WIMP Dark Matter Detection

Millenium Dark Matter Simulation

Dark matter is all around us

Dark matter dominates structure formation in the Universe. Our galaxy is in the middle of a huge dark matter halo, indeed it would not have formed without it. Thus we might be able to establish the prescence of the dark matter particles zooming through our laboratory (and going through the Earth) by detecting the rare interactions between them and our target in a sensitive low-background experiment. Dark matter particle models that predict these type of detectable interactions are generally called Weakly Interacting Massive Particles, or WIMPs.

WIMPs are a very well motivated dark matter model, as it arrises naturally from many supersymmetric theories which predict that the lightest supersymmetric particle (LSP) is stable. If this particle exists and is neutral, i.e. it is a neutralino, a sneutrino, or a gravitino, it could be the dark matter. If it is a gravitino, its cross section with the standard model is severely suppressed and direct detection would be unlikely. If the dark matter is a neutralino or a sneutrino, however, and the cross section to the standard model is sufficiently high, direct detection in the laboratory is possible.

Direct detection requirements

Neutral WIMPs ineract with our detectors through a nuclear recoil. Integrating the differential recoil spectrum we find that the expected rate in a dark matter detector is less than one event per kg of detector per year. Backgrounds in a normal laboratory are millions of times higher. Thus, we operate our detectors far underground to shield from cosmic rays, and use extensive shielding around our detector to shield from the radioactivity in the laboratory. Furthermore, the purity of the materials used for the detector, its housing, any nearby electronics, etc. must be very tightly controlled.

Since the vast majority of backgrounds are photons and electrons and thus produce electron recoils, a dark matter detector than can discriminate between electron and nuclear recoils will have better background control. A massive target is clearly preffered. Lower energy thresholds will increase the sensitivity of the experiment to lighter dark matter candidates, since the recoil spectrum gets steeper with lower dark matter mass.

In summary, when designing a dark matter direct detection experiment, the requirements are:

  1. Low background rate
  2. Large exposure (mass × time)
  3. Low energy threshold
  4. Discrimination between signal and backgrounds

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