(main image) Karl van Bibber, Physics 2, 2 (2009) ;(inset) Fermilab

Subtle signals.A dark matter particle interacting with the particle detectors (photo) in the CDMS experiment will produce both an electrical signal and a heat signal in the form of vibrations called phonons.

Wimpy Evidence for Dark Matter Particles

Staff Writer

Tiger Woods's philandering notwithstanding, the truth often proves less exciting than the gossip preceding it. So it is with the rumor that physicists had detected dark matter--the mysterious stuff whose gravity appears to hold the galaxies together. Today, physicists working with an underground particle detector reported that they have indeed seen a couple of candidate particles, just as Internet buzz had it. However, researchers cannot rule out the possibility that the two were ordinary particles, and they make no claim to have discovered dark matter.

"Nobody should be attempting to say that this is evidence" for dark-matter particles, says Richard Gaitskell, a physicist at Brown University who was not involved in the experiment.

The results come from the Cryogenic Dark Matter Search (CDMS), an array of exquisitely sensitive particle detectors lurking 747 meters down in the Soudan mine in Minnesota. The detectors are semiconducting disks designed to detect so-called weakly interacting massive particles (WIMPs), hypothetical particles thought to make up elusive dark matter. Because WIMPs (if they exist) are heavy, vast clouds of them could provide the gravity needed to hold galaxies like our Milky Way together. Yet WIMPs would be nearly undetectable otherwise, as they would interact with ordinary atoms so feebly.

Since 2003, CDMS team members have been trying to spot the distinctive electrical and heat signals of WIMPs knocking into the atomic nuclei in their detectors. And for the first time, researchers have seen at least some events that look like WIMPs, members of the 80-person collaboration reported today in simultaneous talks at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, and SLAC National Accelerator Laboratory in Menlo Park, California. The CDMS team sees two such events, the researchers report.

Unfortunately, ordinary particles may wander into the detector and create "background" events that mimic WIMPs. In fact, the reason detectors operate deep underground is to shield them from cosmic rays that might produce those backgrounds. Even taking precautions to weed out such events, researchers expected to see about one background event in a data set of the current size, and there's a 25% chance of seeing two or more.

So should the CDMS result be taken as an observation of dark matter? "Absolutely not," says Edward Thorndike, an experimental particle physicist at the University of Rochester in New York state. The 25% chance that the purported "signal" is actually just a few extra background events is far too big to justify any claim of discovery, he says. "If you're going after something that's going to send you to Stockholm, [that probability] better be well below 1%."

Even so, Joseph Lykken, a theorist at Fermilab, says he's relieved that CDMS has finally seen something. WIMPs are predicted to exist by theories involving a principle called supersymmetry, which posits a heavy partner for every particle currently known. Had CDMS continued to see nothing, the results would have undermined those theories. So seeing something is better than seeing nothing, Lykken says.

Gaitskell disagrees. Statistically speaking, he notes, with an expected background of one event, the probability of seeing zero events is almost the same as the probability of seeing two, so both are equally consistent with no WIMPS at all, he says. "As Christmas presents, they're both a pair of socks."

All agree that much more data is needed. That could come quickly, as the much-larger XENON100 detector in Gran Sasso, Italy, and other detectors come on line. At the same time, astrophysical observation could provide evidence of dark-matter particles annihilating each other, and the world's largest atom smasher, Europe's Large Hadron Collider, could make such particles by blasting protons together at ultrahigh energy. All of these efforts could pay off soon, Lykken says. "Next year will be the year of dark matter," he says. "I'll be very surprised if it isn't." But are the chances of disappointment 25% or less than 1%?

Posted in Physics