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Searching for WIMPs

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].

next up previous
Next: Gas Detectors Up: WIMPs Previous: WIMPs

Dave Spergel
Wed Mar 6 14:02:15 EST 1996