 
New
cryogenic detectors probe recent evidence for dark matter particle
29
Feb 2000
By
Robert Sanders, Public Affairs
LOS
ANGELES--A new generation of particle detector that operates
at temperatures near absolute zero has proven extremely accurate
in identifying the particles that crash through it, an international
team of scientists reported last week.
The
novel detector, buried 35 feet underground on the Stanford University
campus, has dedicated itself for more than a year to the search
for exotic and elusive particles that, according to some theories,
make up more than 90 percent of the mass of the universe.
Though
the device has yet to find evidence of such particles - known
collectively as WIMPs, or weakly interacting massive particles
- it has proved to have a keen ability to discriminate between
the different kinds of particles that zip through it.
Developed
over the past 11 years by scientists at 10 institutions and
coordinated by the Center for Particle Astrophysics at the University
of California, Berkeley, it uses germanium or silicon semiconductors
cooled to several hundredths of a degree above absolute zero,
the coldest possible temperature. The scientific team goes by
the name Cryogenic Dark Matter Search, or CDMS.
One
of the more common particle detectors today relies upon sodium
iodide crystals that give off a burst of light when a particle
passes through it. When a particle passes through the new semiconductor
detector, the detector is both ionized and heated. Measurement
of both allows much better discrimination and identification
of particles.
UC
Berkeley assistant research physicist Rick Gaitskell reported
the consortium's results and conclusions Feb. 25 at the Fourth
International Symposium on Sources and Detection of Dark Matter
in the Universe in Marina del Rey, Calif.
Another
group of researchers based in Rome and Beijing and calling themselves
the DArk MAtter (DAMA) group, reported just the opposite at
the same meeting. Using a detector containing 100 kilograms
of sodium iodide, they claim to see evidence for the existence
of a neutralino, a type of WIMP predicted by the most popular
theory of particle physics, supersymmetry.
"Given
our own results and our level of sensitivity, I have to disagree
with the interpretation of the DAMA results in terms of Weakly
Interacting Massive Particles," said Bernard Sadoulet,
a member of the CDMS team. Sadoulet, a professor of physics
at the University of California, Berkeley, is director of the
Center for Particle Astrophysics and has worked for more than
15 years on WIMP searches.
"True,
since we are using different target materials we both could
be right, but within the currently favored theoretical framework
our results appear seriously incompatible. We are simply not
observing enough events in CDMS above our neutron contamination,
and we have found no experimental effect that could account
for this."
The
American team's novel detector is comparable in sensitivity
to the sodium iodide detector of the Italian group, but is superior
in its ability to distinguish background "events"
- interactions in the detector that result from known particles
- from likely dark matter interactions.
"The
discrimination capability of these detectors is amazing,"
said Sunil Golwala, a UC Berkeley graduate student who did a
large fraction of the work reported this week. "We're literally
looking for a needle in a haystack. We start with 6.4 million
events and end with 13 events that are of interest for dark
matter searches. And these 13 events are, in fact, what we expect
based on our simulations of the neutron background in our facility."
"What
is most exciting to me is that we've been able to employ a new
type of detector, developed explicitly to do this type of measurement,
to obtain a result with implications for a fundamental question
in cosmology," said Tony Spadafora, associate director
of CfPA. "These measurements are difficult and take a long
time because WIMP signals are thought be very small and infrequent".
The
CDMS collaboration is supported jointly by the Department of
Energy and by the National Science Foundation in a collaboration
that includes groups from UC Berkeley, Stanford, UC Santa Barbara,
Lawrence Berkeley National Laboratory, Fermi National Accelerator
Laboratory (Fermilab), Case Western Reserve University, Santa
Clara University, the National Institute of Standards and Technology
in Boulder, Co., the University of Colorado at Denver and Princeton
University.
Their
findings have been submitted to Physical Review Letters, and
are available at http://arXiv.org/abs/astro-ph/?0002471.
For
more than a decade, various experiments around the world have
searched for dark matter in the form of WIMPs. Theoretically,
a million WIMPs would pass through an area the size of a thumbnail
each second, but only about one per day would interact in a
one-kilogram germanium detector.
Detectors
capable of recording one such event per day per kilogram, or
less, are sufficiently sensitive to search for a particular
type of WIMP suggested by supersymmetry - the neutralino.
Last
week, the DAMA collaboration reported on more than three years
of data collected with a 100-kilogram sodium iodide detector
operated deep underground in the Gran Sasso National Laboratory
in Italy. In a statistical analysis of tens of thousands of
events, they claim to see a modulation in the event rate with
the same period as Earth's solar year, with a maximum in June
and a minimum in December.
This
annual modulation is expected to result from the motion of the
sun and earth system through a massive cloud of WIMPs as our
solar system rotates about the center of our galaxy.
The
CDMS experiment is based on about half a kilogram of detector
mass operated over one year and producing about 12 kilogram
days of data.
The
CDMS experiment is housed in a cave; the dirt helps shield the
cryogenic detectors from cosmic radiation. Further shielding
consists of lead, polyethylene and active plastic scintillator,
which produces a tiny light flash for every particle interaction.
These allow the influence of cosmic and terrestrial radiation
to be reduced by a factor of about 10,000.
The
only particles that can get through and mimic WIMPs such as
the neutralino are neutrons, which also bounce off atomic nuclei
in the detectors. During its one-year run, CDMS observed 13
single scattering nuclear recoils, all of which were identified
as neutrons and not WIMPs.
"The
method used by DAMA is both very clever and very demanding,"
Sadoulet said. "The rate variation that they are measuring
over the year is about one percent deviation from the mean,
and it is extremely difficult to guarantee that it is not due
to instrumental effects. Our Italians colleagues have tried
their best and so far have found no other explanation than a
WIMP signal. Given the significance that such an interpretation
would have, it is very important to keep looking for more mundane
explanations. Many things vary seasonally, for instance the
temperature of the laboratory, the level of water in the Gran
Sasso mountain, which could influence in subtle ways the response
of the DAMA apparatus. To be frank, although I have not enough
information to identify the possible cause, I am not totally
convinced by the DAMA arguments claiming to rule out such effects.
"
These
reports are not the end of the story, the researchers say. CDMS
and DAMA both plan expanded new experiments. DAMA will increase
its detector mass from 100 to 250 kilograms, and the newly approved
CDMS-II experiment will move deep underground to the Soudan
mine in northern Minnesota. CDMS-II will utilize more than 10
times the present detector mass in an environment where the
neutron background has been reduced by nearly a factor of 1,000.
"We
are beginning to probe interesting territory for WIMPs and are
looking forward to a factor of 100 increase of sensitivity with
our second-generation experiment," Spadafora said.
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