogram/day, is
far below most backgrounds. There are two
potentially experimental signatures that can
aid in the WIMP search: a roughly 10% annual
modulation of the event rate due to the Earth's
motion around the Sun [24]
and a large (
) asymmetry in
the direction of the WIMP flux due to the
Sun's motion through the galactic halo [63].
The first generation of WIMP experiments were
rare-event experiments that were adapted to
search for dark matter. The first set of experiments
were ultra-low background
germanium semiconductor experiments [1,16,54]
that
were developed as
double beta-decay experiments and modified into
dark matter detectors. In these experiments,
a recoiling Ge nucleus produces 
-hole pairs
that are detectable down to recoil energies 
keV.
These experiments have been limited by microphonics,
electronic noise, and by cosmogonic radioactivity.
We are now entering the era of second generation experiments that have been designed primarily as dark matter detectors. In this section, I will highlight several of the promising experimental technologies.
The Heidelberg-Moscow
germanium experiment is
a modification
of the early germanium experiments. It consists of
6 kilogram of purified 
Ge detector in Gran Sasso Tunnel.
Since it does not contain 
Ge, it has a reduced cosmogonic
background. In this experiment,
electronics and microphonics are the dominant background.
This experiment places the best current limits
on the halo density of WIMPs more massive than 50 GeV [11].
Rather than detecting the electron-hole pairs produced by recoiling nuclei, the Stanford silicon experiment [76] detected the ballistic phonons produced by recoiling silicon nuclei. This experiment has been calibrated by neutron bombardment. The Munich group is developing a silicon detector that will detect the ballistic photons with an SIS junction [48].
At Berkeley, the CfPA group is developing a
detector that is sensitive to
both phonon and electron-hole pairs. This
dual detection allows much better background rejection as
electrons excited by radioactive decays have a different
photon and electron-hole pair signature than nuclear recoils.
Neutron bombardment experiments suggest that this dual
detection technique can reject
of radioactive background [58].
A more massive experiment that utilizes this
technique has the potential to probe into
interesting region of parameter space in supersymmetric
theories.
Several groups are developing scintillators that
are potential WIMP detectors. There
are several scintillator experiments currently
under development:
a 36.5 kg NaI experiment in Osaka that has
begun to place interesting limits on
heavy neutrinos [27,26];
the Rome/Beijing/Saclay experiment [13], a smaller detector,
with sensitivities similar to the Osaka experiment;
and a Munich sapphire scintillator experiment
that is designed to be sensitive to
low mass (m < 10 GeV) WIMPs.
This technology has several advantages
over the germanium and silicon semiconductors;
the material is
sensitive to spin-dependent coupling (although,
this is now thought to be less important
for supersymmetric dark matter detection [37])
and it is relatively easy to build very large mass
detectors.
The challenge for these experiments is
to improve their background rejection. Spooner
& Smith [65] suggest that it might be possible
to have
some rejection of radioactive 
's in these
NaI scintillators
through measurements of UV and VIS signatures of recoils
[65].