The detection of neutrinos for the first time by Reines and Cowan in 1956 was an exciting and important step for observational experiments. Neutrinos interact with matter in only the weakest fashion. This makes them difficult to detect, but it also makes them a very specialized probe.

Neutrinos are the only particle which can travel from the core of our Sun to Earth without being blocked by the large amount of matter in between. Thus neutrinos carry information which is directly related to conditions in the inside of a star.

Using a Standard Solar Model (SSM) and a host of experimental data it is possible to predict how many neutrinos that originate in the sun one would expect to see here on Earth. This requires knowledge of the reactions taking place in the Sun to create the neutrinos as well as knowledge of how the neutrinos interact after their creation. Armed with this knowledge Ray Davis set up a tank of chlorine at the Homestake mine in South Dakota to detect neutrinos.

What he saw was startling. His results indicated a surprising lack of neutrinos reaching Earth. So was the Solar Neutrino Problem born.

Follow-up experiments included Kamioka, SAGE, GALLEX, and Super-K. Each of these experiments found a deficit of detected neutrinos compared to SSM calculations. Even more interesting is that the number of "missing" neutrinos was not constant between various experiments. It appears to vary with energy. This information presents us with three distinct possibilities:

1. We don't understand the detectors we are using.
2. We don't understand the reactions that are creating the neutrinos.
3. We don't understand the neutrinos we are detecting.

1. The first possibility seems rather doubtful. Each experiment has been checked many times for possible errors. In addition the different experiments vary greatly in complexity and technique, making it extraordinarily unlikely that they are all making similar mistakes.

2. For neutrino physics it is especially important to have the most reliable possible predictions of the expected solar neutrino flux. The most uncertain nuclear physics ingredient in such a calculation is the reaction rate for the fusion of protons with ⁷Be nuclei to form ⁸B nuclei, which then decay to produce high energy neutrinos. It is this reaction which we are presently measuring.

Even with with refined data from the Sudbury Neutrino Observatory (SNO), our understanding of the expected neutrino production rate in the Sun will continue to play an important role in neutrino physics, especially if it turns out that solar neutrinos can oscillate or change into another type of neutrino which does not interact with normal matter at all, called a sterile neutrino. It now becomes apparent that these three cases are not completely separate.

3. The third possibility is being studied further by large-scale experiments. Evidence of neutrino oscillations in higher energy "atmospheric" neutrinos has been reported at Super-K, while both Super-K and now SNO are trying to determine whether solar neutrinos oscillate, and if so some of the parameters of the oscillations.

The idea that neutrinos may oscillate, or change from one neutrino type (called flavor) to another as they travel through free space, or through matter, is allowed in quantum mechanics under certain conditions, but not in classical physics. A growing number of physicists suspect that this may be the explanation of the Solar Neutrino Problem.

Though many Non-standard Solar Models have been proposed to explain the Solar Neutrino Problem none have been created that show as consistent agreement with solar observations as do the SSMs. SSMs enjoy particular success with regards to the helioseismological frequencies of pressure mode oscillations in the Sun.

A Standard Solar Model utilizes many factors to make predictions about conditions within our Sun, balancing the outward pressure created by radiation moving outward through the plasma with the inward tug of gravity. The opacity of molecules and atoms to varying wavelengths of light is combined with typical interaction lengths and mean free paths in a complex but well thought out code that once required a super-computer to run and now can be operated in some form on a powerful desk top PC.

Some of the most important parameters in a solar model are the cross-sections of the reactions that power the Sun. A cross-section indicates the probability for an interaction to occur. The reactions' cross-sections thereby determine how hot and how dense the Sun must be, as well as other factors.

Many of the cross-sections involved are either well measured or constrained by conditions in the Sun. If they were to be even slightly different from the value we currently believe them to have we would see a drastically different star before us.

Nearly all of the energy in the Sun is produced in a process called the P-P Chain in which four hydrogen nuclei combine to form a helium nucleus. It is to one particular branch of this chain that neutrino physicists pay particular attention.

Detectors of different sorts are sensitive to neutrinos of different energies, but for most all of the large neutrino detectors (excepting those that utilize gallium) one fact is true: they are all predominantly sensitive to neutrinos coming from the decay of the ⁸B produced in the ⁷Be(p,g)⁸B reaction.

Unfortunately the cross-section for this reaction is not well known at the energies which predominate in the core of the Sun. Though we typically think of the Sun as being very hot, by the standards of nuclear physics the stellar core is a very low energy region.

Because nuclei are all positively charged they repel each other. Because of this large energies are needed to push the nuclei close enough together to cause them to fuse and form a single nucleus. Because of this important dependence on the energy of the nuclei involved cross-sections fall very rapidly as energy decreases below a certain point. Dividing out this energy dependence leaves something nearly independent of energy called the S-factor that depends only on the structure of the nuclei involved, and not on energetic considerations. Because this gets at the heart of the physics of interest and is easier to compare between different experiments carried out at differing energies it is the S-factor that nuclear-astrophysicists refer to in experiments of this sort.

At low energies the cross-section is not well studied and the S-factor is not known with precision. The goal of the collaboration between the Nuclear Physics Lab at the University of Washington and TRIUMF in Vancouver is to measure this S-factor to a precision of better than five percent.

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