IggDawg
04-29-02, 12:25 PM
From AIP:
THE SOLAR NEUTRINO PROBLEM HAS BEEN CLOSED 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. 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 numbers.
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 boson.
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.
THE SOLAR NEUTRINO PROBLEM HAS BEEN CLOSED 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. 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 numbers.
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 boson.
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.