Case Western Reserve University physicists and others from the Cryogenic Dark Matter Search experiment announced they have regained the lead in the worldwide race to find the particles that make up dark matter. The CDMS experiment, located a half-mile underground in a Minnesota mine, again sets the world's best constraints on the properties of dark matter candidates.
So far, a dark particle hit has not rung the crystal bell in the detectors in the mine. "We are the best in the world at finding nothing! But this also has major scientific implications about the presence of dark matter in the universe and how far scientists have to go in their search to find it. It tells us about which hypothetical models for dark matter can be ruled out or falsified," said Daniel Akerib, professor and chair of the physics department and member of the CDMS team.
Weakly Interacting Massive Particles, or WIMPs, are hypothetical particles that can be thought of as the glue that keeps all the matter in the universe together through their gravitational forces. They are leading candidates for the building blocks of dark matter, which accounts for 85 percent of the entire mass of the universe. Hundreds of billions of WIMPs may have passed through your body as you read these sentences.
This dark matter search comes under the direction of the Fermi National Accelerator Laboratory, with support from U.S. Department of Energy, the National Science Foundation and from member institutions, including collaborators from Canada and across the U.S. Fifty scientists from 16 institutions are involved in the CDMS experiment.
The CDMS team has defined the perimeters of dark matter existence by increasing the capacity to detect particles through the installation of 30 detectors, many of them tested in a near absolute zero refrigeration chamber in the Rockefeller Building by Akerib and his research team. Team members from the university include research associate professor Dr. Michael Dragowsky and doctoral students, Cathy Bailey and Raul Hennings-Yeomans, whose graduate work is related to the dark matter search.
The campus researchers tested puck-like detectors of germanium crystals and characterized signals of different particles in the universe. If a WIMP interacts in the germanium, it should set off a reaction similar to what happens in billiards when the cue ball strikes a billiard ball, causing it to recoil. In the astrophysical case, the dark matter particle would strike the nucleus of a germanium atom and set off a recoil action. The detectors would pick up that signal. (Germanium is similar to the semiconductor silicon, but is expected to be more sensitive to dark matter recoils owing to its larger size.) The researchers describe it as ringing the crystal germanium bell.
When the experiment first got underway in 2002 only 12 detectors were operating. In 2004, the CDMS experiment increased the number to 30. The increased detectors allowed researchers to, in effect, perform a more sensitive "time exposure" and to filter out unwanted sources of background particles from radioactivity and cosmic rays. Akerib likens the installation of more detectors to increasing the size of a telescope that gives astronomers a deeper look into the night sky.
"With our new result we are leapfrogging the competition," said Blas Cabrera of Stanford University, co-spokesperson of the CDMS experiment, for which the Department of Energy's Fermi National Accelerator Laboratory hosts the project management. "We have achieved the world's most stringent limits on how often dark matter particles interact with ordinary matter and how heavy they are, in particular in the theoretically favored mass range of more than 40 times the proton mass. Our experiment is now sensitive enough to hear WIMPs even if they ring the 'bells' of our crystal germanium detector only twice a year. So far, we have heard nothing."
If they exist, WIMPs might interact with ordinary matter at rates similar to those of low-energy neutrinos, elusive subatomic particles discovered in 1956 by physicist Frederick Reines while on the faculty at Case Western.
But to account for all the dark matter in the universe and the gravitational pull it produces, WIMPs must have masses about a billion times larger than those of neutrinos. The CDMS collaboration found that if WIMPs have 100 times the mass of protons, they collide with one kilogram of germanium less than a few times per year; otherwise, the CDMS experiment would have detected them.
The CDMS collaboration presented its results to the scientific community at the Eighth UCLA Dark Matter and Dark Energy symposium on Feb. 22. The Department of Energy's Fermi National Accelerator Laboratory hosts the project management for the experiment.
The CDMS experiment resides in the Soudan Underground Laboratory, shielded from cosmic rays and other particles that could mimic the signals expected from dark matter particles. Physicists expect that WIMPs, if they exist, travel right through ordinary matter, rarely leaving a trace. If WIMPs crossed the CDMS detector, occasionally one of the WIMPs would hit a germanium nucleus. Like a hammer hitting a bell, the collision would create vibrations of the detector's crystal grid, which scientists could detect. Not having observed such signals, the CDMS experiment set limits on the properties of WIMPs.
The discovery of WIMPs would require extensions to the Standard Model of particles and their forces. The CDMS result tests the viability of new theoretical concepts that have been proposed.
A new phase of the CDMS experiment with 25 kilograms of germanium is planned for the SNOLAB facility in Canada.
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