A hotly anticipated announcement regarding a possible signature of dark matter delivered some grist for the physics mill Thursday but failed to produce the blockbuster result some had predicted. In a Webcast talk from Stanford University, Jodi Cooley, a particle physicist at Southern Methodist University, presented the latest results from the Cryogenic Dark Matter Search 2 (CDMS-2), a series of detectors buried deep underground in a former iron mine in northern Minnesota. (The first CDMS experiment was located at Stanford, much closer to the surface.) CDMS-2, she said, detected two signals that fit the bill for the passage of dark matter particles, but other possibilities could not be ruled out.
Dark matter is thought to make up roughly a quarter of the universe but has never been directly observed. In present-day estimates of the universe's makeup, ordinary atoms (such as those we detect as the visible universe) contribute only about 5 percent; the bulk of the cosmos takes the form of so-called dark energy, under whose influence the universe is expanding at an increasing clip. Dark matter's presence has for decades been inferred from its gravitational effects on large-scale structures such as galaxy clusters, but because it does not interact much with ordinary matter and does not emit or absorb light—hence the "dark" moniker—it has so far proved impossible to observe firsthand.
Many models of dark matter hold that the mysterious stuff comes in the form of weakly interacting massive particles, or WIMPs—that is, particles that interact via the weak nuclear force in addition to gravity. (The weak force, which acts only over very short distances, is the force of nature responsible for radioactive decay, among other phenomena.) CDMS-2, 780 meters below ground in Minnesota's Soudan Underground Laboratory, is on the lookout for such WIMPS. The experiment monitors germanium detectors, cooled to a fraction of a degree above absolute zero, for subtle vibration and ionization effects that would be produced by WIMPs colliding with germanium nuclei.
The detectors are shielded from contaminating radiation by several layers of insulation. One of those layers is 4.5 centimeters of lead salvaged from the ballast of a centuries-old French shipwreck, whose age ensures that most of the potentially contaminating radiation from the decay of unstable lead isotopes is long gone. CDMS-2's underground location further insulates the experiment from cosmic rays and other background radiation sources.
Even so, Cooley and her colleagues calculated that the detectors could be expected to pick up approximately 0.5 WIMP-mimicking background events in the course of acquiring the newly unveiled CDMS-2 data set, a run that spanned 2007 and 2008. Even with a more cautious look at background sources, the CDMS team came to an estimated background contribution of 0.8 event. Tantalizingly, the detectors registered two events that looked like WIMPs—more than would be expected from background noise. What is more, Cooley noted, "the two events occurred during a time of nearly ideal detector performance." And they were registered in different months by different detectors.
Nevertheless, the CDMS collaboration cautions that there was a roughly 25 percent chance of seeing two background events. "Our results cannot be interpreted as significant evidence for WIMP interactions," Cooley said. Still, she added, the team "cannot reject either as a signal." In a summary of results released by the CDMS team Thursday (pdf), the researchers noted that the detectors would have needed to register five events to present a rock-solid case for dark matter detection.
Some will no doubt find the CDMS-2 announcement disappointing. Rumors had roiled the blogosphere since Resonaances, a particle theory blog, inaccurately reported December 7 that the CDMS collaboration would publish its results in the journal Nature, which would hint at a discovery with broad appeal and acceptance beyond the particle-physics community. (Scientific American is part of Nature Publishing Group.)
SuperCDMS, which will involve more detectors and hence more material for WIMPs to bump into, is already taking shape at Soudan and may reach full deployment by summer 2010, the collaboration noted. The unambiguous direct detection of dark matter, one of the biggest prizes in physics, may still be up for grabs when it does.