Sensitive measurement by SNO observes solar neutrinos in a new way

A team of scientists from Canada, the United States and the United
Kingdom today announced the results of a unique new measurement of
the total number of neutrinos of all known types reaching the Earth
from the Sun. Using data entirely from the Sudbury Neutrino Observatory
(SNO) in Canada they are also able to determine that the observed
number of electron neutrinos (the type produced by the Sun) is only a
fraction of the total number. This shows with great certainty that
neutrinos from the Sun change from one type to another before reaching
the Earth.

Says SNO Project Director Art McDonald of Queen’s University,
“These new results show in a clear, simple and accurate way that
solar neutrinos change their type. The total number of neutrinos we
observe is also in excellent agreement with calculations of the
nuclear reactions powering the Sun. The SNO team is really excited
because these measurements enable neutrino properties such as mass to
be specified with much greater certainty for fundamental theories of
elementary particles.”

Neutrinos are particles with no electric charge and very little
mass. They are known to exist in three types related to three
different charged particles – the electron and its lesser known
relatives the muon and the tau. The Sun emits electron-neutrinos,
which are created in the thermonuclear reactions in the solar
core. Previous experiments have found fewer electron-neutrinos than
suggested by calculations based on how the Sun burns – the famous
“solar neutrino problem”.

SNO uses the unique properties of heavy water – where the hydrogen has
an extra neutron in its nucleus – to detect not only
electron-neutrinos through one type of reaction, but also all three
known neutrino types through a different reaction.

The results presented today at the Joint American Physical
Society/American Astronomical Society meetings in Albuquerque, New
Mexico, show that the number of electron-neutrinos observed is only
about one third of the total number reaching the Earth. This shows
unambiguously that electron-neutrinos emitted by the Sun have changed
to muon- or tau-neutrinos before they reach Earth.

Dr. Andre Hamer of Los Alamos National Laboratory told the meeting,
“In order to make these measurements, we had to restrict the
radioactivity in the detector to minute levels and determine the
background effects very accurately to show clearly that we are
observing neutrinos from the Sun. The care taken throughout this
experiment to minimize radioactivity and the careful calibration and
analysis of our data has enabled us to make these neutrino
measurements with great accuracy.”

In June 2001, results from the detection of electron-neutrinos in
SNO first indicated, with a certainty of 99.9%, that neutrinos change
type on their way from the Sun, thus solving the long-standing
problem. However, these conclusions were based on comparisons of
results from SNO with those from a different experiment, the
Super-Kamiokande detector in Japan. The new results, obtained
entirely from the SNO, are so accurate that it is 99.999% probable
that solar neutrinos change type before reaching Earth. The results,
which have been submitted to Physical Review Letters, are of great
importance because the way in which the neutrinos – long thought to
be massless particles – change types is thought to be linked to
neutrino mass and mass differences between various neutrino types.

Says Professor Hamish Robertson of the University of Washington,
“It was a dramatic and exciting moment for us when we first saw the
neutrons being produced by this type of neutrino interaction and
realized there were three times as many as you would get if only
electron neutrinos were coming from the Sun. There’s absolutely no
question the neutrino type changes and now we know quite precisely
the mass differences between these particles.”

Further background information.

For further information

Reference

SNO Participating Institutions

Funding for SNO

• Natural Sciences and Engineering Research Council Canada
• U.S. Department of Energy
• U.K. Science and Engineering Research Council
• National Research Council Canada
• Northern Ontario Heritage Fund Corporation
• Industry Canada
• INCO Limited

SNO Sponsors

• INCO Limited
• Atomic Energy of Canada Limited
• Northern Telecom
• Ontario Hydro / Ontario Power Generation
• Sun Microsystems
• Agra-Monenco/Canatom Limited
• CVD Manufacturing Inc.

SNO Prime Contractors and Suppliers

• Agra-Monenco/Canatom Limited (Project Management and design)
• INCO Limited (Cavity excavation and support systems)
• Dynatec International (Civil construction)
• Reynolds Polymer Technology Incorporated (Acrylic vessel)
• Lawrence Berkeley National Laboratory (Photomultiplier support structure)
• Hamamatsu Photonics (Photomultipliers)
• Schott Glass Incorporated (Photomultiplier glass bulbs)
• CVD Manufacturing Inc. (CVD nickel for neutral current detectors)
• Mirotech Limited (CVD nickel for neutral current detectors)