|
The
Super-Kamiokande detector, half-filled
with pure water. (Courtesy of
the Institute for Cosmic Ray Research,
the University of Tokyo.) |
For many years, scientists believed
that neutrinos, released with electrons
when a nucleus undergoes beta decay,
might be massless. But in 1995, after
collecting three years of data, Los
Alamos National Laboratory reported
evidence for neutrino oscillations,
or transformations of one type of
neutrino into another. This finding
was supported recently when an international
team, including Americans, working
at Japan's Super-Kamiokande detector
(supported in part by the Office of
Science) demonstrated that the muon
type of neutrinos oscillate, probably
changing into the tau type, as they
pass through the Earth. Such oscillations
can occur only if neutrinos have mass,
albeit the smallest yet observed for
elementary particles, and there is
"mixing" among certain particlesphenomena
that contradict the Standard Model,
physicists' current theory of matter
and the forces of nature. The difference
in mass of the two types of neutrinos
was measured based on energy and flight
distance from neutrino production
in the atmosphere by cosmic radiation
to an underground instrument.
Scientific Impact:
When verified, this discovery will
force a revision in the Standard Model.
Furthermore, scientists might have
some help in answering a question
posed by current models of the universe,
which suggest that more matter is
present than can be detected by observing
electromagnetic radiation; neutrinos
might constitute the missing "dark
matter."
Social Impact: This
discovery, because it may alter estimates
of the total mass of the universe,
has implications for human understanding
of the origin and eventual fate of
the universe. The measured neutrino
mass is sufficient that the relic
neutrinos made in huge numbers at
the time of the Big Bang creation
of the universe would account for
much of its mass.
Reference: "Tau
Neutrinos Favored over Sterile Neutrinos
in Atmospheric Muon Neutrino Oscillations,"
The Super-Kamiokande Collaboration,
Phys. Rev. Lett. 85: 3999-4003
(2000).
URL:
http://www.phys.washington.edu/~superk/
Technical Contact:
Dr. James Stone, james.stone@science.doe.gov
Press Contact: Jeff
Sherwood, DOE Office of Public Affairs,
202-586-5806
SC-Funding Office:
Office of High Energy and Nuclear
Physics |