Friday, December 18, 2009
Scientists working on the Cryogenic Dark Matter Search (CDMS), in the disused Soudan iron ore mine in Minnesota, announced last night that they had detected two events with the signature of the weakly-interacting massive particles (WIMPs) that are thought to make up dark matter.
If the signals 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 CDMS researchers stressed, however, that they were not claiming to have discovered dark matter because there is a good chance that the events were caused by something else.
The probability that the signals were caused by ordinary radioactive decay or cosmic rays is about one in four, they said. A one in 1,000 chance of a random event would be needed to confirm that dark matter has finally been seen.
“In the new data set there are indeed two events seen with characteristics consistent with those expected from WIMPs,” the CDMS team said in a statement. “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, in essence that the ratio of signal to background events must be large enough that there is no reasonable doubt.
“Typically, there must be less 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 background events, so we can make no claim to have discovered WIMPs.”
The nature of dark matter has challenged scientists for more than 70 years. It has been known since the 1930s that the visible matter we can see in stars, galaxies and clouds of cosmic dust accounts for less than 5 per cent of the total mass of the Universe.
Some 25 per cent of the mass is thought to be dark matter, and another 70 per cent to be still more mysterious dark energy. They are known principally from their gravitational effects on galaxies and other normal matter.
The standard explanation for dark matter is that it is composed of particles that barely interact with normal matter, effectively passing straight through the Earth while leaving no trace. This quality — which explains the WIMP acronym — also makes the particles exceptionally difficult to observe.
Several laboratories, including one at Boulby in North Yorkshire, are hunting dark matter using highly sensitive detectors that can pick up very rare incidences where a WIMP collides with a normal atom and scatters particles. All are situated deep underground, to minimise interference from cosmic rays and background radiation.
The Soudan laboratory is currently upgrading its detectors, which should enable it to look for WIMP signals with much greater accuracy.
Pier Oddone, director of the Fermilab physics laboratory near Chicago, which contributed to CDMS, said: “While this result is consistent with dark matter, it is also consistent with backgrounds.
“In 2010, the collaboration is installing an upgraded detector at the Soudan mine with three times the mass and lower backgrounds than the present detectors. If these two events are indeed a dark matter signal, then the upgraded detector will be able to tell.”