Physicists from Canada, the UK and the US are today announcing
that their first results provide a solution to a 30-year old
mystery- the puzzle of the missing solar neutrinos. The Sudbury
Neutrino Observatory (SNO) finds that the solution lies not with
the Sun, but with the neutrinos, which change as they travel from
the core of the Sun to the Earth.
Neutrinos are elementary particles of matter with no electric
charge and very little mass. There are three types: the
electron-neutrino, the muon-neutrino and the tau-neutrino.
Electron-neutrinos, which are associated with the familiar
electron, are emitted in vast numbers by the nuclear reactions
that fuel the Sun. Since the early 1970s, several experiments
have detected neutrinos arriving on Earth, but they have found
only a fraction of the number expected from detailed theories of
energy production in the Sun. This meant there was something
wrong with either the theories of the Sun, or the understanding
of neutrinos.
“We now have high confidence that the discrepancy is not caused
by problems with the models of the Sun but by changes in the
neutrinos themselves as they travel from the core of the Sun to
the earth,” says Dr. Art McDonald, SNO Project Director and
Professor of Physics at Queen’s University in Kingston, Ontario.
“Earlier measurements had been unable to provide definitive
results showing that this transformation from solar electron
neutrinos to other types occurs. The new results from SNO,
combined with previous work, now reveal this transformation
clearly, and show that the total number of electron neutrinos
produced in the Sun are just as predicted by detailed solar
models.”
The SNO scientists present their first results today in a paper
submitted to Physical Review Letters and in presentations at the
Canadian Association of Physicists Annual Conference at Victoria,
B.C. and at SNO Institutions in the U.S. and the U.K. “It is
incredibly exciting, after all the years spent by so many people
building SNO, to see such intriguing results coming out of our
first data analysis – with so much more to come.” says UK
Co-spokesman Prof. David Wark of the Rutherford/Appleton
Laboratory and the University of Sussex.
The determination that the electron neutrinos from the Sun
transform into neutrinos of another type is very important for a
full understanding of the Universe at the most microscopic level.
This transformation of neutrino types is not allowed in the
Standard Model of elementary particles. Theoreticians will be
seeking the best way to incorporate this new information about
neutrinos into more comprehensive theories.
The direct evidence for solar neutrino transformation also
indicates that neutrinos have mass. By combining this with
information from previous measurements, it is possible to set an
upper limit on the sum of the known neutrino masses. “Even though
there is an enormous number of neutrinos in the Universe, the
mass limits show that neutrinos make up only a small fraction of
the total mass and energy content of the Universe.” says Dr.
Hamish Robertson, U.S. Co-Spokesman and Professor of Physics at
the University of Washington in Seattle.
The SNO detector, which is located 2000 meters below ground in
INCO’s Creighton nickel mine near Sudbury, Ontario, uses 1000
tonnes of heavy water to intercept about 10 neutrinos per day.
The results being reported today are the first in a series of
sensitive measurements that SNO is performing. From this initial
phase, the SNO scientists report on an accurate and specific
measurement of the number of solar electron neutrinos reaching
their detector, by studying a reaction unique to heavy water
where a neutron is changed into a proton. They combined these
first SNO results with measurements by the SuperKamiokande
detector in Japan of the scattering of solar neutrinos from
electrons in ordinary water (offering a small sensitivity to
other neutrino types), to provide the direct evidence that
neutrinos oscillate.
At the beginning of June the SNO scientists began the next phase
of their measurements, by adding salt to the heavy water, to
study another neutrino reaction with deuterium that provides a
large sensitivity to all neutrino types. Their further
measurements can address the transformation of neutrino type with
even greater sensitivity, as well as studying other properties of
neutrinos, of the Sun and supernovae.
