Solar neutrino problem

The solar neutrino problem

In general scientists make progress by comparing what they see, either directly in nature or in an experiment they perform, from what they currently understand about the laws of nature. Very often, scientists combine the many bits of information about what they currently understand into a "model". For example, scientists have for many years been constructing a model of the Sun. No, they don't use balsa wood and glue to make this - the "model" in this case is actually a description of the inside of the Sun. The model is a list of things such as the chemical composition, temperature, pressure and density of matter inside the Sun at different places ranging from the core of the Sun all the way out to the visible surface.

An agreement between the experiment (or observation) and the model, tells scientists they are on the right track. In other words, the assumptions that went into the construction of the model are correct. But, sometimes there is a disagreement. This is when things can get really interesting, because it may mean that there is something going on in nature which lies outside of our current understanding. For many years there has been such a disagreement with the standard solar model and observations of what are called neutrinos.

This famous discrepency is called the solar neutrino problem. Although many scientists have participated, the principal problem may in many ways be described as a thirty year disagreement between the observations and the models made by these two men:

A neutrino is a particle (not to be confused with the neutron) which is believed to have no mass and travels at the speed of light. Neutrinos are produced in the cores of stars like the Sun as a by product of nuclear fusion. They interact only weakly with matter, and speed out of the core of the star essentially unimpeded. This is important for scientists, because it means neutrinos coming out of the Sun come directly from the core and can give us direct information about the nuclear fusion going on there.

Observing neutrinos, either from the Sun or from anywhere else in the universe, is difficult because most neutrinos aren't stopped by most matter. Scientists detect them by collecting large amounts of certain kinds of matter which are somewhat better at stopping the neutrinos, allowing them to be counted. One of the substances used is a certain type of chlorine, present in cleaning fluids. In the first experiment started more than 30 years ago, scientists placed a huge tank of cleaning fluid deep underground in a gold mine in South Dekota. (The tank was placed here to shield it from other types of particles from space called cosmic rays).

At first, the scientists didn't detect any neutrinos at all in the tank. Then, slowly at first, they began to see them. On average they saw about one per day! However, it soon became clear that were just not seeing as many as they thought they would, based on what they understood from the amount of nuclear fusion going on in the standard solar model.

To test that the observations were correct, scientists built larger and more elaborate experiments. The most sensitive experiment built to date is the "Super-Kamiokande" detector in Japan. This consists of 50,000 tons of pure water filled in an underground tank surrounded by 11,000 special tubes (called photomultiplier tubes). (note the men in the small raft to the right of the photo - when the experiment is running the entire tank is filled with water) The tubes can detect the extremely faint light that is emitted when neutrinos interact with the water. (Photo courtesy the Institute for Cosmic Ray Research, University of Tokyo).

By now these and other independent experiments have confirmed what was already indicated by the first experiment: the Sun is producing approximately a factor of 2 fewer neutrinos than we expect that it should be.

Over the years scientists have considered two possible explanations of the solar neutrino problem:

1. Perhaps we don't understand the Sun well enough. Maybe a better theory of the internal structure of the Sun would predict fewer neutrinos, in agreement with the measurements.

2. Perhaps we don't understand neutrinos well enough; maybe they have some features beyond the standard theory of neutrinos that account for the problem.

At present it is difficult to accept (1) because the standard solar model is very successful at describing many other aspects of the Sun (for example, the results from helioseismology). Thus much recent attention has been focussed on the possibility that neutrinos do something unusual. Most speculation centers on some varation of a theory that there are three kinds (called "flavors") of neutrinos, and that their passage through matter (like the Sun!) can cause one neutrino flavor to "oscillate" into another.

It can be shown that if this is arranged in just the right way it could account for the observed deficit of solar neutrinos, because the detectors only measure one flavor of neutrino. Evidence for the oscillation of neutrinos has in fact been accumulating. However, the existence of these neutrino oscillations contradicts another standard model in physics, called the standard electroweak model. Therefore, until scientist have something satisfactory to replace this model, the "solar neutrino problem" will no doubt remain with us as one of the more fascinating mysteries of the Sun!