The controversy surrounding evidence for the discovery of “dark matter” particles has heated up following two conflicting talks given at a conference at the end of February. The papers were presented at the 4th International Symposium on Sources and Detection of Dark Matter/Energy in the Universe held in Marina del Ray, California. (Most of the transparencies from the conference are available on the Web.)

For almost 70 years astronomers have known that dust, gas and other ordinary matter cannot account for almost 90% of the mass of many galaxies. The galaxies must contain other “dark” matter to explain the orbital motions of stars around their centres. Many astrophysicists, cosmologists and particle physicists have conjectured that this seemingly empty space could be populated by a dense body of massive, but very weakly interacting, particles called WIMPs. Such particles would then provide the gravitational fields needed to keep the stars moving as observed.

Since the results of the first experimental efforts to detect these particles were published in 1987, literally dozens of experiments have been performed around the world. Two of the most sensitive experiments to date are the DAMA experiment at the Gran Sasso laboratory in Italy, and the CDMS experiment at Stanford University in the US. The DAMA collaboration – which includes physicists from the University of Rome Tor Vergata, the University of Rome La Sapienza and the Chinese Academy in Beijing – has been searching for WIMPs for several years using a large array of sodium-iodide detectors located 1400 m below ground. The CDMS experiment uses cryogenic detectors and is located just 10 m underground. The collaboration includes researchers from several centres in the US and Russia.

Assuming that they do exist, a WIMP will occasionally strike a nucleus in the detector material head-on and cause an elastic recoil. The recoil energy depends on the mass and velocity of the WIMP, together with the mass of the target nucleus. This energy can be measured in several ways, depending on the detector used. A scintillation photon may be emitted, electric charge may be liberated, or a phonon could cause a slight rise in temperature in the cryogenic material.

The challenge for dark-matter experiments is to discern the small effects produced by the rare WIMP interactions from the vast number of background events. This background includes cosmic rays and radioactive particles released from the detector and its surroundings. As a result, great care is taken to shield the detectors from comic radiation and to ensure that any radioactive impurities in the detector or shielding material are removed (see “The search for dark matter” by Nigel Smith and Neil Spooner Physics World January 2000).

Claims and counterclaims

For several years, the Italian group has claimed that it is has found evidence for WIMP interactions as the Earth and the Sun move through a sea of cold dark-matter particles in the halo of the Milky Way. The relative velocity with which the Earth moves through the dark-matter halo changes because of the way the planet rotates around the Sun. This means that the WIMP signal in June should be higher than that in December. The DAMA detectors have been used to search for this annual modulation in the signal.

The Italian group has accumulated thousands of hours worth of data using a 100 kg array of sodium-iodide detectors to measure scintillations of light. After several years of operation, the researchers see evidence for a seasonally dependent signal that is consistent with the position and velocity of the Earth throughout the year (see figure). This experiment is extremely difficult and requires complex data analysis to uncover the very subtle evidence of such a modulation. Researchers who have doubted the results in the past have suggested that a more mundane process – such as the ambient temperature – could give rise to the same effects.

In the conference session devoted to the experimental search for dark matter, which I chaired, Pierreluigi Belli of the DAMA collaboration presented a paper entitled “Searching for the WIMP annual modulation signature at Gran Sasso: results and perspectives”. During his 30-minute talk, Belli discussed the experimental details in great depth. He paid particular attention to the constant checks that were carried out to monitor the quality and control the stability of the experiment. The conclusion was that the DAMA experiment demonstrates definite evidence for the existence of a signal consistent with that expected from WIMPs with a mass between 44 and 62 GeV c^-2.

The question-and-answer session that followed Belli’s report lasted almost 25 minutes. There were many questions concerning the experiment – particularly about the analysis of the data – and Belli addressed each one in detail. While the entire audience may not have been completely satisfied with some of his responses, Belli left no question unanswered.

The next talk by Richard Gaitskell of the CDMS collaboration, “Recent results on direct searches”, was intended to demonstrate that the experimental evidence presented by the Italian group was, in fact, not real. The CDMS experiment is based on an entirely new type of detector technology that uses germanium and silicon crystals cooled to low temperatures. The detectors simultaneously measure the ionization and heat produced by nuclear recoils. The technology has clearly demonstrated that it is capable of separating events caused by a WIMP displacing a nucleus in the crystal from background events caused by gamma rays and X-rays from natural radioactivity in the surrounding materials.

The CDMS experiment would have definitely refuted the DAMA results if it were not for 13 events in which the germanium nuclei recoiled after being struck by some massive particle. Gaitskell presented details of a number of experiments and calculations designed to show that all 13 of these events were most probably caused by background neutrons produced from cosmic rays entering their laboratory. From their analyses, the CDMS researchers concluded that almost all of these events were definitely from neutrons, and should not be attributed to scattering events from dark-matter WIMPs. (The transparencies from the talk and a paper that has been submitted to Physical Review Letters are available on the Web at cdms.berkeley.edu).

Nevertheless, the discussion that arose during the question session that followed centred on the ability of the collaboration to guarantee that these were, in fact, neutron events. Doubting Thomases of the neutron-identification schemes maintained that one could not conclude that these were definitely neutrons. Furthermore, if these events do not originate from neutrons, the results of the CDMS collaboration are, in fact, exactly as one would expect if the DAMA results are correct. At present, many in the field believe that a stalemate exists. Of course, a large number of people at the conference took one side or the other.

Breaking the deadlock

In the next year or two, both collaborations plan significant upgrades and improvements to their detectors. The DAMA collaboration is planning to increase the mass of its sodium-iodide detector to 250 kg, which will make the experiment far more sensitive to the annual-modulation signal. Meanwhile, the CDMS collaboration plans to move its detector to the Soudan Underground Laboratory, which is located approximately 700 m underground in an abandoned iron mine in Ely, Minnesota. Previous experiments by other groups at the Soudan lab have shown that the background from neutrons will essentially be eliminated. We should therefore learn for certain whether the 13 events observed in the shallow underground laboratory at Stanford are due to neutrons or not.

The increased sensitivity of the two experiments – together with a number of other experiments – will allow physicists to search for dark matter in greater detail. The experiments will either discover WIMPs or rule out particles in a large mass range and with a wide range of interaction properties.

The discovery of cold dark-matter particles would be one of the most important in the history of physics. It would clarify many questions concerning the birth, evolution and final destiny of our universe. A definitive confirmed discovery would certainly merit a Nobel prize and a distinguished place in history for those who provided the intellectual leadership.