Posted: Mar 08, 2010 4:13 pm
At a Mine’s Bottom, Tantalizing Hints of Dark Matter - NYTimes.com
Particle may be leading candidate for mysterious dark matter
Cryogenic Dark Matter Search - Wikipedia, the free encyclopedia
CDMS Home Page - contains some PDF's of some slideshows and some video of some talks by members of the CDMS team
Since starting in 2006, the CDMS team has found 2 events that fit what they expect dark matter to have produced. They also calculate a probability of 1/4 that their results are due to background events like cosmic rays or radioactive decay.
They have made a lot of effort to try to block out these sources of noise. To block off cosmic rays, the CDMS team has placed their experiments in the Soudan mine in Minnesota, about 2341 ft / 713 m below the surface. This is equivalent to about 2100 m / 1.3 mi of water. They also have some additional shielding around the detectors to block of radioactivity in the surrounding rocks.
Not surprisingly, they hope to place their upcoming experiments even deeper, like in the Sudbury mine, which is about 3 times deeper.
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The detectors work by detecting the sound that a single Weakly Interacting Massive Particle (WIMP) makes as it hits the detector material. A WIMP will bounce off one of the particles, giving it some recoil momentum. It then kicks the surrounding material, making a ultrasound wave that spreads to the sound detectors.
It is an extremely weak sound wave, and to keep it from getting swamped by thermal noise, the detector must be chilled to 40 millikelvin, or 0.04 K. In its sound detectors, a phonon (quantum of sound) kicks an electron loose, making it wander until it is picked up and recorded.
Most of the background events recorded are electron hits, but WIMP's and neutrons mostly hit nuclei, and it's possible to distinguish electron hits and nucleus hits.
-
There are even more sensitive experiments in the works, and they should be sensitive enough to settle the question of whether the recent results were due to WIMP's or background sources. If such experiments succeed, then we will be able to test various hypotheses about the WIMP's, like how they interact with neutrons relative to protons, and how massive they are.
WIMP's will likely react differently with up and down quarks, making them react differently with protons (uud) and neutrons (udd). In the simplest case, the interaction rate will be proportional to
(gp Z + gn N)2
for appropriate values of gp and gn. Different nuclides have different proportions of protons and neutrons, with light ones having 1 neutron per proton and heavy ones having as much as 1.5 neutrons per proton. So detecting WIMP's with both light and heavy elements should help in sorting out these effects.
There's a complication when the arriving WIMP's have a momentum wavelength comparable to the size of the nucleus - scattering off of different nucleons will produce interference. That is a well-known effect from nuclear physics, and one can use the "form factors" known from that to work out what to expect.
Using different materials may also make it possible to measure the mass of the WIMP's. The average recoil kinetic energy of a nucleus is
mN mW2 / (mN + mW)2 <vW2>
It is greatest when the nucleus's mass, mN, is equal to the WIMP particle's mass, mW, and falls off for greater or lesser mass.
Supersymmetric theories generally predict WIMP masses at somewhere around 100 GeV or so, around the mass of a silver nucleus. However, that should not be treated as a precise prediction; it could also be 200 GeV or so, around the mass of a lead nucleus.
One will also get the average velocity of the WIMP's, which is expected to be around 300 km/s relative to the Sun, or about 0.001 c.
Fortunately, present and upcoming experiments have been using a variety of materials: germanium (Ge), perfluorobutane (C4F10), carbon disulfide (CS2), sodium iodide (NaI), calcium tungstate (CaWO4), argon (Ar), xenon (Xe). Their proton and neutron contents are:
C(6,6), O(8,8), F(9,10), Si(14,14), S(16,16), Na(11,12), Ar(18,22), Ca(20,20), Ge(32,38.40.42), I(53,74), Xe(54,74.77.78.80.82), W(74,108.109.110.112)
which should be plenty.
Some upcoming experiments also get directional information, though because of the WIMP's' scattering, that is a blurred version of the WIMPs' original directions. But it should be enough to see what the Earth's motion relative to the WIMP's is.
