Search for the Dark Matter of the Universe

Reliable data indicate that most of the matter in the universe emits no detectable light. The `dark' matter is inferred, generally, from the gravitational pull that it exerts on the matter that does emit light. For example, the atoms that orbit many spiral galaxies move at speeds that are much faster than the gravitational pull of the light-emitting matter alone in those galaxies would cause. An extended and more technical review, by Mark Srednicki, can be found here (ps format) (pdf format).

A simple explanation would be that the dark matter consists of protons and other atomic nuclei, which for some reason do not emit light. This explanation seems to be inconsistent with our understanding of the Big Bang, however. In particular, the observed abundances of light nuclei such as deuterium, the isotopes of helium, and those of lithium, are by current understanding incompatible with dark matter that consists of conventional nuclei. An extended and more technical review, by K. Olive and D. Schramm, can be found here (ps format) (pdf format).

Alternate explanations for the dark matter usually involve some type of `new physics'. Among the more popular possibilities are neutrinos which have rest mass, and new types of fundamental particles.

If the dark matter consists of a new type of massive fundamental particle, then about 1000 to 100,000 of these dark matter particles pass through your thumbnail every second.

Although it might seem that the possibilities are endless for the concoction of new types of fundamental particles, there are some important constraints that arise, again, out of our understanding of the Big Bang. One notable constraint is that the new particles should interact with protons and other nuclei, in addition to gravitational pull, via a new type of force. The presence of that new force is deduced from the relative amounts of dark and light-emitting matter, as inferred from current data; if the new force were absent, the universe, today, would have had even more dark matter than is measured.

The UCSB particle physics group is involved in a search for the dark matter. The detection mechanism exploits the implied new force. Should a dark matter particle pass close to an atomic nucleus, the new force would cause the atomic nucleus to feel a push, and then recoil with an appreciable velocity. The magnitude of that velocity should be approximately 200,000 meters/second, which would give the nucleus a kinetic energy that would be a bit less than the energy of an X-ray used in medical applications.

In distinction to the X-ray, however, most of the energy of a recoiling nucleus ends up creating sound; an X-ray creates free charge, or ionization, in addition.

The dark matter detectors we use have been carefully developed by our colleagues at Berkeley and Stanford, so as to be sensitive to the acoustic energy of the recoiling nucleus.

Those detectors are part of a larger apparatus, assembled by a collaboration of scientists named 'CDMS' for Cryogenic Dark Matter Search. The team includes scientists from UCSB, Santa Clara University, Princeton, UC Berkeley, Stanford, Livermore, Colorado, NIST, Fermilab, and Case Western Reserve University.

The UCSB responsibilities are for: shielding the detectors from phenomenon that could fake the dark matter signal, for example, from radioactive backgrounds; and for computing and data acquisition.

Currently, the preliminary version of the apparatus, called CDMS-I (ps format) is functioning in a tunnel on the Stanford campus. The diagram (ps format) shows the tunnel that houses the experiment, 10.5 meters underground, shown at the left of the diagram. At the right is the apparatus; the Active Muon Veto, Pb Shield, and Polyethylene were all simulated, designed, fabricated and are operated by UCSB. Two UCSB staff scientists and two graduate students currently work on that facility.

The current sensitivity (exclusion zone, in magenta) from that apparatus is summarized in a plot (ps format) of cross section (in centimeters2, or cm2) versus rest energy of the dark matter particle (in 109 electron-volts, also denoted GeV).

For a feeling as to what the numbers on that plot mean; there is sensitivity to a `cross section' that is larger than about 10-40cm2, for a rest energy of the dark matter particle between 10 and 1000 GeV.

The area of your thumbnail is approximately 1 cm2. If every time a dark matter particle passed through your thumbnail, the particle pushed on your thumbnail via the new force, then your thumbnail would have a `cross-section' of approximately 1 cm2. However, the new force is far, far weaker than needed to cause a push for every dark matter particle that passes through your thumbnail. If a push occurred only 1 in 1040 times, we would say that your thumbnail had a `cross-section,' as distinct from `area,' of 10-40cm2. For a crude comparison, the area of a typical atom is about 10-16cm2, and that of a typical atomic nucleus is about 10-24cm2. The reason that the cross section for a dark matter particle to push on an atomic nucleus is so tiny, is, the new force only occurs when the dark matter particle and the atomic nucleus happen to come incredibly close to one another.

To get a feeling for what it means that the dark matter particle has a rest energy between 10 and 1000 GeV, for comparison, the rest energy of a proton is about 1 GeV. The rest energy of an atomic nucleus of carbon is about 12 GeV, and the rest energy of the atomic nucleus of a silver atom is about 108 GeV, and the rest energy of the heaviest known atomic nucleus is about 270 GeV. The rest energy of the heaviest known fundamental particle, the top quark, is about 175 GeV.

Sometime in the year 2000, we will begin operating the ultimate apparatus, called CDMS-II, in a deep mine in Minnesota (Soudan). That experiment should provide decisive sensitivities to the one of the most likely set of dark matter hypotheses, which involve the `lightest stable supersymmetric partner (LSSP).' A projection of the ultimate sensitivity of CDMS-II, is summarized in a plot (ps format) of cross section (in centimeters2) versus rest energy of the dark matter particle (in 109 electron-volts, also denoted GeV). The expected region for minimal supersymmetric models (MSS) is shown in blue at the bottom of the plot.

There are opportunities for enthusiastic students to work on the CDMS experiments.

(David O. Caldwell and Harry N. Nelson)

Recent publications:

A Search for Dark Matter using Cryogenic Detectors (CDMS), talk given by Dan Bauer at Vancouver (ICHEP '98) (4.5 Mbytes, due mainly to 4 large images; here is the version with no figures, which is 0.19 Mbytes, and here are the four figures: (1, 1.3 Mb), (2, 1.4 Mb), (3, 0.90 Mb), (4, 0.71 Mb)).

T. Shutt, et al., "First Data from the Cryogenic Dark Matter Search (CDMS) Experiment," to appear in the
proceedings of the 18th Texas Symposium on Relativistic Astrophysics,16-20 December 1996, Chicago.

"Neutron Background for a Dark Matter Experiment at a Shallow Depth Site" Nucl. Instr. and Meth. A 354, 553 (1995).

"Experiments Searching for Nonbaryonic Dark Matter" Nucl. Phys. B 38, 394 (1995).

A longer list of CDMS publication is here.