Joshua Klein
Department of Physics and Astronomy
University of Pennsylvania
Unraveling the Solar Neutrino Problem at the Sudbury Neutrino Observatory
Thirty years ago, Ray Davis and his colleagues hoped to learn
something new about the Sun by observing its neutrinos---particles which
interact so weakly that they travel from the solar core to the Earth undisturbed
by any of the intervening matter. What Davis found was a surprise: while he
could see the neutrinos, there were far fewer of them than predicted by models
of solar energy production. While this was ostensibly a `setback' for solar
astronomy, it was a great opportunity for particle physics, because it raised
the possibility that the apparent deficit was the result of new properties of
neutrinos, and that the studies of the Sun could elucidate those properties.
Six experiments followed Davis's, and all saw a neutrino deficit and
found---even more suggestively---that the deficit was energy-dependent. The
most natural explanation for all the results was that the electron-flavor
+neutrinos produced by the Sun were changing into another (unobserved) flavor
+before
reaching the detectors on Earth. The Sudbury Neutrino Observatory (SNO) was
designed to determine whether this hypothesis was correct, by directly
comparing exclusive measurements of the flux of electron neutrinos with the
inclusive measurement of the flux of all neutrino flavors. SNO's first results,
combined with earlier measurements by the Super-Kamiokande collaboration,
in fact do show a significant difference between these two measurements,
providing direct evidence that neutrinos from the Sun do change flavor. In
addition to causing us to re-think our fundamental ideas about neutrinos,
these results allow us to re-start the project Ray Davis began: using
neutrinos to understand the Sun.