[EAS]Neutrino Flavors

pjk pjk at design.eng.yale.edu
Thu Apr 25 00:06:30 EDT 2002

Mail*Link¨ SMTP               Neutrino Flavors

Fascinating measurements from the Canadian neutrino observatory.
PHYSICS NEWS UPDATE                         
The American Institute of Physics Bulletin of Physics News
Number 586  April 24, 2002   by Phillip F. Schewe, Ben Stein, and
James Riordon

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.  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 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.  Solar neutrinos, setting out from
the same place, flee unhindered, thus providing the most
unadulterated proxy of activity at the core.  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. 
Suspicion naturally fell on the experiments and on the standard
solar model (SSM) used to calculate the flux.  Soon, however, the
neutrinos themselves were implicated.  If on their journey to Earth
some of the neutrinos (basically solar reactions produce
electron-neutrinos exclusively) 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
    SNO scrutinizes a particular reaction in the sun: the decay of
boron-8 into beryllium-8 plus a positron and an e-nu.  SNO's
gigantic apparatus consists of 1000 tons of heavy water (worth $300
million Canadian) 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
interactions.  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 (this
so-called elastic scattering (ES) is only poorly sensitive to muon
and tau neutrinos) or (2) combines with the 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
     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.  Now the definitive result has been tendered by SNO
scientists at this week's joint meeting of the American Physical
Society (APS) and the American Astronomical Society (AAS) in
Albuquerque.  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 D2O for the weakly-bound neutron
inside each D.  This interaction, called a neutral-current (NC)
reaction because the operative nuclear voltage spreads in the form
of 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 upshot: all the nu's from the sun are directly accounted
for.  The missing nu-e flux shows up as an observable mu-nu and tau-
nu flux.  This conclusion is established with a statistical surety
of 5.3 standard deviations, compared to the less robust 3.3 of a
year ago.  The measured e-nu flux (in units of one million per sq.
cm per second) is 1.7 while that for the mu-nu and tau-nu combined
is 3.4.  (When one includes the neutrinos from other reactions, the
flux from the sun is billions/sqcm/sec.)
     Even the issue of how the neutrino changes from one flavor to
another can be addressed by viewing the day-night asymmetry of
neutrino flux.  When the whole of the earth is between the sun and
the detector (night viewing) the oscillation process, which depends
on a density of matter through which the nu proceeds, should be
speeded up.  This type of measurement will also contribute to the
eventual study of neutrino mass.  An experiment like SNO can measure
not mass but the square of the mass difference between nu species. 
Even if the nu mass is quite small (much lighter than the previously
lightest known particle, the electron) it might still have played a
large role in cosmology, where it might have been instrumental in
shepherding galaxies; in supernovas, neutrinos might carry away as
much as 99% of an exploding star's energy.
      The SNO team has submitted its results to Physical Review
Letters; preprints are available at the online preprint server:
nucl- ex/0204008 and 0204009; see also www.sno.phy.queensu.ca.

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