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New cryogenic detectors probe recent evidence for dark matter particles
Researchers from 10 institutions reported today that they have achieved the world's best discrimination in the search for dark matter, which scientists have postulated makes up more than 90 percent of the mass of the universe.
The collaboration, called CDMS (Cryogenic Dark Matter Search), uses an entirely new type of detector technology that employs crystals kept at cryogenic temperatures to detect potential dark matter particles. This powerful technology derives its advantage from being able to distinguish background "events" that result from many of the known particles interacting in the crystals from those that are likely to be dark matter interactions. This discrimination allows an unambiguous identification of events in the crystals caused by any new form of matter.
The results of the search will be reported Friday, Feb. 25, at the Fourth International Symposium on Sources and Detection of Dark Matter in the Universe in Marina del Rey, Calif.
Within the currently favored theoretical models these results appear incompatible with the evidence for dark matter reported by the DAMA collaboration last week. The DAMA (DArk MAtter experiment) is based in Rome and Beijing. The two searches use very different techniques for detecting dark matter particles.
The CDMS collaboration includes groups from UC-Berkeley, Stanford, UC-Santa Barbara, Lawrence Berkeley National Laboratory, Fermi National Accelerator Laboratory (Fermilab), Case Western Reserve University, Santa Clara University, the National Institute of Standards and Technology (Boulder), the University of Colorado at Denver and Princeton. Their findings have been submitted to Physical Review Letters (and http://xxx.lanl.gov/archive/astro-ph). The research is supported jointly by the Department of Energy and by the National Science Foundation in a collaboration coordinated by the Center for Particle Astrophysics.
For more than a decade, a number of experiments around the world have searched for dark matter in the form of weakly interacting elementary particles (WIMPs). Theoretically, a million WIMPs would pass through an area the size of a thumbnail each second, but only about one per day would interact (be deflected) in a one-kilogram germanium detector and produce an event that can be measured from the small amount of recoil energy imparted to a single nucleus.
Detectors capable of recording one such event per day per kilogram, or less, are sufficiently sensitive to search for a particular type of WIMP suggested by supersymmetry -- by far the most popular extension of the Standard Model of particle physics theory. Most supersymmetric models predict the existence of such particles, which are called neutralinos. The neutralinos and their interaction rates are at or below this level.
The discovery of WIMPs would confirm 70 years of combined astrophysics and particle physics research that suggests most of the matter of the universe is dark and is not made of ordinary atoms. The existence of WIMPs is predicted from very general considerations about the big bang and the early universe. If this intriguing hypothesis turns out to be correct -- many have called it the ultimate Copernican revolution -- we are not at our solar system's center, not at our galaxy's center, not in a particularly distinguished galaxy, and not even made of the most dominant form of matter in our universe.
Last week, the DAMA collaboration reported on more than three years of data collected with a 100-kilogram sodium iodide detector operated deep underground in the Gran Sasso National Laboratory in Italy. Their detector produces a flash of light each time there is a particle interaction within its volume. In a statistical analysis of tens of thousands of events, they claim to see a modulation in the event rate with the same period as Earth's solar year, with a maximum in June and a minimum in December.
This annual modulation is expected to result from the motion of the sun and earth system through a massive cloud of WIMPs as our solar system rotates about the center of our galaxy. In this picture the galaxy would be embedded in the much larger cloud of WIMP dark matter. The modulation arises because our solar system is moving through that cloud at about 232 kilometers per second along with the spinning galactic disk. In addition, as the Earth moves around the sun in December the Earth is moving against the solar system motion at 30 kilometers per second, and in June it is moving with the motion. Thus, much like a bicycle rider in the rain gets wetter when riding into the wind than when riding with the wind, DAMA expects to have more events in June than in December exactly what they see. DAMA has accumulated more than 58,000 kilogram days of data backing their claim of the existence of a neutralino with mass about 50 times the proton mass.
The CDMS experiment, releasing its first major scientific report, has developed an entirely new type of cryogenic detector technology. Their report is based on about half a kilogram of detector mass operated over one year and producing about 12 kilogram days of data.
How is it possible for 12 kilogram days to be competitive with more than 58,000 kilogram days? The answer lies in the new detector technology that allows the rejection of most background events on an event-by-event basis. All things are bombarded by naturally occurring radiation from the materials around us. In addition, particles coming in from outer space bombard us continuously.
The CDMS experiment is housed in a cave, 10.5 meters below the surface on the Stanford campus; the dirt helps shield the cryogenic detectors from cosmic radiation. Further shielding consists of lead, polyethylene, and active plastic scintillator (which produces a tiny light flash for every particle interaction). These allow the influence of cosmic and terrestrial radiation to be reduced by a factor of about 10,000.
The remaining events that reach the detectors are primarily gamma and beta rays. These produce electron recoils within the detectors. However, WIMPs (neutralinos are a subset of WIMPs) would only interact with nuclei and not with electrons. These detectors can tell whether the recoiling particle was an electron or a nucleus. Detectors are made using crystals of germanium or silicon and are cooled to within 0.1 degrees above absolute zero (the coldest possible temperature).
For each event the researchers simultaneously measure the ionization within the germanium or silicon semiconducting crystals, much as conventional radiation detectors do, and the heat produced by each event, which can only be measured at these ultra low temperatures. The heat production is a good measure of the actual energy deposition, while the ionization is about three times higher for a recoiling electron than for a recoiling nucleus with the same energy. Now all electron recoils can be removed from the data sets, and only one background remains neutrons. These also interact only with nuclei just like neutralinos would.
CDMS has observed 13 single scattering nuclear recoils, which unfortunately were identified as neutrons and not as WIMPs. The residual signal originated from neutrons that are produced outside the active shield. They can penetrate the shielding and interact with the detectors. Confirmation of the neutron hypothesis comes from the number of events in the silicon detector versus those in the germanium detector, and from the number interacting in only one detector versus those interacting in more than one.
CDMS concludes that after eliminating the electron recoil events, the remaining nuclear recoil events are nearly all from neutrons.
In order to compare with the DAMA experiment, which uses a different target material, a theoretical model is needed. Within the currently favored models where the interaction rates scale as the square of the mass of the target nuclei, the CDMS results are incompatible with the DAMA results and the WIMP interpretation of the seasonal modulation they observe.
From the DAMA measurement, CDMS would typically expect 20 WIMP events in addition to the observed neutron events. However, perhaps an unexpected form of dark matter particle could interact differently from what is expected in the sodium iodide DAMA detectors versus the CDMS germanium and silicon detectors.
These reports are not the end of the story, the researchers say. CDMS and DAMA both plan expanded new experiments. DAMA will increase its detector mass from 100 to 250 kilograms, and the newly approved CDMS-II experiment will move deep underground to the Soudan mine in northern Minnesota. CDMS-II will utilize more than 10 times the present detector mass in an environment where the neutron background has been reduced by nearly a factor of 1,000.
Eileen Walsh, Stanford News Service (650)
Robert Sanders, UC Berkeley, Office of Public
Affairs, (510) 642-6998,
Lynn Yarris, Lawrence Berkeley National
Laboratory, Public Information Department (510)
Gail Brown, UC Santa Barbara, News & Media
Relations (805)-893-7220, (805)-893-2191,
Judy Jackson, Fermilab News Service (630)
Susan Griffith, Office of Univ.
Communications, Case Western Reserve University,
Steven Schultz, Princeton Office of
Communications, (609) 258-5729;