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Richard Schnee, a research associate
with the CDMS project, analyzes data in an
underground particle detector 35 feet below
Panama Street. Coutesy of CDMS |
In fact, say scientists, if you add up all the apparent dark matter in the universe, it would weigh at least nine times more than all the stars, planets, moons and cosmic dust combined.
For decades, researchers at dozens of institutions, including Stanford, have been trying to identify the subatomic particles that make up dark matter.
Last week, scientists from around the globe announced their latest findings at the Fourth International Symposium on Sources and Detection of Dark Matter in the Universe, held in Marina del Rey, Calif.
Two of the most exciting --- and controversial -- presentations involved separate, ongoing experiments at Stanford and at Italy's Gran Sasso National Laboratory.
Both experiments use underground particle detectors to search for what many experts believe is the most likely component of dark matter -- the WIMP, or Weakly Interacting Massive Particle.
Related Information:
- Latest CDMS findings (report number 0002471)
By subatomic standards, a WIMP is indeed massive -- perhaps 50 times bigger than a proton or neutron found in the nucleus of a "normal" atom.
Despite their relatively large size, WIMPs are believed to be so energetically weak that they probably pass right through atoms and molecules like tiny ghosts. Theoretically, a million WIMPs could penetrate an area the size of your thumbnail each second without being noticed.
The challenge for researchers has been to figure out a way to detect individual WIMPs at the subatomic level.
The experiment located at Stanford, known as the Cryogenic Dark Matter Search (CDMS), is a collaboration among Stanford and nine other American institutions (see sidebar).
"We believe we have the best apparatus in the world in terms of being able to identify WIMPs," says Stanford physicist Blas Cabrera.
To prevent contamination by cosmic rays and other forms of matter, the CDMS detectors are housed in a heavily shielded, ultracold chamber located in a tunnel 35 feet below the Stanford campus. The idea is to screen out neutrons and other ordinary particles so that only ghostly WIMPs are able to pass through the chamber and interact with the extremely sensitive germanium and silicon crystals inside.
If a WIMP strikes the nucleus of a germanium or silicon atom, it should cause the nucleus to move or "recoil," changing the atom's electrical charge and producing a slight amount of heat that can be measured only at ultralow temperatures.
In a study presented at the Dark Matter Symposium on Friday, CDMS scientists announced that 13 nuclear recoils were detected at Stanford between 1998 and 1999.
What caused the recoil events? The likeliest explanation is that a handful of ordinary neutrons seeped into the underground apparatus and collided with the detectors, just as CDMS scientists predicted.
"Our neutron identification comes from a statistical argument," explains Cabrera.
He says that, on four separate occasions, recoil events were recorded by more than one germanium detector at the same time. According to statistical models, what probably happened is that a single neutron bounced off one detector then collided with another one sitting a few inches away.
But WIMPs rarely interact with anything, so if a WIMP made contact with the first detector, the odds of it colliding with a second one nearby are infinitesimally small.
However, Cabrera notes, "It is statistically possible that a few of the recoils were caused by WIMPS. We certainly can't rule it out."
The cautionary tone of the CDMS report was in sharp contrast to a more controversial study issued earlier in the week by scientists from DAMA --- the Dark Matter Experiment (DAMA).
The DAMA team, which includes researchers from the University of Rome and the Chinese Academy of Sciences, uses an underground sodium iodide detector at Italy's Gran Sasso Lab. Instead of recoiling, the sodium iodide crystal flashes a short beam of light when struck by a neutron, WIMP or other particle.
The controversy erupted when DAMA scientists announced that they had detected "the possible presence" of WIMPs during a three-year period.
DAMA's analysis was based on the theory that our galaxy, the Milky Way, is embedded in an enormous dark halo of WIMPs.
As our solar system rotates around the center of the Milky Way, it passes through the dark halo.
Theoretically, the Earth should encounter different quantities of WIMPs depending on the season. In June, for example, the Earth is moving directly into the halo and therefore should be bombarded by more WIMPS -- much like a bicyclist in the rain who gets wetter when riding directly into the wind. December should have the opposite effect, because the Earth is moving in the same direction as the dark halo.
According to DAMA scientists, that's exactly what they observed. Their detectors flashed more times in June than in December --- a roughly 1 percent seasonal difference that, according to DAMA, is a good indication that the elusive WIMP finally has been found.
But other researchers remained skeptical, pointing out that a 1 percent difference is not significant.
According to Cabrera, DAMA's results "were statistically incompatible with the CDMS findings."
Based on DAMA's model, he says, the expected number of WIMPs in the CDMS experiment should have been 24. That's three times more than the maximum number of WIMPs that could possibly have been detected at Stanford, according to a statistical analysis by CDMS.
Although absolute proof for the existence of WIMPs remains inconclusive, both CDMS and DAMA plan expanded new experiments. DAMA intends to more than double the amount of sodium iodide crystals in its Gran Sasso detector. A newly approved CDMS-II experiment will move deep underground to the abandoned Soudan mine in northern Minnesota, using 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.
Meanwhile, researchers will continue using Stanford's underground lab to test prototypes for the CDMS-II Soudan detectors.
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. If this intriguing hypothesis turns out to be correct, say CDMS scientists, it will show that we are not even made of the most dominant form of matter in our universe.
"We now have detectors sensitive enough to detect the dark matter particles," University of Chicago cosmologist Michael Turner told the New York Times. "These are the first two experiments to touch that promised land." SR
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. 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.
The latest CDMS findings
are posted on the following website: http://xxx.lanl.gov/archive/astro-ph (report number 0002471).
