 SNO’s Arthur B. McDonald on Nailing Neutrino Mass
Yes? No? Mass? No
mass? These are the questions that have obsessed physicists
investigating the elusive entities known as neutrinos. If the
neutrino has mass, then it represents the first evidence of
physics beyond the Standard Model and a clue in how to proceed.
The conundrum of dark matter in the universe might also be solved.
Since
the late 1960s, experiments have hinted at a more-than-massless
neutrino. Theoretical models of the sun predict that neutrinos
should be made in staggering numbers. Neutrino detectors on the
Earth, however, have repeatedly seen less than expected. Because
neutrinos come in three varieties—known as electron, muon, and
tau neutrinos—and because solar neutrino detectors have been
primarily sensitive only to electron neutrinos, the preferred
explanation over the years is that those "missing"
neutrinos had changed, or oscillated, into a flavor for
which the detectors had little or no sensitivity. And if a
neutrino oscillates, according to the laws of quantum
mechanics, then it must have a mass. And a massive neutrino, no
matter how small that mass may be, is the kind of thing that makes
life interesting for a physicist.
The
evidence has been building for years, most notably from the Super-Kamiokande
experiment in Japan. Then in August 2001, the Sudbury Neutrino
Observatory (known as SNO), a detector facility located 6,800 feet
underground in a mine outside Sudbury, Ontario, checked in with a
direct observation suggesting that electron neutrinos from the sun
really were oscillating into muon and tau neutrinos. SNO published its
report in the August 13, 2001, issue of Physical Review Letters...
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