Direct Detection of Dark Matter


A leading hypothesis on the nature of Dark Matter is that it is comprised of Weakly Interacting Massive Particles, or WIMPs, which were produced moments after the Big Bang. If WIMPs are the dark matter, then their presence in our galaxy may be detectable via scattering from atomic nuclei in detectors as shown in these cartoons (not to scale):

       

The Cryogenic Dark Matter Search (CDMS) uses silicon and germanium "solid state" detectors that are cooled close to absolute zero, and are sensitive to very small temperature changes when a dark matter particle transfers energy to a nucleus. In the top image, a WIMP scatters from a germanium nucleus and the deposited recoil energy slightly warms the detector, producing vibrations of the crystal lattice, or phonons, that are sensed with superconducting thermometers. Since the detector material is a semiconductor, electron-hole pairs are also produced and measured with charge-sensitive amplifiers.

The LUX-ZEPLIN (LZ) detectors are filled with liquid xenon, which produces small flashes of scintillation light as well as some fully ionized atoms. As shown in the bottom image, ionization electrons are drifted upward and produce a second flash of light when the electrons are pulled into the xenon gas above the liquid surface. The light signals measured with photomultiplier tubes, and convey the event type and location. 

In both CDMS and LZ, the measurement of two independent signals provides crucial information in distinguishing between the rare events from WIMPs (no convincing signal has yet been seen!) and the higher rate of events from radioactivity in the environment and detector and from cosmic rays. The materials used in the detectors and the surrounding shields are carefully selected to minimize radioactivity, and the experiments themselves are located deep underground to shield from cosmic rays.

In addition to the event type (nuclear scatter) and measured energy, further information that can help to confirm a WIMP signal occurs due to the relative motion of Earth through the dark matter halo. An annual modulation is expected to produce about a percent variation since we move slightly faster through the dark matter in June and slightly slower in December. A daily modulation could also be detected if the direction of the recoiling nucleus is measured --- but that is the quarry of future technology.