Cosmological Mysteries Part 2

Some notes on the experimental side of dark matter (Part 2 of 3):

  • Supersymmetry: A symmetry relating fermions and bosons in which every “ordinary” particle has a corresponding “superpartner” which differs in spin by half a unit. This theory attempts to find a common explanation for forces.

  • One possible explanation for dark matter is WIMPs (weakly interacting massive particles). A possible candidate for WIMPs are supersymmetric particles. These particles have a very low probability of interaction (less than that of the lighter neutrino). Fortunately supersymmetric particles interact with baryons via nuclear recoils (billiard-ball interactions).

  • Current detectors can detect ~1 recoil per kg per year. To detect a WIMP a ~1 recoil per ton per year is required.

  • There are a number of problems in creating a detector sensitive enough to detect WIMPs. The primary problem is background radiation. This radiation can come from cosmic sources, radon from the ground, naturally occurring isotopes in metals used for constructing the detector, and even potassium 40 found in human sweat. What’s most unfortunate is that WIMPs are low energy which corresponds directly with this background radiation.

    Interesting factoid: radon pools if ventilation is not adequate. Radon levels were ~2x greater than recommended in the underground “red button” rooms of France’s nuclear agency due to poor air circulation. These are the unknown casualities of the cold war.

  • CDMS (Cryogenic Dark Matter Search) is a detector for WIMPs. It is housed in the Soudan underground laboratory.

  • DRIFT is an experiment to eliminate background radiation by exploting properties of the WIMP “wind”. It is assumed that WIMPs are isotropic throughout the universe. Because the earth and sun are moving through the galaxy, there should be a “wind” of WIMPs. The wind will have a particular signature due to the rotation of the earth about the sun and the earth about its axis (For example during July the earth will be moving in the direction of the motion of the sun through the galaxy whereas in December it will be moving in the opposite direction). This signature caused by the WIMP wind should be different than that seen by any other background radiation.

  • MINOS (Main Injector Neutrino Oscillation Search) involves a gun at Fermi that shoots neutrinos at a detector in the Soudan mine. This will be the first attempt to detect neutrinos from a source other than cosmic rays.

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