On Thursday, physicists in China reported the latest result in the search for particles of dark matter, the mysterious stuff whose gravity holds the galaxies together. Researchers with the Particle and Astrophysical Xenon (PandaX) detector spotted no sign of their quarry, which isn't surprising because PandaX isn't yet as sensitive as a detector already running in the United States that hasn't seen anything either. Still, the finding is notable because the PandaX detector features a clever design that might enable it to vie for the sensitivity lead in the next year or so.
The new work "is very credible," says Richard Gaitskell, a physicist at Brown University and a member of the team working with the Large Underground Xenon (LUX) detector at the Sanford Underground Research Facility in Lead, South Dakota, the current leader in sensitivity. Rafael Lang, a physicist at Purdue University in West Lafayette, Indiana, and a member of the collaboration building an even more sensitive detector known as XENON1T in Italy's subterranean Gran Sasso National Laboratory, agrees. PandaX researchers "are doing a great job in [catching up] and making extremely fast progress," he says.
These detectors stalk hypothetical weakly interacting massive particles, or WIMPs. Weighing a few to hundreds of times as much as a proton, such particles would interact with ordinary matter only through their gravity and the extremely feeble weak nuclear force, making them ideal candidates for dark matter. Our galaxy could be floating in a vast cloud of WIMPs. Physicists might detect WIMPs on the rare occasions when one bounces off an atomic nucleus in an exquisitely sensitive detector.
In fact, over the past couple of decades a number of detectors have reported possible sightings of WIMPs in the low-mass range. But those signals have generally not agreed on the precise mass of the WIMPs or the strength with which they interact with ordinary matter. And LUX and XENON100, the predecessor to XENON1T, claim to have ruled out those signals.
So do the new data from PandaX. Like LUX and the XENON detectors, PandaX consists of a tank of frigid liquid xenon. Were a WIMP to crash into a xenon nucleus, it would send the nucleus flying, producing an immediate flash of light that could be pinpointed by photodetectors on the top and bottom of the tank. The collision would also liberate the electrons from the xenon atom. They would float to the top of the tank and out into the gaseous xenon at the top, where they would produce a second, confirmatory flash of light. In 17 days of data taking, PandaX researchers spotted no WIMPs in their detector, which contained 120 kilograms of liquid xenon, 37 of which served as the target "fiducial" mass, as the team reports in a paper in press at Science China Physics, Mechanics & Astronomy.
It would have been more surprising had PandaX seen a signal. That's because LUX, which contains 370 kilograms of xenon with a fiducial mass of 118 kilograms, already ruled out low-mass WIMPs last October. Still, the result is intriguing because it shows that the PandaX team is quickly catching up to the rest of the world. The collaboration, which now numbers roughly 40 members, started in 2010, when XENON100 was already running and LUX was under development. "We started off basically from nothing," says Xiangdong Ji, a physicist at Shanghai Jiao Tong University in China and the University of Maryland, College Park. "We didn’t have a group, we didn't have equipment, we didn't have anything."
One thing they did have, however, was the world's deepest underground laboratory. The 2400-meter-deep China JinPing underground Laboratory was built in just 18 months in 2009 and 2010, next to highway and water tunnels through a mountain with the help of the Ertan Hydropower Development Company Ltd.
The first PandaX results better those of XENON100 in terms of sensitivity, Ji says. And PandaX researchers aim to take a stab at the lead in the race to discover WIMPs. Their detector was designed so that to make it bigger, they need only raise the lid of their cylindrical tank and pour in more liquid xenon, Ji explains. So researchers hope to have it up and running with a total mass of 500 kilograms and a fiducial mass of 200 kilograms early next year. That might just give the PandaX team a shot at catching up to LUX in sensitivity. And it could enable it to get a result out before the first result from XENON1T, which will have a total mass of 3200 kilograms and a fiducial mass of 1000 kilograms. "We have a window of about a year," Ji estimates.
Lang says he's skeptical that PandaX will really take the lead. But Gaitskell says he thinks PandaX can be a player—if researchers can show that they've adequately calibrated their detector and can run it stably for hundreds of days. "If they're going to leave the peloton for the breakaway, they're going to have to demonstrate that they can pedal for more than 17 days."