University of Minnesota
School of Physics & Astronomy


Looking for a few good WIMPS

Prisca Cushman
Professor Prisca Cushman

Professor Prisca Cushman is looking for WIMPs, or Weakly Interacting Massive Particles, as co-spokesperson on the Cryogenic Dark matter Search (CDMS), which is located in the Soudan Mine in Northern Minnesota.

CDMS recently achieved the highest sensitivity of any detector looking for this highly sought-after Dark Matter candidate. There is a lot of indirect evidence for the existence of dark matter, and for it making up 80% of all the mass in the Universe, but no one has actually seen one interact with a detector, nor measured the mass of one WIMP.

Because WIMPs may only interact via the gravitational and weak forces they are extremely difficult to detect. The CDMS detector is made up five towers containing 6 germanium and silicon crystals, each about the size of a hockey puck. They are kept extremely cold to avoid thermal vibration, in order to detect the minuscule vibrations in the crystal matrix generated by a nucleus being "kicked" by a WIMP. In order to distinguish a WIMP kick from background particle interactions, the detector must be operated deep underground to shield from cosmic rays and encased in lead and polyurethane to shield from natural radioactivity in the surrounding rock. Even those muons which do make it down half a mile to the detector are tagged by a plastic scintillation detector, which then tells the experiment to ignore the event. Some radiation still makes it to the detectors, but by looking at how many electrons are shaken off the atom during the interaction, they can be distinguished from the expected WIMP signature.

CDMS finished a year-long run in August and the collaboration recently completed analyzing the data from that run. CDMS did not detect any WIMP signals, but set a new limit on WIMPs in our galaxy. Cushman says we should not be surprised that CDMS still has not detected a WIMP. The limit set by this run only eliminates models that predict high densities of dark matter particles in our neighborhood. Most models say that we will have to wait another couple years before one or two WIMPs interact with the CDMS detector (out of the billions that pass through each second without interacting). "Meanwhile, seeing zero events means that our background is under control and ensures our success in identifying the WIMPs when they do appear.”

Detection of WIMPs is only the first step. Next comes measuring their properties, like mass, and understanding the distribution of WIMPs in our galaxy, i.e., WIMP Astronomy. If the WIMP mass is not too high, then these particles could also be created in proton collisions when the Large Hadron Collider turns on later this year in Switzerland. That would link the dark matter particles observed by CDMS to new symmetries in nature, going a long way toward understanding how to unify the fundamental forces of nature using theories of supersymmetry.

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