DARK MATTER: Racing to Capture Darkness
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Thu, 05 Jul 2007 23:28:05 GMT
Science 6 July 2007:
Vol. 317. no. 5834, pp. 32 - 34
DOI: 10.1126/science.317.5834.32
DARK MATTER: Racing to Capture Darkness
Adrian Cho and Richard Stone
http://www.sciencemag.org/cgi/content/full/317/5834/32
YANGYANG, SOUTH KOREA, AND BATAVIA, ILLINOIS--Deep inside Korea's
Jeombong Mountain, in a vault suffused with an eldritch red glow, a
giant black cube begins to unfold. One thick, lead-lined wall filled
with mineral oil, along with the box's base, inches away from the
rest of the structure to reveal a smaller cube of shimmering copper.
A young man steps up and pulls a chain, hand over hand, and
gradually, amid the clatter of steel, the face of the copper cube
rises. The rarest of coins or the relics of a saint might be accorded
such sanctity, but here, in an anteroom to a tunnel delved for a
hydropower station in northeastern Korea, the treasure is precious
only to a particle physicist. Inside the copper cube are a dozen
blocks of crystalline cesium iodide, doped with thallium and wired
with electronics that will register the tiniest scintilla of light
produced inside the crystals. Researchers are making a few final
tweaks to their crystal array before sealing it up again and
beginning an otherworldly quest.
The 15 centimeters of gamma ray-blocking lead and neutron-quenching
oil in the black cube, the 10 centimeters of copper that absorb
x-rays from the lead, the nitrogen piped into the copper box, the red
light, and the 700 meters of rock between the chamber and the
outdoors all have a singular purpose: to minimize the number of
spurious flashes inside the crystals. Here at the Korea Invisible
Mass Search (KIMS) experiment, researchers are hoping to be the first
to spot what no one--indisputably--has seen before: particles of dark
matter.
After years of preparation, physicist Kim Sun Kee of Seoul National
University and his KIMS colleagues began taking data here last month
with a 100-kilogram array of crystals. Each day they hope to record
one or two instances of weakly interacting massive particles
(WIMPs)--prime candidates for dark matter--tickling cesium and iodine
nuclei in a way that liberates a flash of light. That's assuming dark
particles tangle with ordinary particles as many models predict. "If
they don't interact with matter, we have no hope to find them," says
Kim.
The KIMS experiment is one of a few dozen experiments racing to
detect dark-matter particles. Like Kim's team, groups in several
countries are engaged in so-called direct searches, striving to spot
the particles jostling ordinary atomic nuclei. Others are turning to
the skies in indirect searches that seek signs of dark-matter
particles annihilating one another in the hearts of galaxies.
Meanwhile, the world's most powerful atom smasher, the Large Hadron
Collider (LHC) near Geneva, Switzerland, could make dark matter as
soon as it turns on next spring.
"This is the epoch in which the central theoretical predictions are
finally being probed," says Blas Cabrera of Stanford University in
Palo Alto, California, who for a decade has stalked dark matter as
the co-spokesperson of the Cryogenic Dark Matter Search (CDMS)
project. "The best guess is within reach." That prospect thrills
researchers. At a recent workshop* at Fermi National Accelerator
Laboratory (Fermilab) in Batavia, Illinois, more than half the 170
attendees wagered that dark-matter particles will be detected within
5 years.
Discovery is not guaranteed. The favored theoretical models suggest
that experimenters should soon have dark matter in their grasp, but
others predict the ghostly particles will be so elusive that
researchers can never hope to snare them. It's a make-or-break
situation, predicts Rocky Kolb, a cosmologist at the University of
Chicago in Illinois: "Either in 5 years we will know what dark matter
is, or we will never know."
Astronomers first sensed dark matter's shadowy presence more than 70
years ago. In 1933, Fritz Zwicky of the California Institute of
Technology in Pasadena calculated that the Coma Cluster of galaxies
contains too little visible matter to hold itself together. Some
unseen matter must supply the extra gravity that keeps the galaxies
from flying into space, he reasoned. That maverick idea gained
credence about 4 decades later when astronomers found that individual
galaxies also lack enough luminous matter to hold on to their stars,
suggesting that each galaxy is embedded in a vast clump, or "halo,"
of dark matter.
Evidence continues to mount. In 2003, researchers with NASA's
orbiting Wilkinson Microwave Anisotropy Probe (WMAP) measured the big
bang's afterglow--the cosmic microwave background--the temperature of
which varies ever so slightly across the sky (Science, 14 February
2003, p. 991). The pattern of hot and cold spots reveals much about
how the universe evolved, and researchers found they could explain
the observed pattern if the universe consists of 5% ordinary matter,
22% dark matter, and 73% weird space-stretching "dark energy," all
interacting through gravity.
