Positive ID. A huge sphere of heavy water caught fugitive neutrinos as they changed from one flavor to another.

Breakthroughs 2002: Neutrinos Nailed

Neutrinos, mysterious and misunderstood, are finally getting the respect they deserve. For years, neutrinos were the terra incognita on the particle chart. Electrons, muons, taus, and quarks had all been analyzed for years, their properties measured and dissected. But neutrinos? Nobody knew even whether they had mass until a few years ago. They were essentially unknowns.

No longer. In the last decade, physicists finally proved that neutrinos have mass, and since then, a flurry of experiments has begun to flesh out the elusive neutrinos' properties.

This year, the Sudbury Neutrino Observatory (SNO), a 1000-ton sphere of heavy water deep inside a nickel mine in Sudbury, Ontario, put the final nail in the coffin of the solar neutrino paradox. The nuclear reactions in the sun should produce a large number of electron neutrinos, but all observations had shown that only about one-third of the expected number were actually reaching Earth.

If neutrinos have mass, they can change flavors--from electron neutrinos into tau or mu neutrinos, for example--and that could explain the missing electron neutrinos. SNO showed, once and for all, that this is the case. In April, scientists at SNO announced that they had measured the abundances of all three types of neutrinos--electron, mu, and tau--by detecting when they split apart atoms of deuterium. When they added up the solar electron, mu, and tau neutrinos streaming through the detector, the total matched the number that should be created by nuclear reactions. Electron neutrinos change flavor during their journey to Earth.

As a bonus, the SNO measurements allowed scientists to drastically limit the "mixing angles" that define the neutrinos' flavor-changing abilities, and in December, the KamLAND experiment in Japan restricted the limits even further--with nuclear-reactor-created antineutrinos instead of solar neutrinos. Although physicists still don't know how much neutrinos weigh, the evanescent beasties are no longer blank spots on the particle chart.

Related sites
More detailed Science article on Sudbury findings
More detailed Science article on KamLAND findings
About neutrinos and the solar neutrino problem
The Sudbury Neutrino Observatory