In the heart of a mountain in China’s Sichuan province, underneath 2400 meters of stone, researchers are powering up the most ambitious effort yet to directly detect some of the strangest stuff in the universe: dark matter. Early this year, the PandaX (Particle and Astrophysical Xenon) experiment will start collecting data in hopes of finding evidence of the elusive particles, thought to constitute more than 80 percent of the matter in the universe.
Physicists first hypothesized the existence of dark matter to explain the “missing mass” problem—the fact that galaxies have a greater gravitational effect than their visible matter can explain. The current theory holds that dark matter is composed of weakly interacting massive particles (WIMPs) that interact with ordinary matter only through gravity and the “weak force,” the extremely short-range fundamental force responsible for nuclear decay. If a WIMP bumps directly into the nucleus of an atom of ordinary matter, the theory goes, it might interact with it and cause the emission of other particles, creating visible evidence. Such interactions, however, would be incredibly rare.
Why look for WIMPs under a mountain in China? “I think that one should go to the place where one can do the best experiments,” said collaborating researcher Wolfgang Lorenzon, a physics professor at the University of Michigan, in Ann Arbor. Lorenzon explains that in direct dark-matter-detection experiments, it’s crucial to shield the detection area from other sources of radiation, which could be mistaken for the WIMP signal.
China’s new underground lab is the deepest in the world, meaning it’s well protected from cosmic radiation; in addition, the rock around it is marble, which is particularly devoid of radioactive materials that could produce false signals. “The big advantage is that PandaX is much cheaper and doesn’t need as much shielding material,” Lorenzon said.
PandaX is one of several dark-matter-detection projects operating on the same general principle. The experiment looks for interactions within a big tank of xenon, which is cooled to a liquid. If a WIMP does collide with a xenon nucleus, the recoiling nucleus will plow through other xenon atoms and cause the emission of both photons (detectable by a light sensor) and electrons, which will travel through the liquid xenon at a known speed to be detected at the top of the tank. By comparing the two signals, researchers can determine where in the tank the particle interaction occurred. Because the walls of the tank emit some trace radiation themselves, only interactions in the center region of the tank are valid signals.
Ji Xiangdong, the principal investigator of the US $8 million project and the director of the Institute of Nuclear and Particle Physics at Shanghai Jiao Tong University, said one unique feature of PandaX is that it’s designed to be scaled up rapidly. If the project’s first phase bears fruit, Ji hopes to gain funding for the second phase, which would involve a tank holding 2.4 metric tons of xenon, allowing for an interior target area of about 1 metric ton, which is twice the amount of xenon that will be in use by the end of the first phase of experiments. “If you have that large a volume, you’re more likely to see something,” Ji said.
That volume would by far exceed the other big dark-matter-detection projects: the Xenon project beneath a mountain in Italy and the Large Underground Xenon (LUX) project at a lab in an abandoned mine in South Dakota. But the field is competitive: The European experiment will also be scaled up to a 1-metric-ton target in the next few years.
The search for the WIMP is a Nobel Prize-worthy pursuit, and Richard Gaitskell, a physics professor at Brown University and a spokesperson for LUX, said he’s pleased to see another entrant in the field. “I’m excited about seeing China developing a fundamental physics program,” said Gaitskell. “What we have to wait and see is the degree to which the experiment can come up to speed on multiple fronts: cryogenics, shielding, internal radioactivity, and so on.”
With PandaX’s first results expected this year, researchers will soon know if the detector works as planned. If so, WIMPs will have one less place to hide.
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