CDMS group/Physics
Today: The
Cryogenic Dark Matter
Search (CDMS) experiment, located a half mile underground
at the
Soudan
mine in northern Minnesota claims to have seen two events
that may be dark matter. The evidence however, is not
conclusive, but does limit the interaction range for seeing
dark matter, and rules out some theories on how dark matter
behaves.A more detailed story will appear on the
Physics Today
Update section on the 28 December.
What is dark matter?Astronomical observations
from telescopes, and satellites, and measurements of the
cosmic
microwave background have led scientists to believe that
most of the matter in the universe neither emits nor absorbs
light.This
dark
matter would have provided the gravitational scaffolding
that caused normal matter to coalesce into the galaxies we see
today. In particular, scientists think that our own galaxy is
embedded within an enormous cloud of dark matter. As our solar
system rotates around the galaxy, it moves through this
cloud.Particle physics theories suggest that dark matter may be
composed of
weakly
interacting massive particles (WIMPs). Scientists expect
these particles to have masses comparable to, or perhaps
heavier than, atomic nuclei.Although such WIMPs would rarely
interact with normal matter, they may occasionally scatter from
an atomic nucleus like billiard balls, leaving a small amount
of energy that might be detectable under the right conditions.
Detecting WIMPSThe CDMS experiment uses 30
germanium
and silicon detectors in an attempt to detect such WIMP
scatters.The detectors are cooled to temperatures very near
absolute
zero.Particle interactions in the crystalline detectors
deposit energy in the form of heat, and in the form of charges
that move in an applied electric field. Special sensors detect
these signals, which are then amplified and recorded in
computers for later study.A comparison of the size and relative
timing of these two signals can allow the experimenters to
distinguish whether the particle that interacted in the crystal
was a WIMP or one of the numerous known particles that come
from radioactive decays, or from space in the form of cosmic
rays.These background particles must be highly suppressed if we
are to see a WIMP signal. Layers of shielding materials, as
well as the half-mile of rock above the experiment, are used to
limit the background "noise."
New resultsThe CDMS experiment has been
searching for dark matter at Soudan since 2003. Previous data
have not yielded evidence for WIMPs, but have provided
assurance that the backgrounds have been suppressed to the
level where as few as one WIMP interaction per year could have
been detected.The CDMS group is now reporting on a new data set
taken in 2007-08, which approximately doubles the sum of all
past data sets.With each new data set, the CDMS group must
carefully evaluate the performance of each of the detectors,
excluding periods when they were not operating
properly.Detector operation is assessed by frequent exposure to
sources of two types of radiation: gamma rays and neutrons.
Gamma rays
are the principal source of normal matter background in the
experiment.
Neutrons are
the only type of normal matter particles that will interact
with germanium nuclei in the billiard ball style that WIMPs
would, although neutrons frequently scatter in more than one of
our detectors.Those calibration data are carefully studied to
see how well a WIMP-like signal (produced by neutrons) can be
seen over a background (produced by gamma rays).The expectation
is that no more than one background event would be expected to
be visible in the region of the data where WIMPs should
appear.Since background and signal regions overlap somewhat,
achievement of this background level required the CDMUS group
to throw out roughly 2/3 of the data that might contain WIMPs,
because these data would contain too many background events.All
of the data analysis is done without looking at the data region
that might contain WIMP events. This standard scientific
technique, sometimes referred to as "blinding," is used to
avoid the unintentional bias that might lead one to keep events
that have some of the characteristics of WIMP interactions but
that are really from background sources.After all of the data
selection criteria have been completed, and detailed estimates
of background "leakage" into the WIMP signal region are made,
the CDMUS group must "open the box" and see if there are any
WIMP events present.In this new data set there are indeed
two events seen with characteristics
consistent with those expected from WIMPs.However, there is
also a chance that both events could be due to background
particles. A strict set of criteria for determine whether a new
discovery has been made, in essence that the ratio of signal to
background events must be large enough that there is no
reasonable doubt.Typically there must be less than one chance
in a thousand of the signal being due to background. In this
case, a signal of about 5 events would have met those criteria.
The CDMS group estimate that there is about a one in four
chance to have seen two backgrounds events, so the CDMS group
is not claiming to have discovered WIMPs.Instead they say that
the rate of WIMP interactions with nuclei must be less than a
particular value that depends on the mass of the WIMP. The
numerical values obtained for these interaction rates from this
data set are more stringent than those obtained from previous
data for most WIMP masses predicted by theories.Such upper
limits are still quite valuable in eliminating a number of
theories that might explain dark matter.
What comes next?While the same set of
detectors could be operated at Soudan for many more years to
see if more WIMP events appear, this would not take advantage
of new detector developments and would try the patience of even
the most stalwart experimenters (not to mention theorists). A
better way to increase the sensitivity to WIMPs is to increase
the number (or mass) of detectors that might see them, while
still maintaining the CDMS group's ability to keep backgrounds
under control.This is precisely what CDMS experimenters (and
many other collaborations worldwide) are now in the process of
doing. By summer of 2010, the CDMS group hopes to have about
three times more germanium nuclei sitting near absolute zero at
Soudan, patiently waiting for WIMPs to come along and provide
the perfect billiard ball shots that will offer compelling
evidence for the direct detection of dark matter in the
laboratory.
