The world’s most sensitive search for dark matter announced today that it has found—nothing. The first results from the Large Underground Xenon (LUX) detector are null, scientists say, indicating that the invisible matter thought to make up a large chunk of the universe is even more elusive than many experts thought.
Buried about a kilometer and a half underground in a repurposed South Dakota gold mine that is now the Sanford Underground Research Facility, the LUX experiment searches for signs of dark matter particles colliding with the atoms in a vat of liquid xenon. During its first three months of operation the detector found no such signals whatsoever. “We looked hard for these dark matter particles and we didn’t see anything,” says physicist Rick Gaitskell of Brown University, LUX co-spokesperson. The results, presented at a seminar today and submitted to Physical Review Letters for publication [pdf], rule out a number of possible masses and characteristics for the particles that make up dark matter. The null result also conflicts with earlier experiments that had reported possible dark matter signals.
About a quarter of the universe seems to be dark matter, which makes its presence felt through gravity, despite the fact that it cannot be seen or touched. A leading explanation for dark matter posits that it is made of particles called WIMPs (weakly interacting massive particles). If they exist, a billion of these WIMPs probably pass through your body every second without any of your atoms noticing. The particles’ reticence to interact with normal matter presents a challenge to physicists who aim to detect dark matter. Hypotheses suggest, however, that once in a very rare while WIMPs should slam into normal atoms instead of passing through the space between them.
LUX researchers hope to catch these scarce impacts by measuring light particles (photons) given off by a xenon atom that has been bumped by dark matter. To reduce the chances of anything else causing the xenon to emit light—such as charged particles from space called cosmic rays—the detector is heavily shielded and buried deep in the mine. In terms of background radioactivity, cosmic rays and other contaminants, the center of LUX’s tank of 370 kilograms of xenon is the “quietest place in the world,” Gaitskell says. The experiment is twice as sensitive as other detectors to hypothetical dark matter particles with large masses, and provides an even greater improvement if dark matter particles are relatively lightweight, the scientists say. The fact that LUX has yet to register any such hits indicates that the particles in the mass range it is sensitive to—between five and 10,000 times the mass of a proton (a unit called a giga–electron volt)—interact extremely rarely with regular matter.
The new LUX results also cast doubt on previous claims of possible dark matter detection. The DAMA (for DArk MAtter) project in Italy claimed to have seen signs of WIMPs more than a decade ago, and more recently the CDMS (Cryogenic Dark Matter Search) and the CoGeNT (Coherent Germanium Neutrino Technology) experiment, both in Minnesota, saw handfuls of events that might be attributable to dark matter. “I’m afraid I can’t see their claims really surviving this,” Gaitskell says.
The other teams, however, are not ready to concede defeat. Juan Collar of the University of Chicago, who heads the CoGeNT project, says he believes that the LUX team has not properly accounted for electric field effects and may therefore have overestimated the sensitivity of the xenon detector for low-mass WIMPs. Blas Cabrera of Stanford University, who leads the CDMS project, also maintains that what his project has seen may still prove to be dark matter. “It is unlikely that LUX has ruled out the entire region of interest” for low-mass WIMPs because xenon is not as sensitive as other materials to dark matter in that mass range, he says. (CDMS uses silicon and germanium detectors.) “In spite of these friendly criticisms we are all excited by the success of a new carefully built WIMP search experiment,” Cabrera says. “We also continue to believe that, given the difficulty of all experiments and the uncertainty of the properties of dark matter particles, it is very important [to use] multiple target materials and different detector technologies.”
The competition is fierce to be the first experiment to find dark matter, but the researchers say they care more about answers than fame. “At this point in my career, I am much less concerned with which experiment discovers the dark matter particles, and much more I would like to know within my lifetime,” Cabrera says.
LUX is the latest in a series of searches that have been running for more than three decades, all without finding conclusive evidence of dark matter. But the scientists are not discouraged. “I can’t say I’m disappointed,” says LUX co-spokesman Daniel McKinsey of Yale University. “We’re just so happy the instrument is performing so well.” LUX continues to gather data, and the researchers are already planning an even bigger xenon detector called LUX-ZEPLIN. “Hopefully, next time we’ll be announcing a [positive] result,” Gaitskell says. “That, as they say, we’ll leave in the lap of the gods.”