The modulation arises because our solar system is moving through that cloud at about 232 kilometers per second along with the spinning galactic disk. In addition, as the Earth moves around the sun in December the Earth is moving against the solar system motion at 30 kilometers per second, and in June it is moving with the motion.

Thus, much like a bicycle rider in the rain gets wetter when riding into the wind than when riding with the wind, DAMA expects to have more events in June than in December - exactly what they see.

DAMA has accumulated more than 58,000 kilogram days of data backing their claim of the existence of a neutralino with mass about 50 times the proton mass.

The CDMS experiment, releasing its first major scientific report, has developed an entirely new type of cryogenic detector technology. Their report is based on about half a kilogram of detector mass operated over one year and producing about 12 kilogram days of data.

How is it possible for 12 kilogram days to be competitive with more than 58,000 kilogram days? The answer lies in the new detector technology that allows the rejection of most background events on an event-by-event basis. All things are bombarded by naturally occurring radiation from the materials around us.

In addition, particles coming in from outer space bombard us continuously. The CDMS experiment is housed in a cave, 10.5 meters below the surface on the Stanford campus; 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 remaining events that reach the detectors are primarily gamma and beta rays. These produce electron recoils within the detectors. However, WIMPs (neutralinos are a subset of WIMPs) would only interact with nuclei and not with electrons. These detectors can tell whether the recoiling particle was an electron or a nucleus.

Detectors are made using crystals of germanium or silicon and are cooled to within 0.1 degrees above absolute zero (the coldest possible temperature).

For each event the researchers simultaneously measure the ionization within the germanium or silicon semiconducting crystals, much as conventional radiation detectors do, and the heat produced by each event, which can only be measured at these ultra low temperatures.

The heat production is a good measure of the actual energy deposition, while the ionization is about three times higher for a recoiling electron than for a recoiling nucleus with the same energy.

Now all electron recoils can be removed from the data sets, and only one background remains - neutrons. These also interact only with nuclei just like neutralinos would.

CDMS has observed 13 single scattering nuclear recoils, which were identified as neutrons and not as WIMPs. The residual signal originated from neutrons that are produced outside the active shield. They can penetrate the shielding and interact with the detectors.

Confirmation of the neutron hypothesis comes from the number of events in the silicon detector versus those in the germanium detector, and from the number interacting in only one detector versus those interacting in more than one.

CDMS concludes that after eliminating the electron recoil events, the remaining nuclear recoil events are nearly all from neutrons.

In order to compare with the DAMA experiment, which uses a different target material, a theoretical model is needed. Within the currently favored models where the interaction rates scale as the square of the mass of the target nuclei, the CDMS results are incompatible with the DAMA results and the WIMP interpretation of the seasonal modulation they observe.

From the DAMA measurement, CDMS would typically expect 20 WIMP events in addition to the observed neutron events. However, perhaps an unexpected form of dark matter particle could interact differently from what is expected in the sodium iodide DAMA detectors versus the CDMS germanium and silicon detectors.

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. Detecting Dark Matter - Part One

SPACE SCIENCE
White Dwarf May Hold Key To Dark Matter
Cambridge - January 31, 2000 - A White Dwarf star, discovered in the constellation of Taurus, has been shown to be one of the coolest and therefore oldest white dwarfs ever found, and has been shown to be a member of a hitherto unobserved and possibly large population of faint stars in the Galactic Halo.