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1Missing Solar Neutrinos

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  • Chris Ashcraft
    Jun 21, 2001

      June 18, 2001 12:15 pm Eastern Daylight Time
      First Results from the Sudbury Neutrino Observatory Explain the
      Missing Solar Neutrinos and Reveal New Neutrino Properties

      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

      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

      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.
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