Neutrinos are tiny, possibly massless,
neutral elementary particles which interact with matter via the weak nuclear
force. The weakness of the weak force gives neutrinos the
property that matter is almost transparent to them. The sun, and all other
stars, produce neutrinos copiously due to nuclear fusion and decay processes
within the core. Since they rarely interact, these neutrinos pass through the sun
and the earth (and you) unhindered. Other sources of neutrinos include
exploding stars (supernovae), relic neutrinos (from the birth of the universe)
and nuclear power plants (in fact a lot of the fuel's energy is taken away by
neutrinos). For example, the sun produces over two hundred trillion trillion
trillion neutrinos every second, and a supernova blast can unleash 1000 times
more neutrinos than our sun will produce in its 10-billion year lifetime.
Billions of neutrinos stream through your body every second, yet only one or
two of the higher energy neutrinos will scatter from you in your lifetime.
In recent years, theoretical models of the
sun have permitted detailed calculations of the number (or flux) of neutrinos
released from the sun. Several neutrino experiments have detected solar
neutrinos and found the flux was too low. It appears that far too few neutrinos
are detected than can be consistent with the known energy output of the sun.
This is known as the "Solar Neutrino Problem" (SNP).
The neutrino was proposed by Wolfgang Pauli
in 1930; but it would be 26 years from then before the neutrino was actually
detected. Pauli proposed the existence of the neutrino as a solution to a
frustrating problem in a nuclear process called beta decay. It seemed that
examination of the reaction products always indicated that some variable amount
of energy was missing. Pauli concluded that the products must include a third
particle, but one which didn't interact strongly enough for it to be detected.
Enrico Fermi called this particle the neutrino which meant "little neutral
one". In 1956 Reines and Cowan found evidence of neutrino interactions by
monitoring a volume of cadmium chloride with scintillating liquid near to a
nuclear reactor. Fred Reines was jointly award the Nobel Prize in physics in
1995 in part for this revolutionary work.
We know that the mass of the neutrino is
approximately zero, but we are unsure how large the masses of the three
individual neutrino types are because of the difficulty in detecting neutrinos.
This is important because neutrinos are by far the most numerous particle in
the universe (other than photons of light) and so even a tiny mass for the
neutrinos can enable them to have an effect on the evolution of the Universe
through their gravitational effects. There are other recent astrophysical
measurements that provide information on the evolution of the Universe and it
is interesting to seek complementary information by direct determinations of
the masses of neutrinos.
Spectrum
of Solar Neutrinos (Bahcall SSM)
Detecting neutrinos from the sun is very
valuable as a way of seeing the sun's interior. The neutrinos produced in the
core of the sun escape unhindered and a very small number may be detected with
suitable apparatus on earth. This is why SNO is often referred to as the
"window on the sun", since the neutrinos act as a probe on the mechanisms
in the solar core. Now since neutrinos only weakly interact with matter, SNO
requires a large detector volume to compensate (1000 tonnes of heavy water).
The flux of solar neutrinos has been measured
by previous experiments; but the results have been inconsistent and perplexing.
The pioneering experiment is Ray Davis's 600 tonne chlorine tank (actually dry
cleaning fluid) in the Homestake mine, South Dakota. His radio-chemistry assay,
begun in 1967, finds evidence for only one third of the expected number of neutrino
events. A light water Cherenkov experiment at Kamioka, Japan, upgraded to
detect solar neutrinos in 1986, finds one half of the expected events for the
part of the neutrino spectrum for which they are sensitive. Two recent gallium
detectors (SAGE and GALLEX), which have lower energy thresholds, find about
60-70% of the expected rate. The clear trend is that the measured flux is found
to be dramatically less than is possible for our present understanding of the
reaction processes in the sun (see the John Bahcall's page on the Standard
Solar Model for details). Furthermore, the neutrino deficit appears
to depend on the energy of the neutrino. The sun produces neutrinos with a
range of energies (see the figure above), and the different detectors are
sensitive to different energy ranges. The options are:
You could argue that all the experiments are
simply wrong, but this is highly unlikely. The different experiments all use
diverse detection techniques, overseen by large collaborations, and have been
calibrated with a variety of sources.
Now is a good time to introduce another fact
about neutrinos; there are actually three types of neutrinos (six types if you
count the anti-neutrinos). The three types (called flavours) are the
electron-neutrino (ne ), the muon-neutrino (nu )
and the tau-neutrino (nt ); they correspond to the three known
"generations" of particles that make up the known roster of
elementary particles. Normal "earthly" matter is made from first
generation particles, protons, neutrons and electrons. The higher generation
particles can be created in particle accelerators (that is how they were
discovered), but they rapidly decay back to the first generation due to their
larger mass.
Now the sun only produces electron neutrinos,
and, to date, detectors on earth have only been sensitive to electron
neutrinos. So if the neutrinos were undergoing a "flavour"
oscillation then the probability of detection would be reduced. There is a
proposed scenario, called the MSW effect, where the large mass densities in the
sun could greatly enhance this oscillation effect. This turns out to be a very
attractive possibility for the solution to the solar neutrino problem.
With the heavy water, the Sudbury Neutrino
Observatory (SNO) can detect all three flavours of neutrinos. So the SNO
detector will be able to observe separately the number of electron neutrinos
and the number of all neutrinos. This allows a determination of the probability
for these flavour oscillations to occur. >From the neutrino flux and shape
of the energy spectrum SNO will be able to determine how strongly the neutrino
flavours mix together, and determine information about the neutrino masses. SNO
started collecting the first data in April 1999, and this will be a very
exciting time for neutrino physics.
N.B. N. Hata and P. Langacker's page "Implications of Solar Neutrino Experiments"
provides a good up to date source of current neutrino data and their agreement
with various models.
URL: sno/neutrino.html (Last revised Jun 13, 2002) |