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Missing Solar Neutrinos
Particle physicists refer to it as the "solar neutrino problem": They
cannot find a set of elementary particles that are crucial to the basic
models of astrophysics - and perhaps to understanding the universe.
These elusive subatomic bits, which are released by the nuclear
reactions within the sun and other stars, have neither mass nor
electrical charge, and although they should in theory pass through the
earth by the quadrillions every second, very sophisticated instruments
have found but a fraction of the neutrinos that physicists expect.
Until last week there were three possible explanations for the missing
neutrinos: The sun does not work the way it is supposed to, something
has gone awry in the detection experiments or physicists do not
understand neutrinos. A paper published in the journal Nature suggests
that the sun is not to blame.
Physicists have long pictured the sun as a giant nuclear-fusion engine
that gives off light and other particles, including neutrinos. But the
scarcity of neutrinos cast doubt on the details of the model. Now,
Georges Isaak, of the University of Birmingham, England, and colleagues
have bolstered the standard model of the sun, using measurements of
shock waves that periodically pulse, like a heartbeat, through the solar
interior. The researchers compiled and analyzed worldwide observations
of solar pulsing to conclude that the sun should indeed be producing as
many neutrinos as theory predicts.
Russian strikeout. That leaves it to particle physicists to explain the
missing neutrinos. Since 1968, scientists have used huge detectors to
search for signs of these subatomic phantasms, catching on average less
than one neutrino a day. Earlier this year, preliminary results from
Baksan, Russia, site of the latest and theoretically the most sensitive
detector, provoked great excitement among physicists: In its first four
months, the Baksan detector saw not a single neutrino.
Unless more of these tiny particles can be measured, some of the most
basic assumptions of physics may have to be re-evaluated. Under current
theory, neutrinos come in three types, or flavors," only one of which,
the electron neutrino, is thought to be produced by the sun. Two
theorists, John Bahcall of the Institute for Advanced Study at Princeton
and Hans Bethe of Cornell, have strengthened speculation that neutrinos
change flavors on their way from the sun, thus escaping detection. The
detector at Baksan was designed to search exclusively for electron
neutrinos.
Physics will be turned on its ear if Bahcall and Bethe are right.
Kenneth Lande of the University of Pennsylvania, a collaborator at
Baksan, says neutrinos that transform flavors are behaving in ways never
observed in particle accelerators, which simulate nuclear reactions at
work in stars. Even more significant, to change flavors, these
evanescent particles must, according to the laws of physics, have at
least an infinitesimal amount of mass. This raises the possibility that
the difficult-to-detect neutrinos filling space could actually be the
"dark matter," the invisible mass astrophysicists believe makes up most
of the universe but which they have until now been unable to find.
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