Particle no-show pans former find
Physicists unbowed as fail to detect
The most powerful search yet for the Universe's missing matter has come up empty handed, contradicting an earlier study that claimed to have seen new particles.
Researchers from the Cryogenic Dark Matter Search II (CDMSII) say they are pleased with their first results, which show that their detector is working and set new constraints on how the so-far undetected matter can behave, if it exists.
For decades, physicists and astronomers have known that conventional particles of the type that make up atoms, stars and people only account for a tiny fraction of the Universe's mass. The rest of the mass is referred to as dark matter, as its identity is unknown. It is thought to come from a variety of heavy particles that rarely interact with regular matter and can pass through conventional objects unseen.
CDMSII has been looking for a type of theoretical particles called weakly interactive massive particles, or WIMPs. If they are detected, this would not only shed light on the mystery of dark matter, but would also be good evidence for supersymmetry, a theory of physics that goes beyond today's standard model.
The new detector is four times more sensitive than any previous experiment. To shield it from high-energy particles from outer space, the machine is based 700 metres underground in an abandoned iron mine in Soudan, Minnesota. The detector is also chilled to within a tenth of a degree of absolute zero to reduce vibrations from surrounding molecules.
The detector itself consists of sensors attached to six germanium and silicon crystals. If a particle strikes one of the crystals, it causes the crystal to ring like a bell, and the sensors detect the vibrations.
However since it started running in November last year, the detector has not seen a single WIMP. The negative result puts an upper limit on the number of interactions that can be occurring over a certain period of time, says Harry Nelson, a physicist at the University of California, Santa Barbara. He presented the results at the American Physical Society's annual meeting in Denver, Colorado, on 3 May.
That effectively contradicts the findings of DAMA, a less sensitive experiment based at the National Laboratory of Gran Sasso, Italy, that has been collecting data since 1996. Researchers there thought they might have detected WIMPs. And they saw more particles in the summer than in the winter, an effect that could have been caused by the Earth travelling with or against the flow of cosmic dark particles.
But Nelson says that if those events were real, then the more sensitive CDMSII should have seen 50 events for each one detected by DAMA. "If the particles had been present, we are sure we would have seen them," he says.
Other results have cast doubt on DAMA's finding, but the data from CDMSII are likely to be seen as conclusive evidence that the Italian experiment did not spot a new particle. "[DAMA's finding] is essentially dead," says Lawrence Krauss, a theoretical astrophysicist at Case Western Reserve University in Cleveland, Ohio.
Physicists with the CDMSII experiment say they will now add another 24 crystals to the detector, increasing its sensitivity tenfold. If they still fail to find something, then theorists who believe in supersymmetry may have to rethink their ideas. "CDMSII isn't exerting an annoying pressure on theorists yet," says Nelson. "But they're starting to feel it."