The Solar
Neutrino Problem

Neutrinos are produced in the cores of stars by processes such as the PP chain. Since neutrinos interact only weakly with matter, they speed out of the core of the star essentially unimpeded. Thus, if neutrinos can be detected from a star, they provide a glimpse directly into the processes going on now in the core of the star, while the visible light emitted at the surface may correspond to energies produced hundreds of thousands of years ago.

Neutrino Detectors

Neutrino experiments are difficult because the neutrinos interact so weakly with matter, but now several independent experiments have confirmed what was already indicated by the earliest such experiments: the Sun is producing approximately a factor of 2 fewer neutrinos than we expect that it should be. This is called the solar neutrino problem. (The standard unit of neutrino flux used in this discussion is called a "solar neutrino unit" or snu. Thus, one says that the Sun is not producing enough "snus".)

Possible Explanations of the Solar Neutrino Anomaly

Once experimental difficulties have been ruled out, the scarcity of snus could be explained in two general ways:
  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 anomaly.
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 mentioned in an earlier section). 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 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. There is now convincing evidence that neutrinos oscillate in this manner:

Further Reading:

The Solar Neutrino Problem