IEEE Spectrum February, 2014 - 20

nEwS

the deepeSt dark
detector
China's PandaX project will look for dark matter
beneath a marble mountain
creating visible evidence. Such interactions,
In the heart of a mountain
in China's Sichuan province, however, would be incredibly rare.
Why look for WIMPs under a mountain in
underneath 2400 meters of
stone, researchers are pow- China? "I think that one should go to the place
where one can do the best experiments," says
ering up the most ambitious effort yet to
collaborating researcher Wolfgang Lorenzon,
directly detect some of the strangest stuff
in the universe: dark matter. Early this year, a physics professor at the University of
Michigan, in Ann Arbor. Lorenzon explains
the PandaX (Particle and Astrophysical
that in direct dark-matter-detection experiXenon) experiment will start collecting data
in hopes of finding evidence of the elusive
ments, it's crucial to shield the detection area
particles, thought to constitute more than
from other sources of radiation, which could
80 percent of the matter in the universe.
be mistaken for the WIMP signal. China's new
Physicists first hypothesized the existence
underground lab is the deepest in the world,
of dark matter to explain the "missing mass" meaning it's well protected from cosmic radiaproblem-the fact that galaxies have a greater tion; in addition, the rock around it is marble,
gravitational effect than their visible mat- which is particularly devoid of radioactive
materials that could produce false signals.
ter can explain. The current theory holds
that dark matter is composed of weakly "The big advantage is that PandaX is much
cheaper and doesn't need as much shieldinteracting massive particles (WIMPs) that
interact with ordinary matter only through
ing material," Lorenzon says.
gravity and the "weak force," the extremely
PandaX is one of several dark-mattershort-range fundamental force responsible
detection projects operating on the same
for nuclear decay. If a WIMP bumps directly
general principle. The experiment looks
into the nucleus of an atom of ordinary mat- for interactions within a big tank of xenon,
ter, the theory goes, it might interact with which is cooled to a liquid. If a WIMP does
it and cause the emission of other particles, collide with a xenon nucleus, the recoil20

fEb 2014

nORTh aMERICan

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ing 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, says 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 says.
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, says he's pleased
to see another entrant in the field. "I'm excited about seeing China developing a fundamental physics program," says Gaitskell.
"What we have to wait and see [about] 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. -eliza str ick l a nd

SCott StephenSon

kInG unDer the MountaIn: In the PandaX
experiment, a vat of liquid xenon is stored beneath
hundreds of meters of rock. With luck, the isolation will
keep things quiet enough to sense signs of dark matter.

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Table of Contents for the Digital Edition of IEEE Spectrum February, 2014