The Solar Neutrino Problem

So What Is the Problem?

For more than twenty years, the Homestake Solar Neutrino Experiment in the Homestake Gold Mine in South Dakota has been attempting to measure neutrino fluxes from space; in particular, this experiment has been gathering information on solar neutrino fluxes. The results of this experiment have been checked against predictions made by standard solar models and it has been discovered that only one-third of the expected solar neutrino flux has been detected. This "Where are the missing neutrinos?" question is known as the Solar Neutrino Problem.

And it is not just the Homestake experiment that is detecting a shortage of neutrinos. Several other experiments, including Kamiokande II, GALLEX, and SAGE, have noticed a definite neutrino shortfall.

Just What Is a Neutrino Anyway?

Neutrinos are subatomic particles produced during nuclear fission and fusion processes. Like electrons (and muons and tauons), neutrinos are classified as leptons. There are three "flavours" of neutrinos: electron neutrinos, muon neutrinos, and tauon neutrinos. At this time it is unknown whether neutrinos have either mass or magnetic moments but recent observations of Supernova 1987A have set an upper limit on any neutrino magnetic moments at less than about 10^(-13) Bohr magnetons. If neutrinos do have a magnetic moment, then they will either be "left-handed" or "right-handed" in orientation.

How Does the Sun Produce Neutrinos?

The Sun produces energy by fusing hydrogen to helium. This may be accomplished in a number of ways but in the Sun, a process known as the proton-proton chain is thought to be primarily responsible for energy generation.

H + H              -->   D + positron + neutrino
H + H + electron   -->   D + neutrino
D + H              -->   He3 + gamma ray
He3 + He3          -->   H + H + He4

He3 + He4          -->   Be7 + gamma ray
Be7 + positron     -->   Li7 + neutrino
Li7 + H            -->   He4 + He4

Be7 + H            -->   B8 + gamma ray
B8                 -->   Be8* + positron + neutrino
Be8*               -->   He4 + He4

H is hydrogen, D is deuterium (heavy hydrogen), He is helium, Li is lithium, Be is beryllium, and B is boron. Numbers indicate different isotopes. The Homestake experiment detects only the highest energy neutrinos produced by the Sun, the neutrinos produced by the beryllium/boron reactions.

What Is the Solution?

Solutions to the solar neutrino problem are usually classified in one of two categories, astrophysical or physical. Solutions that require a change in the way we think about the Sun are termed astrophysical solutions while solutions that require a change in the way we think about neutrinos are called physical solutions.

Astrophysical Solutions: One way to solve the solar neutrino problem is to lower the central temperature of the Sun by a few percent. This will mean fewer high-energy nuclear reactions occurring in the solar core and thus, fewer neutrinos being produced and hence detected. There are a number of ways to lower the central solar temperature. Mixing will cause fresh fuel to be brought into the core, and thus a lower temperature will be needed to maintain equilibrium. Rotation, convection, or other instabilities such as the helium 3 instability could cause mixing in the core. Other more exotic solutions rely changing the metallicity of the core (that is, changing the relative abundances of the heavy elements) and using WIMPs (Weakly Interactive Massive Particles).
Physical Solutions: A current theory in particle physics states that it is possible for neutrinos to transform from one type to another. The Mikheyev-Smirnov-Wolfenstein (MSW) effect claims that electron neutrinos may transform or oscillate into either muon or tauon neutrinos. Other theories state that left-handed neutrinos may precess into right-handed neutrinos, or that neutrinos of one flavor and orientation may transform into neutrinos of another flavor and orientation. If these transformations take place in a vaccum, then they are called vacuum oscillations. Transformations taking place in matter are called, reasonably enough, matter oscillations. The neutrino experiments currently running on Earth only detect left-handed electron neutrinos. Therefore, if neutrino oscillations are taking place, then some, perhaps two-thirds, of the electron neutrinos produced by the Sun are being transformed into something that we are not detecting.

Neutrinos and Sunspots: Any Correlation?

The Homestake experiment has been running for over two solar activity cycles (1 activity cycle = 11 years approximately) and it has been noticed that the neutrino fluxes are not constant. Many researchers have tried to link solar surface activity with neutrino fluxes and, depending upon whether you believe their statistical arguments, have succeeded. It has been claimed that the neutrino flux is correlated to solar radius and solar wind mass flux; and anti-correlated to line-of-sight magnetic flux, p-mode frequencies, and (you guessed it) sunspots. (If two quantities are correlated, then they increase and decrease together. If two quantities are anti-correlated, then when one increases, the other decreases, and vice versa.)

Many of these parameters are (anti-) correlated with each other and are internally consistent. The solar activity cycle is usually defined by sunspot numbers but sunspots are related to magnetic activity in the Sun. Many of these other parameters are also directly affected by magnetism. If these correlations really exist, then it would seem that neutrinos are reacting with the magnetic fields in the heliosphere and magnetosphere. Thus, from this evidence, the solution to the solar neutrino problem is a physical one.

Another possibility, rarely discussed, is that the solar neutrino flux is actually constant and it is the cosmic ray background that is varying. Cosmic rays are more likely to get through to the Earth during periods of low solar activity. Therefore, neutrinos generated in the Earth's atmosphere by cosmic rays will increase in number during these times. If this cosmic background flux is not correctly subtracted from the total detections, then it will appear that the solar flux is indeed varying with the solar cycle.

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