Researchers have never captured a speck of dark matter, however. Like
a cosmic Cheshire Cat, the stuff hides in plain sight, presumably
floating through our galaxy and the solar system and showing only its
gravity as its grin. That coyness vexes physicists, who assume that
dark matter must consist of particles. "This is the best evidence we
have of new physics," says Jonathan Feng, a theorist at the
University of California, Irvine. "It's simply a fact that there is
dark matter, and we don't know what it is." Theorists have dreamed up
dozens of possibilities. Dark matter could be particles that would
exist if space has minuscule extra dimensions. Or it could be
particles called axions that have been hypothesized to patch a
conceptual hole in the theory of the strong force that binds the
nucleus.
Most promising may be the idea that dark matter consists of particles
predicted by supersymmetry, a theoretical scheme that pairs every
known particle with a heavier, undiscovered superpartner. The
lightest superpartner, expected to be a few hundred times as massive
as a proton, could be the long-sought WIMP. And if it interacts with
ordinary matter as anticipated, then a simple calculation shows that
roughly the right amount of WIMPy dark matter should remain from the
big bang. That uncanny coincidence, or "WIMP miracle," suggests that
supersymmetry is more than another stab in the dark, Feng says.
The proof is in the particles. The most obvious way to find them is
to catch them bumping into ordinary matter, and the KIMS experiment
joins more than a dozen experiments that are hunting for collisions
with ever greater sensitivity--including one that claimed a signal.
Spotting dark matter is easier said than done, however. The particles
should interact with ordinary matter even more feebly than do
neutrinos, which can zip through Earth unimpeded. Researchers must
also shield detectors from cosmic rays and other ordinary particles
so that they may perceive the soft cries of dark particles amid the
din of ordinary collisions.
In the race to capture darkness, the frontrunner for the past few
years has been an experiment called CDMS, which runs in the Soudan
Mine in northern Minnesota. Its 5-kilogram "cryogenic" detector
consists of stacks of germanium and silicon wafers cooled to within a
fraction of a degree of absolute zero. If a WIMP crashes into a
nucleus, it should knock loose several electrons and produce a tiny
pulse of heat. Analyzing both the charge and heat signals,
researchers can look for dark-matter particles and weed out neutrons
and other red herrings.
Now, another experiment has taken the lead in sensitivity. The
XENON10 experiment, which resides in a tunnel in Gran Sasso, Italy,
consists of a tank filled with 15 kilograms of liquid xenon. When
pinged by a WIMP, a xenon nucleus should rebound through the liquid
to produce a flash of light and knock free a handful of electrons. In
April, the XENON10 team, led by Elena Aprile of Columbia University,
reported that it had searched with five times the sensitivity of
CDMS--and found nothing.
To go head to head with such efforts, the KIMS team had to start from
scratch. A decade ago, Korea did not have a particle physics
facility. "We always had to go abroad for research and training,"
says Kim, who cut his teeth at Japan's KEK accelerator laboratory in
Tsukuba in the 1980s. When South Korea's science ministry launched a
Creative Research Initiative in 1997, Kim, with colleagues Kim Hong
Joo of Kyungpook National University in Daegu, South Korea, and Kim
Yeong Duk of Sejong University in Seoul, pounced. Thrice the trio of
Kims submitted their aptly named KIMS proposal, and thrice they
failed. Finally, in 2000, they opted for a novel cesium iodide
detector--and got funded. They caught a second break when during
construction of the Yangyang Pumped Storage Power Plant, a small
section off one tunnel caved in, and plant officials were amenable to
hosting the experiment. "We were very lucky," says Kim Sun Kee. The
collapse "opened up just enough space for the experiment."
Since then, the most arduous task has been to develop a detector
largely free of trace radioactive isotopes. The KIMS team has also
spent 3 years studying the scintillation signals of gamma rays and
stray cosmic rays, which cause chain reactions in the atmosphere that
give rise to a background "noise" of hurtling neutrons. "The neutron
signal is very similar to what we expect a WIMP signal to look like,"
Kim explains, so the experimenters must find ways to screen it out.
So far they have reduced it by 99.999%, he says.
KIMS won't immediately rival CDMS and XENON10 for overall
sensitivity. But KIMS will excel in one important regard: If the
WIMP-nucleus interaction depends on how each particle spins, KIMS
will have a better chance of seeing the effect. "That makes KIMS
complementary with CDMS and XENON10," Kim says.
KIMS can also test one of the more spectacular recent claims in
physics. In 1997 and again in 2000, researchers with the Italian DAMA
experiment at Gran Sasso reported evidence of WIMPs in a 100-kilogram
array of sodium iodide crystals (Science, 3 March 2000, p. 1570). The
team found that the rate of flashes went up and down with the
seasons. That would make sense if the galaxy turns inside a cloud of
WIMPs so that the solar system faces a steady WIMP wind. As Earth
circles the sun, it would alternately rush into and away from the
wind, causing the collision rate to rise and fall.
No other experiment has reproduced the DAMA signal, however, and most
physicists dismiss the sighting. Because KIMS employs a similar
detector array--with cesium iodide instead of sodium iodide--many
experts say it can provide an unambiguous test of the DAMA results.
