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Physicists Adopt Complementary Approaches in Dark Matter Search

Bubble chamber dark matter detector
A bubble chamber dark matter detector at the Chicagoland Observatory for Underground Particle Physics experiment at the University of Chicago
Physicists are adopting a varied range of complementary approaches in the experimental search for the elusive dark matter. According to speakers at the APS April Meeting in St. Louis, these approaches include using liquid noble gases as a detecting medium; solid state devices incorporating germanium and silicon crystals cooled to cryogenic temperatures; and resurrecting the relatively old technology of bubble chambers in the search of searching for dark matter.

The two leading contenders for dark matter are massive astrophysical compact objects (MACHOs) and weak interacting massive particles (WIMPs). The former would be black holes, neutron stars, brown dwarfs, and other celestial objects that emit little or no radiation and therefore escape detection. WIMPs would be an entirely new type of matter that almost never interacts with regular matter, making them even more difficult to detect than MACHOs since they only interact through the gravitational and weak nuclear forces.

Therefore, physicists searching for dark matter are going deep underground, using Earth as a natural shield to filter out the background noise from radiation emitted by other particles, such as neutrinos and cosmic rays. WIMPs seem to share certain qualities with neutrinos, which also only rarely interact with other subatomic particles, so many neutrino experiments can be modified to search for WIMPs.

Tom Shutt of Case Western Reserve University is spearheading the Large Underground Xenon (LUX) experiment, housed in the abandoned Homestake gold mine in Lead, South Dakota, in the very same cavern where physicist Ray Davis conducted his seminal solar neutrino experiments in the 1950s. Noble gases are excellent scintillating materials for the purpose of detecting collisions between atoms and WIMPs because they block the passage of many radioactive particles that could interfere with detecting dark matter signals. LUX will use xenon, the heaviest noble gas, which liquifies at -108 degrees Celsius.

The detector will have both a large pool of liquid xenon, and a layer of the gaseous version just above it. Should a WIMP strike a xenon atom, it will emit a flash of light, which will be recorded by photosensitive detectors. Electrons will be bumped off the atom at the time of impact, and pulled through an electric field out of the liquid and into the gaseous layer, emitting a second flash of light when they encounter the gaseous xenon atoms.

Those two flashes of light will comprise a telltale “signal” for a collision between a xenon atom and a WIMP, as opposed to another type of particle, such as a neutrino or cosmic ray. The signal will be different in part because a WIMP should strike the nucleus of an atom, whereas cosmic rays or neutrinos would strike the electrons orbiting the nucleus. This will change the “recoil” behavior and thus comprises a unique signature.

The Cold Dark Matter Search (CDMS) collaboration has moved its experimental headquarters to the Soudan Underground Laboratory, an abandoned iron mine 700 meters below ground in Eli, Minnesota, according to Jodi Cooley of Stanford. The site also houses the Main Injector Neutrino Oscillation Search (MINOS) facility. As cold as it gets in Minnesota during the winter, joked Cooley, it’s still not cold enough for the cryogenics of their experiment.  

The germanium and silicon crystals they use in their detectors are the size of hockey pucks, cooled down to about 50 milliKelvins. When a WIMP passes through a crystal, it sets off tiny vibrations whenever it bumps into an atom, which can be detected via a layer of tungsten-aluminum metal. Of course, the detector also picks up vibrations from other sources as well, so the team uses lead and copper for additional shielding to further reduce background noise.

In March, Cooley’s team announced new results they say set an upper limit on certain key parameters, thereby excluding several of the numerous theoretical models that have been proposed for where the dark matter signal would likely be seen. Cooley said it is the best upper limit achieved thus far, and that any model predicting values above that (a mass of 60 GeV/c2 and a size of 4.4 x 10-44 cm2) could be safely excluded “because we would have seen it.” The detectors are currently being upgraded to conduct even more sensitive experimental measurements in 2009.

Juan Collar at the University of Chicago is taking a very different approach, using bubble chambers to search for dark matter in his Chicagoland Observatory for Underground Particle Physics (COUPP) experiment, located 350 feet underground in a tunnel on the Fermilab site. Bubble chambers were nearly extinct in high-energy physics labs before Collar put them to use in the COUPP experiment. However, “This is not your daddy’s bubble chamber,” he insisted.

COUPP’s bubble chamber detector is a glass jar filled with a liter of a fire-extinguishing liquid (iodotrifluoromethane). When a WIMP hits an atomic nucleus, it triggers an evaporation of a small amount of that liquid, producing a tiny bubble. The bubble is initially too tiny to see, but it grows, and that growth can be recorded with digital cameras. Once the bubble reaches about 1 millimeter in size, the COUPP scientists can study the images for telltale statistical variations between photographs. Ideally, this will enable them to distinguish whether a bubble resulted from background radiation, or from a dark matter particle.

Like the CDMS collaboration, Collar’s group has succeeded in placing some fundamental limits on certain predicted properties for WIMPs. Next on the agenda is to increase the bubble chamber detector’s sensitivity by increasing the amount of liquid from one liter to around 30 liters. Collar has also just installed a new germanium-based compact neutrino detector in the sewers of Chicago, renting this unusual lab space from the city. The design has been modified to detect WIMPs.

Several days after the APS April Meeting, the DAMA-LIBRA collaboration in Gran Sasso, Italy, announced confirmation of a controversial earlier experimental result of a statistically significant signal of the sort one would expect from the collision of WIMPs with the detector. DAMA-LIBRA is an upgrade of a 2000 experiment producing what the Italian scientists believed to be a “clear” signal for dark matter (WIMPs).

Other physicists disagreed, arguing that the original findings were probably a systematic error stemming from the high degree of background noise associated with DAMA’s particular experimental approach: looking for a tiny signal variation in a sodium iodide detector over the course of one year. The tiny variation is believed to be due to the orbital motion of Earth through the cosmic dark matter background. Subsequent experiments at a French underground experiment called EDELWEISS and at CDMS failed to confirm DAMA’s original results.

Collar and many other scientists say that the latest DAMA-LIBRA results, while intriguing, still must be confirmed by other dark matter searches using complementary approaches before scientists can definitively conclude that this is indeed a direct detection of dark matter. “There is no perfect dark matter detector out there,” Collar said, and each approach has its own strengths and weaknesses.

“We all weigh in from different directions,” and then compare results, according to Shutt. That includes upcoming experiments at the Large Hadron Collider at CERN, which will look for missing energy in its collisions as a possible signal for direct detection of the dark matter.

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