FOR more than three decades, physicists have been puzzling over the fact that
their detectors only seem able to pick up about one-third of the neutrinos
thought to be produced by the Sun鈥檚 nuclear furnace. Now an experiment in Japan
may have provided the answer: neutrinos transform, or oscillate, from one type
to another during flight, and so evade detectors hunting for just one type.
鈥淲e鈥檝e been trying to demonstrate neutrino oscillations with terrestrial
experiments for 28 years鈥this experiment] may finally have done it,鈥� says
David Wark of the Rutherford Appleton Laboratory near Oxford.
The new results, announced at a high energy physics meeting in Osaka last
week, were eagerly awaited as they are the first to come from a new generation
of 鈥渓ong-baseline鈥� neutrino experiments. The Japanese experiment involved firing
a beam of neutrinos from the KEK particle physics lab in Tsukuba 250 kilometres
through the Earth to SuperKamiokande, the world鈥檚 largest neutrino detector.
Similar experiments are planned in the US and Europe.
As well as explaining the shortage of solar neutrinos, the fact that some
change type, or 鈥渇lavour鈥�, has major implications for models of fundamental
particles. Neutrinos were originally thought to have no mass, but theorists say
that if they oscillate, they must have mass.
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The by-product of nuclear reactions, neutrinos almost never interact with
other subatomic particles. To catch just a handful of them, scientists have
built huge detectors deep underground to screen out the signals from other
particles. In 1967, neutrino pioneer Raymond Davis, then at Brookhaven National
Laboratory, found that his detector in the Homestake Goldmine in South Dakota
did not pick up the expected number of solar neutrinos. Other detectors have
found a similar shortfall.
Physicists took a hard look at their theories of how the Sun works, but
eventually decided that, as well as being elusive, neutrinos must also be
masters of disguise. During their journey from the centre of the Sun, some of
the neutrinos oscillate into a flavour that is invisible to the under-ground
detectors. Neutrinos come in three flavours: electron neutrinos, muon neutrinos
and tau neutrinos. Tau neutrinos are so hard to detect that the first positive
sighting was reported only two weeks ago
(快猫短视频, 29 July, p 6).
The idea of neutrino oscillation gained strong support when SuperKamiokande
started looking for neutrinos produced as cosmic rays strike particles in the
Earth鈥檚 atmosphere. It detected a difference in the ratio between muon and
electron neutrinos that come from the atmosphere above the detector and those
that pass through the Earth from the far side.
To confirm that they were seeing oscillating neutrinos, researchers at KEK
aimed a beam of muon neutrinos at SuperKamiokande. From June 1999 to June 2000,
the detector counted muon neutrinos. If neutrinos do not oscillate, it should
have spotted 40 during that period, reports spokesperson Koichiro Nishikawa from
KEK. Just 27 muon neutrinos were detected.
Nishikawa warns that these are initial figures, but other physicists are
enthusiastic. 鈥淭his is an achievement for the Japanese, in the construction of
the beam and the detectors,鈥� says Herv茅 de Kerret of the Coll猫ge
de France in Paris. There is even a relaxed attitude to the implications of
weighty neutrinos for the standard model of fundamental particles. 鈥淚鈥檓 quite
sure there will be no problem in extending the standard model to take account of
this,鈥� says Wade Allison of Oxford University.