DAMA group leader Rita Bernabei, a physicist at the University of
Rome Tor Vergata, disagrees. "No direct comparison will be possible,"
she argues, because cesium iodide is less sensitive to low-mass
dark-matter particles than DAMA's detectors were. In 2003, Bernabei's
group fired up an upgraded 250-kilogram detector called DAMA/LIBRA.
Its initial findings are due to be released next year.
The competition among dark-matter experiments is heating up. The CDMS
team has already collected enough data to retake the sensitivity lead
this summer. Meanwhile, researchers in North America, Europe, and
Asia are deploying or planning a gaggle of ever more ambitious
detectors, including XMASS, an 800-kilogram spherical liquid xenon
detector that won funding this year and will be built in Kamioka,
Japan. "For the first time, the direct detection experiments are
moving into a regime where theorists would say that a priori you
would expect to see something," says Lawrence Krauss, a theorist at
Case Western Reserve University in Cleveland, Ohio.
Meanwhile, astronomers are searching for signs of dark-matter
particles in the heavens. When two WIMPs in a galactic halo collide,
theory says they can annihilate each other to produce high-energy
gamma ray photons or other ordinary particles. The emerging
generation of gamma ray "telescopes" should be well-suited to search
for such signs. Since 2004, the European-funded High Energy
Stereoscopic System (HESS) in Namibia, Africa, has used its four
detectors to look for light created when a gamma ray smashes into the
atmosphere and triggers an avalanche of particles. Similarly, the
Very Energetic Radiation Imaging Telescope Array System (VERITAS) at
the base of Mount Hopkins in Arizona began taking data earlier this
year. "The gamma ray observations are really the only way to measure
the halo distribution and tie this all together," says James Buckley,
an astronomer at Washington University in St. Louis, Missouri, who
works on VERITAS.
HESS has already mapped the gamma ray glow coming from the heart of
our Milky Way galaxy, the most obvious place to look for dark matter.
Unfortunately, those gamma rays come overwhelmingly from more mundane
sources, such as hot gas. So researchers may have to turn away from
the central glare and look at so-called dwarf spheroidal galaxies
that orbit our galaxy. Those galaxies should come into fuller view
when NASA's Gamma-ray Large Area Space Telescope (GLAST) blasts into
orbit, perhaps as early as this winter.
Dark-matter annihilations would produce other particles, too. The
Russian-Italian satellite PAMELA is looking for antiprotons and other
antiparticles born in the process. And IceCube, an array of 4200
light sensors being lowered into the South Pole ice, could spot
neutrinos from annihilations in the sun. Zipping along with
tremendous energy, measured in billions of electron volts or GeV, a
few would interact with the ice to create flashes of light. A stream
of 100 GeV neutrinos coming out of the sun would be a sure sign of
dark matter huddling there, says Francis Halzen, a physicist at the
University of Wisconsin, Madison. "How else do you get a 100 GeV
neutrino out of the sun?"
Before researchers find dark-matter particles, they may be able to
manufacture them. The European LHC will smash protons together at
energies seven times greater than any previous collisions,
recreating, in billions of tiny explosions, conditions that haven't
existed since the big bang. If superpartners exist, the LHC should
crank them out by the thousands, says Alex Tumanov of Rice University
in Houston, Texas, who works on an LHC particle detector. "Most of
these models predict that we will find or exclude the dark matter
particles within 1 or 2 years," he says. "That's why everyone is so
excited. We're on the doorstep."
Even if the LHC spews out new particles, however, it might not reveal
enough about them to nail down which of the many versions of
supersymmetry nature plays by, says Michael Schmitt of Northwestern
University in Evanston, Illinois. That would require another collider
that could study particles in greater detail: the proposed
40-kilometer-long International Linear Collider.
Ultimately, all three methods--direct detectors, telescopes, and
colliders--may have to strike pay dirt before scientists can say what
dark matter is. "It's really going to require that we detect the
particles in our galaxy and produce them in the lab, and that we
convince ourselves that they are the same thing," says Edward Baltz,
a theorist at Stanford University. In the race to spot dark matter,
he says, "You don't win until everybody finishes."
Of course, the efforts may not come together so harmoniously. Direct
searches might spot particles so massive that the LHC can't generate
them. Or, in spite of the "WIMP miracle," dark matter might turn out
to comprise several different types of particles. Researchers also
face a psychological challenge if they do see something. "The first
thing that you would say would be, 'Is this real?' " says Daniel
Akerib, a CDMS team member from Case Western. "The first thing we
would have to do is to try to make it go away" and prove it was a
spurious signal, he says. That could be tricky, as it would require
checking every conceivable way an ordinary particle might mimic a
WIMP.
Still, that's a problem most researchers, including Kim Sun Kee,
would love to have. Kim hopes that within a year, his team members
will have accumulated enough data in their Korean crypt to reveal a
convincing WIMP signal. The form of a WIMP behind that Cheshire grin
is another question. "We don't know what a WIMP will look like," says
Kim. They may soon find out--and solve one of the bigger mysteries in
physics.
See: http://www.sciencemag.org/cgi/content/full/317/5834/32
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