for today’s top stories, visit
sn Today at www.sciencenews.org
the simulated dark matter. “My best bet
is that something has gone astray in the
measurements or analysis,” says Flynn,
a referee of the paper. “I was able to show
to my own satisfaction that there was a
problem of some kind in the method,
but couldn’t diagnose where the prob-
lem really is.”
Moni Bidin says future observations
of millions of stars should help sort out
what’s going on. Right now, he’s hesitant
to infer too much from the results and
notes that scientists trying to capture
dark matter particles on Earth might be
fighting a losing battle. If the solar neigh-
borhood really is a dark matter desert,
there are broader consequences for
the distribution of dark matter around
the Milky Way.
Instead of being spherical, the dark
halo might be elongated and shaped
more like a cigar, a squashed beach ball or
a rugby ball. A cigar or rugby ball oriented
perpendicularly to the galactic plane
would fit the absence of dark matter near
the sun. “But models and theories have
great trouble explaining the observation
of this shape,” Moni Bidin says.
Back Story | SEARCHING FOR WIMPS
Physicists suspect that dark matter
takes the form of weakly interacting
massive particles, called WiMPs. as
the name suggests, WiMPs don’t generally mingle with ordinary matter. But
their sheer abundance should make
them detectable to experimenters on
Earth. scientists trying to directly detect
dark matter particles generally bury
detectors deep underground —to avoid
contaminating background signals from
cosmic rays and other sources —and
use a variety of substances capable of
registering a collision with a WiMP. at
least one team has found evidence for
a WiMP-wind— or seasonal fluctuation
in dark matter particles, the product
of Earth’s orbit carrying it through the
galactic dark matter halo. But those
reports are inconsistent with other
experiments’ negative results, and the
WiMP sightings to date have been less
than definitive, leaving scientists puzzled. so far, no Earth-based experiment
set up to capture WiMPs has produced
a confirmed detection.
The two tunnels at the entrance of the underground Gran Sasso Laboratory in
Italy, where the XENON100, CRESST and DAMA experiments are housed.
CDMS, the cryogenic Dark Matter search,
looks for WiMPs using germanium and
silicon crystals chilled to nearly absolute
zero. it’s located in an old mine shaft
hundreds of meters beneath soudan, Minn.
in 2009, it announced the detection of
two possible WiMP particles, a finding
that might have been due to background
interference and so remains unconfirmed.
CoGeNT, in the same Minnesota cavern as
cDMs, uses a wedge of germanium in its
detector. in 2010, the team reported hundreds
of potential WiMP particles, still unconfirmed.
in 2011, researchers described suspected
evidence for a seasonal fluctuation in dark
matter, which is also still subject to debate.
XENON100, located beneath Gran sasso
mountain in italy, relies on vats of liquid
xenon. in 2011, researchers reported finding
no evidence of WiMPs.
CRESST, another Gran sasso experiment,
uses calcium tungstate crystals. This team
reported a potential signal in september
2011 that is still unconfirmed.
DAMA, also at Gran sasso, uses a sodium-iodide detector. first in 1998 (and again in
2008 and 2010), the DaMa team reported
detecting a seasonal dark matter fluctuation
that remains unconfirmed by other groups.