Resolution of the Solar Neutrino Problem

Once experimental difficulties have been ruled out, the scarcity of solar neutrinos can 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.

By the late 1990s it was difficult to accept explanation (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 the discussion of the Sun's properties). In addition, it became increasingly clear that a solution in terms of changing the Solar model required mutually contradictory things (for example, that the Sun simultaneously be both hotter and colder at its center than was assumed in the Standard Solar Model!). As a consequence, attention shifted increasingly to the possibility that neutrinos do something unusual.

Neutrino Flavor Oscillations
Most speculation concerning unusual neutrino behavior as a possible resolution of the solar neutrino problem centered on some version of a theory that the three flavors of neutrinos (and three of antineutrinos) could change one into the other as they moved through empty space or through matter. This conjectured process is called neutrino flavor oscillation, because it permits one neutrino flavor to "oscillate" into another.

In this explanation of the solar neutrino problem, the Sun could be producing neutrinos at the expected rate but the detectors would see only a portion of the neutrinos produced because the detector design is such that they are sensitive to only the electron neutrinos. They cannot see muon or tau neutrinos, so if an electron neutrino produced in the Sun changes into one of those by the time it gets to the detector, the detector will miss it.

Implications for the Standard Model of Elementary Particle Physics

The Standard Model of elementary particle physics (not to be confused with the Standard Solar Model), which is the most accurate description that we have of elementary particles and their interactions, requires that the neutrinos have exactly zero mass. Therefore, the stakes are higher in the search for neutrino oscillations than just the resolution of the solar neutrino problem. A demonstration that neutrinos undergo flavor oscillation is implicitly a demonstration that at least one flavor of neutrino has mass. That in turn would demonstrate a clear failure of the Standard Model of elementary particle physics and point the way toward revision of that model.

Neutrino Masses and Oscillations
One important property of neutrinos is whether they have mass or whether they are exactly massless particles (like photons). This is significant for the neutrino flavor oscillation hypothesis because for such oscillations to occur, at least one flavor of neutrino must have a nonzero mass. (If all neutrinos had exactly zero mass, theory indicates that there would be no observable consequence of a neutrino flavor oscillation.)

Vacuum Oscillations and Matter Oscillations
The issue of whether neutrinos undergo flavor oscillations is even more complex than outlined above. Theory predicts that if neutrinos have mass they can undergo flavor oscillations as they travel through empty space at near the speed of light. These are called vacuum oscillations. But if neutrinos travel through matter (for example, through the interior of the Sun), there is the possibility of an additional enhancement of the vacuum oscillations that is called the MSW resonance (named for the first letters of the last names of the three scientists who predicted this effect: S. Mikheyev, A. Smirnov, and L. Wolfenstein). These are called matter oscillations. Though the details are too complicated for our discussion, we may say qualitatively that the MSW effect can occur basically because the electron neutrino interacts in more ways with the electrons in the Sun than the tau or muon neutrinos can.
Resolution of the Solar Neutrino Problem: Neutrinos Oscillate!
Several new experimental results obtained between 1998 and 2003 appear to have resolved the solar neutrino problem.

1. First, experiments at the Japanese detector Super-Kamiokande demonstrated rather conclusively that neutrinos produced in the atmosphere by cosmic rays were oscillating in flavor. The data suggested that the oscillation was between muon and tau neutrinos. Since the Sun produces electron neutrinos, the Super-Kamiokande results are not directly relevant to the solar neutrino problem. However, they are indirectly relevant because they demonstrate that at least some neutrinos have mass, a necessary ingredient in a neutrino oscillation solution to the solar neutrino problem.
2. Another experiment called Kamland, also running in Japan, indicated that electron antineutrinos produced in nuclear reactors were oscillating in flavor before they reached the Kamland detectors.
3. A new detector built in Canada called the Sudbury Neutrino Observatory (SNO) used a detection method based on what are called neutral weak currents to look for all three flavors of neutrinos coming from the Sun (electron, muon, and tau neutrinos). Remarkably, the SNO results appear to have solved the solar neutrino problem completely. When SNO looked at the electron neutrinos only, it found the same suppression of the neutrino flux found in the earlier solar neutrino experiments that could detect only electron neutrinos. But, when SNO looked at all three flavors of neutrinos coming from the Sun and added up their flux, the prediction of the Standard Solar Model was reproduced within 1 percent! A graph illustrating the SNO results is shown in the following figure.



Evidence for the MSW Resonance

The totality of the new results on neutrino oscillations also points strongly to the matter oscillations caused by the MSW resonance in the body of the Sun (not the vacuum oscillations between the Sun's surface and Earth) as the primary source of the solar neutrino flavor conversion that gives rise to the "solar neutrino problem".

Interpreting SNO
The SNO results in the preceding figure are a little complicated to explain completely. We shall be content with noting that the vertical axis is the sum of tau plus muon neutrino fluxes, the horizontal axis is the electron neutrino flux, and the labels Charged Current, Neutral Current, and Electron Scattering refer to different neutrino detection methods. The most important observation concerning this diagram is that the meeting of the various bands on the plot at approximately the same point (denoted by the star) is the indication that the total flux of all flavors of neutrinos coming from the Sun is the same as that predicted by the Standard Solar Model.

Neutrinos Have Mass and We Understand the Sun
We conclude that the apparent solar neutrino problem was generated by the insensitivity of earlier experiments to flavors other than electron neutrinos. The (initially electron) neutrinos coming from the Sun are oscillating into other flavors, so only if one detects all three flavors simultaneously can one see the full original flux of neutrinos produced by the Sun. These three remarkable experiments (Super-Kamiokande, SNO, and Kamland) then seem to have demonstrated that the Standard Model of elementary particle physics is incomplete because the neutrinos have (very small but not zero) masses, and solved the solar neutrino problem at the same time!