Dark matter claim meets resistance
Feb 25, 2000
Physicists working at the Gran Sasso underground laboratory claim to have observed the first direct evidence for dark matter particles in experiments. The so-called weakly interacting massive particle or WIMP has a mass about 50 times the mass of the proton. If confirmed the findings will have immense implications for particle physics and cosmology.
The DAMA collaboration, which is lead by Rita Bernabei of the University of Rome, will present its results at the Fourth International Symposium on Sources and Detection of Dark Matter/Energy in the Universe at Marina del Rel, California, later today. The team's latest paper, which is available at the Gran Sasso Web site, claims that a "cumulative analysis of all the available data favours the possible presence of a WIMP" with a mass between 44 and 62 GeV. However, many physicists remain sceptical about the results.
Up to 90% of the mass in the universe is thought to consist of so-called "dark matter" - matter that cannot be seen and only reveals its presence through its gravitational pull on visible matter. Failed stars such as brown dwarfs and so-called massive compact halo objects or MACHOs are known to comprise some of the dark matter in our galaxy. However, many physicists believe that exotic particles left over from the big bang are also responsible for some of the dark matter. These include WIMPs and other particles not included in the Standard Model of particle physics. WIMPs rarely interact with ordinary matter, which makes them exceedingly difficult to detect.
The DAMA experiment, which involves physicists from Italy and China, consists of a 100 kilograms of extremely pure sodium iodide (NaI) placed some 1400 metres underground in the Gran Sasso lab. The rocks overhead protected the NaI from cosmic rays, which can mimic a dark matter signal. It is also essential to isolate the detector from all background sources of radiation. On those rare occasions when a WIMP interacts with a nucleus in the NaI crystal, a small flash of light is released.
Over a three-year period the DAMA team looked for a seasonal variation in the detection rate as the Earth orbits the Sun, which itself is moving through the dark matter halo of our Milky Way galaxy. The number of WIMPs detected in June should be greater than the number detected in December by between about 5 and 10 percent. However, a large number of other processes also vary like this throughout the year, so there is a possibility that the variation seen by the Gran Sasso team is not due to dark matter particles.
"It is very difficult to ensure that you have got rid of every seasonal modulation, which is why there is a healthy level of scepticism over the results," says Nigel Smith of the Rutherford Appleton Laboratory. Smith is a member of the UK Dark Matter Collaboration that is also using NaI detectors to search for dark matter.
The US-based Cryogenic Dark Matter Search (CDMS) is expected to announce at the symposium that it has not seen any WIMPs in the mass range predicted by the DAMA group.