Detecting halo weakly interacting massive particles with a cryogenic phonon detector

If dark matter in the halo is made of weakly interacting massive particles (WIMPs), it may be detected with a cryogenic phonon detector. Such a detector must have low energy threshold, high energy resolution, and, more importantly, good background rejection capability. It has been proposed that a good background rejection ratio can be obtained by a simultaneous measurement of phonons and ionization. To test this experimentally, we have built a 60 g Ge detector and have measured simultaneous phonon and ionization signals, each with 4 keV energy resolution. We present a simple estimate of the rejection ratios, and WIMP event rates that may be achieved by such a detector.