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    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

    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

    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


      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


      Sudbury Neutrino Observatory Institute

      Creighton Mine, Lively Ontario

      (705) 692-7000

      FAX (705) 692-7001

      Dr. Doug Hallman

      Director of Communications

      Sudbury Neutrino Observatory

      Laurentian University

      (705) 675-1151 Ext. 2231

      FAX (705) 675-4868

      Dr. Eugene Beier

      U.S. Co-spokesman

      University of Pennsylvania

      Philadelphia, PA, USA

      (215) 898-5960

      FAX (215) 898-8512

      Dr. David Wark

      U.K. Co-spokesman

      RAL/University of Sussex

      Sussex, UK

      01 235 445094

      FAX 01 235 446733