Third experiment sees hints of dark matter

Dark-matter detectors are buried deep beneath Gran Sasso (Image: Max Planck Institute)

A third experiment has detected tantalising signs of dark matter. The finding raises more questions than answers, however, as two other experiments have found no sign of the mysterious stuff, which is thought to create the gravity that holds spinning galaxies together, accounting for about 85 per cent of all matter in the universe.

The new result comes from an experiment called CRESST II, which uses a few dozen supercooled calcium tungstate crystals to hunt for dark matter from deep beneath the Gran Sasso mountain in Italy. When a particle hits one of the crystals, the crystal gives off a pulse of light, and sensitive thermometers gauge the energy of the collision.

A cold finger (CF) links the cryostat (CR) to the experimental volume,where the detectors are arranged in a common support structure, the so-called carousel (CA). This volume is surrounded by layers of shielding from copper (CU), lead (PB), and polyethylene (PE). The copper and lead shieldings are additionally enclosed in a radon box (RB). An active muon veto (MV) tags events which are induced by cosmic radiation.

The vast majority of hits come from garden-variety particles such as cosmic rays. These rain down on Earth from space in such large numbers that they strike CRESST&II – which is shielded by a kilometre of rock – at a rate of about one per second. This shield should have little effect on dark-matter particles because they are thought to interact very weakly with normal matter.

Now researchers led by Franz Pröbst and Jens Schmaler of the Max Planck Institute for Physics in Munich, Germany, say the experiment detected around 20 collisions between June 2009 and last April that may not have been caused by known particles.

The collisions may have involved dark matter, says team member Federica Petricca, also of the Max Planck Institute. She reported the results yesterday at the Topics in Astroparticle and Underground Physics conference in Munich.

Particle minnows

If so, the energy measurements of the collisions can be fed into dark-matter models to produce estimates of the particles’ mass. Using the leading theoretical model of dark matter, which posits that it is made of weakly interacting particles called neutralinos, the CRESST II result suggests they weigh between 10 and 20 gigaelectronvolts.

This is on the lighter end of previously predicted values, which fall between roughly 10 and 1000 GeV. The range is based on estimates of how many particles that ultimately decayed into neutralinos were created in the early universe.

Two other experiments have previously detected signs of low-mass dark matterCoGeNT, located in a mine in Soudan, Minnesota, and DAMA, also buried inside Gran Sasso, have both seen signals interpreted as being caused by particle minnows with masses between 7 and 20 GeV.

But the new results conflict with two other dark-matter experiments, CDMS II, located in the Soudan mine, and XENON100 inside Gran Sasso. Both have seen no sign of dark matter at all.

New landscape?

CRESST II’s 20 potential detections are not a strong enough finding to settle the confusion and claim a dark-matter detection – they could still be known particles such as cosmic rays. “We simply do not know enough yet to say anything conclusive. We need more data,” says Belli Pierluigi of Italy’s National Institute of Nuclear Physics in Rome, who is part of the DAMA group.

CRESST II team members will continue the experiment and hope to present more sensitive results next year. Rafael Lang, a member of the XENON100 team at Purdue University in Indiana, is eager to see them. “If their signal stays, that will be very interesting indeed,” he says.

“The CREST II results and the apparent disagreements may be the first glimpses of something completely unexpected,” he says. “It may be something totally new. Rather than dark matter, we could be seeing just the highest peaks of some extraordinary new physics landscape.”

The result closely follows one from NASA’s FERMI satellite, which confirmed a hint that there is more antimatter than expected coming from space. Frustratingly, the result also ruled out previous suggestions that dark matter was the source.

Journal reference: arxiv.org/abs/1109.0702
http://www.newscientist.com/

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