What to look for?

So far we have observed dark matter only through its gravitational interaction on very large scales. To better understand the nature of dark matter we are searching for a more direct signal from dark matter particles through their interaction with ordinary matter.

We may have a chance to observe such interactions since dark matter is part of our own galaxy where it is more or less smoothly distributed. We can therefore expect the dark matter particles to cross the path of the earth and interact with the detectors we set up for dark matter detection.

The interaction rate of dark matter particles with ordinary matter depends on the mechanism by which they interact. However, the effect that an interaction would have depends (to first order) only on the velocity distribution of the particles and on their mass. The velocity distribution - which can be estimated from the fact that the particles are gravitationally bound to our galaxy - is centred about a few hundred km/s. The mass of WIMPs is expected to be in the range between a few and a few thousands of GeV/c² (for comparison: atoms have masses between one (hydrogen) and ~240 GeV/c² (uranium)).

Atoms, the constituents of ordinary matter, consist of a (heavy) nucleus surrounded by light electrons. With the kinematic information from the previous paragraph (velocity and mass of the WIMPs) we can calculate the expected energy distribution of events from interacting dark matter. We find that the typical energy transferred to an electron would be rather small (of order of a few tens of eV) while the typical energy transferred to atomic nuclei would be in the range of a few tens of keV. The measurement of tens of keV is challenging, but definitely possible, while it would be extremely difficult to search for interactions in the range of a few tens of eV, especially when using large detectors. Therefore experiments searching for direct signals from dark matter WIMPs typically look for recoiling atomic nuclei (nuclear recoils) as opposed to electron recoil events.

Main experimental challenges

The first challenge is the low expected interaction rate. As discussed in the previous chapter we can get a very rough estimate of this rate from the total abundance of dark matter in the universe. This estimate gives rates as low as a few interactions per year in several hundred kg of detector material. If more sophisticated theories are applied we find variations of this rate by a few orders of magnitude, but even in the most optimistic scenarios the rates are rather low (a few events per kg per year).

The second challenge is that all kinds of other particles also interact with the detector material, typically at rates much higher than that expected from WIMP interactions; these interactions that are not from WIMPs are called background. The sources of such particles are the cosmic radiation and radioactive materials in the surrounding of the experiment, the experimental set up or even the detector material itself. To get an idea of the magnitude of this problem we just have to keep in mind that in a cubic meter of regular room air we have typically a few tens of radioactive decays per second. If unprotected, an experiment search for WIMP interactions would be completely swamped by the natural radioactivity and would have no chance to observe any interesting signal.

Experiments

To reduce the background from cosmic radiation, all experiments searching for direct interactions of dark matter WIMPs are located in underground laboratories, laboratories located typically a few hundred to several thousand meters below the Earth surface. To reduce the background from environmental radioactivity the experiments are surrounded by lots of shielding material. All materials for constructing the experimental setup are carefully screened to make sure they do not contain too much radioactivity which would compromise the measurement. Keep in mind that almost all materials contain trace amounts of radioactivity which are so small that they are not usually of any concern, but could be devastating for these extremely sensitive experiments. Finally the detector material itself has to be selected carefully and kept under the cleanest possible conditions during the construction and commissioning of the experiment.

In the end all the care taken to produce an ultra-clean setup is not sufficient and an additional means of distinguishing between the background and the expected signal is necessary. The key to this discrimination is that (as discussed above) the WIMP interactions these experiments are looking for are interactions with the atomic nuclei, while most of the background radiation interacts primarily with the electrons in the material. This can produce different types of signals which potentially allow a very efficient discrimination. Different experiments choose different techniques to accomplish this goal. Often the experiments measure two different signals where the ratio of the signal amplitudes depends on the type of interaction. In other experiments signals from nuclear recoils (signal) and electron recoils (background) show a different pattern in time and can thus be distinguished.

Dealing with the background is the most challenging point in this field of physics and the success of an experiment depends critically on its ability to handle this problem.

Below you find a table with an (incomplete) list of dark matter experiments. They are grouped by the technology they are using and their status (past/present, upcoming and further in the future). For more details about each experiment please refer to their respective web pages or publications.