SEASONAL variations in the number of particles colliding with the Earth
strongly suggest that WIMPs exist, say Italian researchers. WIMPs, otherwise
known as weakly interacting massive particles, may be the dark matter that many
astronomers believe makes up 90 per cent of the mass of the Universe.
Last month, at a meeting on dark matter at Marina del Rey in California, the
team said that three years of observation have confirmed its initial claim to
have discovered WIMP-like signals
(New 杏吧原创,16 January 1999, p 24).
But American researchers still remain unconvinced.
Since 1997 a Chinese-Italian team has been using a particle detector in the
Gran Sasso Laboratory deep inside a mountain in the Apennines to detect heavy
particles. After analysing three years of data, the researchers say that about 7
per cent more particles with a mass of about 50 protons鈥攖he size that
WIMPs are expected to be鈥攈it their detector in summer than in winter.
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Because of its orbit around the Sun, the Earth moves faster relative to the
centre of the Galaxy in June than in December. The researchers argue that the
Earth therefore sweeps up more of these particles during the northern
summer.
However, at the same meeting, an American researchers said that after a
year-long run of their WIMP detector they haven鈥檛 picked up any seasonal
variation in their signals. The team, which includes researchers from seven
universities and three national laboratories, conclude that they had not found
any WIMPs.
鈥淭he two experiments appear to be inconsistent,鈥� says the Americans鈥� project
manager, Roger Dixon of the Fermilab particle physics lab near Chicago. 鈥淭his
doesn鈥檛 mean necessarily that one is wrong and one is right. We have to go back
and understand the difference,鈥� he says.
One difficulty is that the two groups use different technologies to detect
WIMPs. While the Italians have a 100-kilogram sodium iodide detector sited
1400 metres underground to shield it from background interference caused by
cosmic radiation, the Americans have an extremely sensitive 0.5-kilogram
germanium and silicon crystal detector about 10.5 metres underground in
California. The Americans use software and additional detectors to get rid of
all events not caused by WIMPs.
Alessandro Bettini, director of the Gran Sasso Laboratory, says because of
the drastic elimination of non-WIMP signals, 鈥渢hey may have thrown out the baby
with the bathwater.鈥� Some of the WIMP-like signals came from neutrons and a few
of them may be WIMPs, admits David Caldwell of the University of California at
Santa Barbara.
But the Americans remain unconvinced by the Italians鈥� efforts to tease out
the seasonal variations from the huge amount of background noise. Typically, a
0.5-kilogram detector would catch one WIMP per week, and this is 鈥渉ard work,鈥�
says Michael Turner, a theoretical cosmologist at Fermilab.
But Turner, who coined the word WIMP, says he feels the dark-matter problem
will be solved very soon: 鈥淲e now have instruments that are sensitive enough to
detect these particles.鈥