AT LAST physicists have solved a problem that鈥檚 been plaguing them for three
decades. Why does the Sun seem to emit fewer neutrinos than it should? It鈥檚
simple. Neutrinos can change from one form to another.
Until now physicists had begun to think that either our understanding of the
structure of the Sun was wrong, or that neutrino detectors did not work
properly. But this week, a team of researchers from Canada, Britain and the US
announced that neutrinos made in the Sun can change to other types en route to
Earth. The other kinds are harder to spot, and so fool the detectors.
The team discovered that this 鈥渘eutrino mixing鈥� was taking place using
results from the Sudbury Neutrino Observatory (SNO), a giant particle detector
buried 2 kilometres underground in a nickel mine in Ontario, Canada.
When they account for the effect, the results agree closely with accepted
theory, says team member Nick Jelley of Oxford University. 鈥淲hen you put the
mixing in you find that the flux of neutrinos starting out in the Sun is
completely consistent with the standard solar model,鈥� he says.
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鈥淚鈥檓 thrilled,鈥� says John Bahcall of the Institute for Advanced Study in
Princeton, New Jersey, one of the authors of the solar theory. 鈥淎 lot of my
colleagues have been giving me high-fives today.鈥�
The results also confirm that neutrinos have mass, as first reported in 1998
by physicists at the Super-Kamiokande particle detector (Super-K) in Mozumi,
Japan. This pokes a sizable hole in the prevailing theory of particle physics,
the Standard Model, which predicts that neutrinos have no mass and cannot change
type.
There are three types of neutrino: the electron neutrino, the muon neutrino,
and the tau neutrino. Physicists are confident that nuclear reactions in the Sun
produce only electron neutrinos, and the SNO experimenters concentrated on this
type. Every second, more than a thousand billion electron neutrinos pass through
the SNO detector, which comprises 1000 tonnes of heavy water surrounded by
10,000 electronic eyeballs called photomultipliers. But each day only a handful
of the neutrinos interact with the heavy water and are detected.
In the interaction, a deuterium nucleus鈥攁 neutron bound to a
proton鈥攁bsorbs an electron neutrino and decays into two protons and an
electron. The electron carries away most of the neutrino鈥檚 energy, producing a
detectable flash of light.
The SNO team measured the flux of electron neutrinos and compared it with
earlier results from Super-K, which used ordinary water. Super-K also detected
muon and tau neutrinos, although not as well as it detected electron
neutrinos.
They found that their electron-neutrino rate was much lower than Super-K鈥檚
more inclusive rate. That means that some of the electron neutrinos generated in
the Sun must be turning into muon and tau neutrinos, and that Super-K detected
some of the converted particles, says Art McDonald of Queen鈥檚 University in
Kingston, Ontario. 鈥淢ore than 60 per cent are being transformed,鈥� he says.
Two years ago, the Super-K team confirmed that muon neutrinos can become tau
neutrinos. The new results are the first to show that electron neutrinos also
鈥渕颈虫鈥�.
The solar neutrino problem has vexed physicists since 1968, when Raymond
Davis Jr of Brookhaven National Laboratory near New York built a neutrino
detector in a gold mine in South Dakota and observed roughly half the neutrinos
predicted by Bahcall鈥檚 calculations. 鈥淚t was fun in the early days,鈥� Davis says.
鈥淲e had a lot of arguments. Some people said it just had to be wrong.鈥�
But experimenters tended to question the theoretical predictions rather than
their own results, says Lawrence Sulak, a physicist at Boston University and a
member of the Super-K team. 鈥淢ost of us didn鈥檛 put too much credence in the
solar theory,鈥� he says. 鈥淏ut it looks like it鈥檚 damn good.鈥�
Hans Bethe, a physicist at Cornell University in Ithaca, New York, agrees
with that assessment: 鈥淛ohn Bahcall went to great lengths to show from other
properties of the Sun that the theory was correct.鈥�
