Dark Matter Particles and Dark Galaxy

1. Scientists working on the Cryogenic Dark Matter Search (CDMS), in a disused iron ore mine in Minnesota, have announced that they had detected two weakly-interacting massive particles (WIMPs), that are thought to make up dark matter.

If they are confirmed by further observations that will begin next year, they would rank as one of the most important recent advances in physics and understanding of the cosmos.

The particles showed as two tiny pulses of heat deposited over the course of two years in chunks of germanium and silicon that had been cooled to a temperature near absolute zero.

The detectors are place half a mile down to avoid them being effected by background radiation.

But the scientists still said there was more than a 20 percent chance that the pulses were caused by fluctuations in the background radioactivity of their cavern, so the results were tantalizing, but not definitive.

This is confirmation of the rumor that started about two weeks ago

the results summary from Dec 17, 2009 from the Berkeley Cryogenic Dark Matter Search (CDMS) experiment (2 page pdf)

In this new data set we indeed see two events with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made. The ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be fewer than one chance in a thousand of the signal being due to background. In this case, a signal of about five events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs. Instead we say that the rate of WIMP interactions with nuclei must be less than a particular value that depends on the mass of the WIMP. The numerical values obtained for these interaction rates from this data set are more stringent than those obtained from previous data for most WIMP masses predicted by theories. Such upper limits are still quite valuable in eliminating a number of theories that might explain dark matter.

What comes next? While the same set of detectors could be operated at Soudan for many more years to see if more WIMP events appear, this would not take advantage of new detector developments and would try the patience of even the most stalwart experimenters (not to mention theorists). A better way to increase our sensitivity to WIMPs is to increase the number (or mass) of detectors that might see them, while still maintaining our ability to keep backgrounds under control. This is precisely what CDMS experimenters (and many other collaborations worldwide) are now in the process of doing. By summer of 2010, we hope to have about three times more germanium nuclei sitting near absolute zero at Soudan, patiently waiting for WIMPs to come along and provide the perfect billiard ball shots that will offer compelling evidence for the direct detection of dark matter in the laboratory.

Soudan Underground Lab

Cryogenic Dark Matter Search Posters

Dark Galaxy Evidence: New evidence has been discovered by an international team led by astronomers from the National Science Foundation’s Arecibo Observatory and from Cardiff University in the United Kingdom that VIRGOHI 21, a mysterious cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth, is a Dark Galaxy, emitting no starlight. Their results not only indicate the presence of a dark galaxy but also explain the long-standing mystery of its strangely stretched neighbour. Skeptics of the dark-matter interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254