![]() The missing nu-e flux shows up as an observable mu-nu and tau-nu flux. The upshot: all the nu's from the sun are directly accounted for. This interaction, called a neutral-current (NC) reaction because the weak interaction is carried by a neutral Z boson, is fully egalitarian when it comes to neutrino scattering unlike last year's ES data, the NC reaction allows e-nu's, mu-nu's, and tau-nu's to scatter on an equal footing. The new findings update last year's CC and ES data and introduce, for the first time, evidence deriving from a reaction in which the incoming neutrino retains its identity but the deuteron (D) is sundered into a proton and neutron this is why SNO went to such trouble and expense of using the D2O for the weakly-bound neutron inside each D. Now the definitive result has been tendered by SNO scientists at the Albuquerque meeting. The SNO data, when supplemented with ES data from the Super Kamiokande experiment in Japan, provided preliminary evidence a year ago for the neutrino-oscillation solution for the solar neutrino problem. Last year SNO reported first results based on reactions in which a solar neutrino enters the detector and either (1) glances off an electron in one of the water molecules, or (2) combines with a deuteron to create an electron and two protons, a reaction referred to as a "charged current" (CC) interaction since it is propagated by the charged W boson. ![]() SNO's gigantic apparatus consists of 1000 tons of heavy water held in an acrylic vessel surrounded by a galaxy of phototubes, the whole residing 2 km beneath the Earth's surface in an Ontario mine, the better to filter out distracting background. The experimental goals are threefold: to prove neutrinos change their flavor to measure the number of neutrinos coming from the Sun and to determine the relative masses of neutrinos. It is designed to scrutinize a particular reaction in the sun: the decay of boron-8 into beryllium-8 plus a positron and an e-nu. SNO is a unique neutrino telescope, the size of a ten-story building, two kilometers underground in INCO's Creighton Mine near Sudbury, Ontario, operated by a 100-member team of scientists from Canada, the U.S. If on their journey to Earth some of the neutrinos had changed into muon-or tau-neutrinos, then terrestrial detectors designed only to spot electron neutrinos (e-nu's) would be cheated of their rightful numbers. Soon, however, the neutrinos themselves were implicated. ![]() Suspicion naturally fell on the experiments and on the standard solar model (SSM) used to calculate the flux. Measurements dating back to the 1960's of this neutrino flux were puzzling: only a fraction of the expected number arrived at detectors on Earth. Solar neutrinos, setting out from the same place, flee unhindered, thus providing the most unadulterated indication of activity at the core. The benevolent sunlight we receive on Earth has its origin in the sun's central fusion furnace, whence the light must fight its way outwards in a series of scatterings that takes, on average, hundreds of thousands of years. This finding gives physicists new confidence that they understand how energy is produced in the sun's core and that neutrinos are just as quirky as we thought. The solar neutrino problem has been settled and the ability of neutrinos to change from one type, or "flavor," to another established directly for the first time by the efforts of the Sudbury Neutrino Observatory (SNO) collaboration.
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