Cryogenic Dark Matter Search
Encyclopedia
The Cryogenic Dark Matter Search (CDMS) is a series of experiments designed to directly detect particle dark matter
in the form of WIMPs. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with terrestrial materials. The first experiment, CDMSI, was run in a tunnel under the Stanford University
campus. The current experiment, CDMSII, is located deep underground in the Soudan Mine
in Minnesota
.
A number of proposed candidates for the missing mass have been put forward over time. Early candidates included heavy baryon
s that would have had to be created in the big bang
, but more recent work on nucleosynthesis seems to have ruled most of these out. Another candidate are new types of particles known as weakly interacting massive particles, or "WIMP"s. As the name implies, WIMPs interact weakly with normal matter, which explains why they are not easily visible.
Detecting WIMPs thus presents a problem; if the WIMPs are very weakly interacting, detecting them will be extremely difficult. Detectors like CDMS and similar experiments measure huge numbers of interactions within their detector volume in order to find the extremely rare WIMP events.
and phonon
s produced by every particle interaction in their germanium
and silicon
crystal substrates. These two measurements determine the energy deposited in the crystal in each interaction, but also give information about what kind of particle caused the event. The ratio of ionization signal to phonon signal differs for particle interactions with atomic electrons ("electron recoils") and atomic nuclei ("nuclear recoils"). The vast majority of background particle interactions are electron recoils, while WIMPs (and neutrons) are expected to produce nuclear recoils. This allows the vast majority of the unwanted background interactions to be rejected, so that any WIMP-scattering events can be identified even if they are very rare.
CDMS detectors are disks of germanium or silicon, cooled to millikelvin temperatures by a dilution refrigerator
. The extremely low temperatures are needed to limit thermal noise which would otherwise obscure the phonon signals of particle interactions. Phonon detection is accomplished with superconduction transition edge sensors (TESs) read out by SQUID amplifiers, while ionization signals are read out using an FET amplifier. CDMS detectors also provide data on the phonon pulse shape which is crucial in rejecting near-surface background events.
, Mark Srednicki and Frank Wilczek
.
CDMS collected WIMP search data in a shallow underground site at Stanford University
through 2002, and has operated (with collaboration from the University of Minnesota
) in the Soudan Mine since 2003. Its results so far are negative, but CDMS is able to provide the most sensitive limits on WIMP dark matter in many interesting models. CDMS is currently/when taking data at Soudan with its full complement of detectors, aiming to increase the sensitivity of WIMP searches by a further factor of 10. The current/when sensitivity is nearing the levels where theoretically the detector should start to detect dark matter.
collisions. It is estimated that such noise would produce two or more events in one quarter of the time. Polythene absorbers were fitted to reduce any neutron background.
A 2011 analysis found little evidence of WIMPs < 9GeV (in conflict with other DM expts).
s.
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
in the form of WIMPs. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with terrestrial materials. The first experiment, CDMSI, was run in a tunnel under the Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
campus. The current experiment, CDMSII, is located deep underground in the Soudan Mine
Soudan Underground Mine State Park
The Soudan Underground Mine State Park is a Minnesota state park at the site of the Soudan Underground Mine, on the south shore of Lake Vermilion. The mine is known as Minnesota's oldest, deepest, and richest iron mine, and now hosts the Soudan Underground Laboratory. As the Soudan Iron Mine, it...
in Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
.
Background
Observations of the large-scale structure of the universe show that matter is aggregated into very large structures that would not have time to have formed under the force of their own self-gravitation. It is generally believed that some form of missing mass is responsible for increasing the gravitational force at these scales, although this mass has not been directly observed. This is a problem; normal matter in space will heat up until it gives off light, so if this missing mass exists, it is generally assumed to be in a form that is not commonly observed on earth.A number of proposed candidates for the missing mass have been put forward over time. Early candidates included heavy baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
s that would have had to be created in the big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
, but more recent work on nucleosynthesis seems to have ruled most of these out. Another candidate are new types of particles known as weakly interacting massive particles, or "WIMP"s. As the name implies, WIMPs interact weakly with normal matter, which explains why they are not easily visible.
Detecting WIMPs thus presents a problem; if the WIMPs are very weakly interacting, detecting them will be extremely difficult. Detectors like CDMS and similar experiments measure huge numbers of interactions within their detector volume in order to find the extremely rare WIMP events.
Detection technology
The CDMS detectors measure the ionizationIonization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
and phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
s produced by every particle interaction in their germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
crystal substrates. These two measurements determine the energy deposited in the crystal in each interaction, but also give information about what kind of particle caused the event. The ratio of ionization signal to phonon signal differs for particle interactions with atomic electrons ("electron recoils") and atomic nuclei ("nuclear recoils"). The vast majority of background particle interactions are electron recoils, while WIMPs (and neutrons) are expected to produce nuclear recoils. This allows the vast majority of the unwanted background interactions to be rejected, so that any WIMP-scattering events can be identified even if they are very rare.
CDMS detectors are disks of germanium or silicon, cooled to millikelvin temperatures by a dilution refrigerator
Dilution refrigerator
A dilution refrigerator is a cryogenic device first proposed by Heinz London. Its refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4...
. The extremely low temperatures are needed to limit thermal noise which would otherwise obscure the phonon signals of particle interactions. Phonon detection is accomplished with superconduction transition edge sensors (TESs) read out by SQUID amplifiers, while ionization signals are read out using an FET amplifier. CDMS detectors also provide data on the phonon pulse shape which is crucial in rejecting near-surface background events.
History
Simultaneous detection of ionization and heat with semiconductors at low temperature was an original idea by Lawrence M. KraussLawrence M. Krauss
Lawrence Maxwell Krauss is an American theoretical physicist who is professor of physics, Foundation Professor of the School of Earth and Space Exploration, and director of the Origins Project at the Arizona State University. He is the author of several bestselling books, including The Physics of...
, Mark Srednicki and Frank Wilczek
Frank Wilczek
Frank Anthony Wilczek is a theoretical physicist from the United States and a Nobel laureate. He is currently the Herman Feshbach Professor of Physics at the Massachusetts Institute of Technology ....
.
CDMS collected WIMP search data in a shallow underground site at Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
through 2002, and has operated (with collaboration from the University of Minnesota
University of Minnesota
The University of Minnesota, Twin Cities is a public research university located in Minneapolis and St. Paul, Minnesota, United States. It is the oldest and largest part of the University of Minnesota system and has the fourth-largest main campus student body in the United States, with 52,557...
) in the Soudan Mine since 2003. Its results so far are negative, but CDMS is able to provide the most sensitive limits on WIMP dark matter in many interesting models. CDMS is currently/when taking data at Soudan with its full complement of detectors, aiming to increase the sensitivity of WIMP searches by a further factor of 10. The current/when sensitivity is nearing the levels where theoretically the detector should start to detect dark matter.
Results
On December 17, 2009, the collaboration announced the possible detection of two candidate WIMPs, one on August 8, 2007 and the other on October 27, 2007. However, due to this low number of events, the team could not claim that the detections were true WIMPs; they may have been false positives from background noise such as neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
collisions. It is estimated that such noise would produce two or more events in one quarter of the time. Polythene absorbers were fitted to reduce any neutron background.
A 2011 analysis found little evidence of WIMPs < 9GeV (in conflict with other DM expts).
Proposed upgrades
SuperCDMS, a proposed successor to CDMS II, may have the sensitivity to explore and detect neutralinoNeutralino
In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...
s.