Particle may be leading candidate for mysterious dark matter
Cryogenic Dark Matter Search - Wikipedia, the free encyclopedia
CDMS Home Page - contains some PDF's of some slideshows and some video of some talks by members of the CDMS team
Since starting in 2006, the CDMS team has found 2 events that fit what they expect dark matter to have produced. They also calculate a probability of 1/4 that their results are due to background events like cosmic rays or radioactive decay.
They have made a lot of effort to try to block out these sources of noise. To block off cosmic rays, the CDMS team has placed their experiments in the Soudan mine in Minnesota, about 2341 ft / 713 m below the surface. This is equivalent to about 2100 m / 1.3 mi of water. They also have some additional shielding around the detectors to block of radioactivity in the surrounding rocks.
Not surprisingly, they hope to place their upcoming experiments even deeper, like in the Sudbury mine, which is about 3 times deeper.
-
The detectors work by detecting the sound that a single Weakly Interacting Massive Particle (WIMP) makes as it hits the detector material. A WIMP will bounce off one of the particles, giving it some recoil momentum. It then kicks the surrounding material, making a ultrasound wave that spreads to the sound detectors.
It is an extremely weak sound wave, and to keep it from getting swamped by thermal noise, the detector must be chilled to 40 millikelvin, or 0.04 K. In its sound detectors, a phonon (quantum of sound) kicks an electron loose, making it wander until it is picked up and recorded.
Most of the background events recorded are electron hits, but WIMP's and neutrons mostly hit nuclei, and it's possible to distinguish electron hits and nucleus hits.
-
There are even more sensitive experiments in the works, and they should be sensitive enough to settle the question of whether the recent results were due to WIMP's or background sources. If such experiments succeed, then we will be able to test various hypotheses about the WIMP's, like how they interact with neutrons relative to protons, and how massive they are.
WIMP's will likely react differently with up and down quarks, making them react differently with protons (uud) and neutrons (udd). In the simplest case, the interaction rate will be proportional to
(gp Z + gn N)2
for appropriate values of gp and gn. Different nuclides have different proportions of protons and neutrons, with light ones having 1 neutron per proton and heavy ones having as much as 1.5 neutrons per proton. So detecting WIMP's with both light and heavy elements should help in sorting out these effects.
There's a complication when the arriving WIMP's have a momentum wavelength comparable to the size of the nucleus - scattering off of different nucleons will produce interference. That is a well-known effect from nuclear physics, and one can use the "form factors" known from that to work out what to expect.
Using different materials may also make it possible to measure the mass of the WIMP's. The average recoil kinetic energy of a nucleus is
mN mW2 / (mN + mW)2 <vW2>
It is greatest when the nucleus's mass, mN, is equal to the WIMP particle's mass, mW, and falls off for greater or lesser mass.
Supersymmetric theories generally predict WIMP masses at somewhere around 100 GeV or so, around the mass of a silver nucleus. However, that should not be treated as a precise prediction; it could also be 200 GeV or so, around the mass of a lead nucleus.
One will also get the average velocity of the WIMP's, which is expected to be around 300 km/s relative to the Sun, or about 0.001 c.
Fortunately, present and upcoming experiments have been using a variety of materials: germanium (Ge), perfluorobutane (C4F10), carbon disulfide (CS2), sodium iodide (NaI), calcium tungstate (CaWO4), argon (Ar), xenon (Xe). Their proton and neutron contents are:
C(6,6), O(8,8), F(9,10), Si(14,14), S(16,16), Na(11,12), Ar(18,22), Ca(20,20), Ge(32,38.40.42), I(53,74), Xe(54,74.77.78.80.82), W(74,108.109.110.112)
which should be plenty.
Some upcoming experiments also get directional information, though because of the WIMP's' scattering, that is a blurred version of the WIMPs' original directions. But it should be enough to see what the Earth's motion relative to the WIMP's is.