Background for the CDMS experiment
What does "setting limits" mean in an experiment?
Experimental physicists, such as those working on CDMS, frequently are
faced with the situation of quantifying what it means when they have
not yet found what they are searching for. Sometimes the result is that
no events have been found, but more often it is the case that events
have been seen which are consistent with expected sources of
backgrounds. In either case, there are mathematical formulae for
calculating "90% confidence level upper limits" on the rate of the
signal which hasn't been clearly detected. Such a limit implies that if
you could do the same experiment 100 times, the result would be the
same (or fewer) events detected in 90 of those experiments.
What is dark matter?
Judging by the way galaxies rotate, scientists have known for 70 years that the matter we can see does not provide enough gravitational pull to hold the galaxies together. There must exist some form of matter that does not emit or reflect light. According to the Wilkinson Microwave Anisotropy Probe, or WMAP, a survey of the microwave background radiation left over from the big bang, ordinary ("baryonic") matter containing atoms makes up only 4% of the energy-matter contents in the Universe. "Dark energy" makes up 73%, and an unknown form of dark matter makes up the last 23%. (Most of the baryonic matter is dark matter too and resides in hydrogen and dust clouds and very dim clumps called massive compact halo objects. MACHOs include planets and cold dead stars like brown dwarfs and black holes.)
We can infer some of the properties of the nonbaryonic dark matter, such as its density, from the WMAP. The Sloan Digital Sky Survey has recently confirmed these results. We know that neutrinos, very light particles left over from the big bang in massive quantities, make up a small amount. WIMPs, or weakly interactive massive particles, may make up the rest.
What's the difference between a neutrino and a neutralino?
Neither has an electric charge, and each probably makes up some of the missing matter but otherwise they have little in common. The neutrino carries almost no mass. It moves at nearly the speed of light, which makes it "hot dark matter." This velocity means that it could not have made galaxies congeal in the early universe. Most neutrinos remain from the beginning of the universe, but particle decay and atomic fusion in stars continue to produce them.
Supersymmetry predicts the existence of the massive neutralino. We know it carries at least 46 times the mass of a proton because otherwise experiments at the Large Electron-Positron collider (LEP) at CERN would have detected their production. If it is a WIMP, it travels through the universe at 1/1000 the speed of light, making it "cold dark matter." Neutralinos were produced at the beginning of the universe but exist in fewer numbers than the neutrino because their great mass makes them harder to produce, and they annihilate each other. They both pass through the Earth in large quantities.
What is a WIMP?
Weakly interactive massive particles may make up most of the dark matter, if they have a mass of 10 to 10000 times the mass of the proton. They only interact via the weak force and gravity (not the strong or electromagnetic force), so they only disturb atoms when they collide with a nucleus. Atoms contain mostly empty space, so this rarely happens. Photons pass right through them. As many as 10 trillion WIMPs should pass through one kilogram of the Earth in a second but perhaps as few as one per day will interact.
What is supersymmetry?
The Standard Model describes all of the particles and forces in the universe, but it does not adequately explain the origin of mass. To solve this problem, in 1982 some theorists proposed an extension to the Standard Model where every mass particle (the quark, electron, etc) and every force-carrying particle (the photon, graviton, etc.) has an associated "superpartner" that differs only in its spin and mass. The undetected superpartners are much more massive than the particles observed so far. The lightest neutral supersymmetric particle is the neutralino. With an expected mass of 50-1000 billion electron volts (GeV)-a proton's mass is 1 GeV-and weak interaction with the baryons (protons and neutrons) that make up everyday matter, they are considered WIMPs.
Why are the detectors underground?
Cosmic rays hit the surface of the Earth and the reactions produce neutrons. Placing the detectors deep underground shields them from most of the cosmic rays that would produce neutrons.
Why are the detectors so cold?
The cryostat uses six nested layers to cool the detectors to 50 millikelvin (thousandths of a degree above absolute zero.) This reduces the background vibrations of the detector's atoms and makes them more sensitive to individual particle collisions.
How do the detectors work?
The experimental set-up for CDMS II contains two towers of detectors. Each tower contains a kilogram of germanium for detecting dark matter and 200 grams of silicon to distinguish WIMPs from neutrons. Supersymmetry models predict that only a few WIMPs per year, one per day at most, will interact with the detectors. The biggest challenge involves sorting them from background interactions due to electrons, neutrons, and gamma rays.
When a WIMP hits a germanium nucleus, the nucleus recoils and vibrates the whole germanium crystal. This warms the thin aluminum and tungsten outer layers, which an electrical circuit measures. Photons and electrons, however, strike the germanium's electrons. A charge collection plate measures ionization resulting from this type of collision and uses it to separate these interactions from those of WIMPs. The ratio of charge to heat for each event tells whether a particle struck the nucleus, as WIMPs do, or simply rattled the electrons surrounding the nucleus, as most background particles do.
Incoming neutrons also strike the germanium nucleus, so they more closely resemble WIMPs. The germanium detectors sit in a stack with detectors made of silicon. A silicon atom has a smaller nucleus, and so will be hit less frequently by WIMPs. The strong nuclear force does not affect WIMPs, but it does affect neutrons and so neutrons will hit nuclei of different sizes at about the same rate. A higher collision rate in the germanium than the silicon will indicate the interaction of WIMPs.
Does CDMS have any competition?
CDMS has several competitors around the world. DAMA, a collaboration between Italian and Chinese scientists in the Gran Sasso tunnel in Italy, has collected data over the past six years and shows seasonal changes in events. They argue that this modulation results from the Earth traveling with or against the flow of cosmic dark matter particles and provides direct evidence for WIMPs. The scientists have addressed concerns that other factors--including temperature, humidity, or radon--may account for the data, but according to Bauer, "they don't address all of the conventional explanations at the level of detail that I think an extraordinary claim like that requires."
The closest competition comes from a French project called EDELWEISS, which also uses a germanium detector and measures heat and ionization. They have collecting data for three years, but they have fewer detectors than CDMS right now and they have less-sophisticated background rejection capability.
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