Background Information on the Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a unique neutrino telescope,
the size of a ten-storey building, 2 kilometers underground in
INCO’s Creighton Mine near Sudbury Ontario planned, constructed
and operated by a 100-member team of scientists from Canada, the
United States and the United Kingdom. Through its use of heavy
water, the SNO detector provides new ways to detect neutrinos
from the sun and other astrophysical objects and measure their
properties. For many years, the number of solar neutrinos
measured by other underground detectors has been found to be
smaller than expected from theories of energy generation in the
sun. This has led scientists to infer that either the
understanding of the Sun is incomplete, or that the neutrinos are
changing from one type to another in transit from the core of the
Sun.
The SNO detector has the capability to determine whether solar
neutrinos are changing their type en-route to Earth, thus
providing answers to questions about neutrino properties and
solar energy generation.
The SNO detector consists of 1000 tonnes of ultra-pure heavy
water enclosed in a 12-meter diameter acrylic plastic vessel,
which in turn is surrounded by ultra-pure ordinary water in a
giant 22-meter diameter by 34-meter high cavity. Outside the
acrylic vessel is a 17-meter diameter geodesic sphere containing
9456 light sensors or photomultiplier tubes, which detect tiny
flashes of light emitted as neutrinos are stopped or scattered in
the heavy water. The flashes are recorded and analyzed to
extract information about the neutrinos causing them. At a
detection rate on the order of 10 per day, many days of operation
are required to provide sufficient data for a complete analysis.
The laboratory includes electronics and computer facilities, a
control room, and water purification systems for both heavy and
regular water.
The construction of the SNO Laboratory began in 1990 and was
completed in 1998 at a cost of $73M CDN with support from the
Natural Sciences and Engineering Research Council of Canada, the
National Research Council of Canada, the Northern Ontario
Heritage Foundation, Industry, Science and Technology Canada,
INCO Limited, the United States Department of Energy, and the
Particle Physics and Astronomy Research Council of the UK. The
heavy water is on loan from Canada’s federal agency AECL with the
cooperation of Ontario Power Generation, and the unique
underground location is provided through the cooperation and
support of INCO Limited.
Measurements at the SNO Laboratory began in 1999, and the
detector has been in almost continuous operation since November
1999 when, after a period of calibration and testing, its
operating parameters were set in their final configuration.
Further information about the SNO detector can be found on
the SNO Detector page.
SNO Participating Institutions
Canada
|
Queen’s University Laurentian University University of British Columbia |
Carleton University University of Guelph Chalk River Laboratories (to 1996) |
United States
|
Lawrence Berkeley National Laboratory University of Pennsylvania Brookhaven National Laboratory University of California at Irvine (to 1989) |
Los Alamos National Laboratory University of Washington Princeton University (to 1992) |
United Kingdom
| Oxford University |
For further information:
|
Prof. Art McDonald, Queen’s University Director Sudbury Neutrino Observatory Institute Creighton Mine, Lively Ontario (705) 692-7000 FAX (705) 692-7001 mcdonald@sno.phy.queensu.ca |
Dr. Doug Hallman Director of Communications Sudbury Neutrino Observatory Laurentian University (705) 675-1151 Ext. 2231 FAX (705) 675-4868 edh@nu.phys.laurentian.ca |
|
Dr. Eugene Beier U.S. Co-spokesman University of Pennsylvania Philadelphia, PA, USA (215) 898-5960 FAX (215) 898-8512 geneb@hep.upenn.edu |
Dr. David Wark U.K. Co-spokesman RAL/University of Sussex Sussex, UK 01 235 445094 FAX 01 235 446733 d.wark1@physics.ox.ac.uk |
Reference:
|
Paul de la Riva, media relations officer Public Affairs Department Laurentian University Sudbury, ON, Canada (705) 673-6566 FAX (705) 675-4840 www.laurentian.ca pdelariva@nickel.laurentian.ca |
Anne Kershaw, manager Public Affairs Department Queen’s University Kingston, ON, Canada (613) 533-6000 Ext. 74038 FAX (613) 533-6652 www.queensu.ca kershaw@post.queensu.ca |
