Number 586, April 24, 2002
by Phil Schewe, James Riordon, and Ben Stein
The Solar Neutrino Problem Has Been Closed
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 cm2 per second) is
1.7 while that for the mu-nu and tau-nu combined is 3.4. (When one includes
all the other types of neutrinos, the flux from the sun is billions/cm2/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 the SNO website.
Dark Matter Detectors
Dark matter detectors will be looking for what is expected to be the
major type of matter in the universe, stuff that interacts with normal
matter only via gravity or the weak nuclear force. Like a neutrino detector,
a dark-matter detector will only work if the incoming flux of sought-after
particles is great enough and if there are enough target atoms in the
detector with a sufficient interaction potential.
Dark matter objects have been observed indirectly: clumps of weakly
interacting massive particles (WIMPs) in our galactic halo seem to have
gravitationally lensed the light of background stars. But one would
like to see the particles directly.
In one prototype detector design using both liquid and gaseous xenon,
an incoming WIMP would interact with a xenon atom (Xe's mass is about
113 GeV, very near to the expected WIMP mass) producing scintillating
light. To calibrate this process one can aim neutrons into the Xe bath.
Physicists at Imperial College in London have used a tabletop plasma
source of neutrons. The neutrons are generated by D-D fusion reactions
between a beam of energetic deuterium ions and stationary deuterium.
The D-D fusion reaction produces a helium nucleus plus a neutron at
the singular energy of 2.45 MeV.
Besides serving as a calibration tool for dark matter research, the
tabletop neutron source (a pulsed device with about 20 million neutrons
per shot) might be handy in the detection of nitrogen-based explosives
or in the transmutation of nuclear waste. (Beg
et al., Applied Physics Letters, 22 April 2002, firstname.lastname@example.org.)