## The Solar Neutrino Problem

The solar core emits neutrinos as a result of the nuclear reactions in the pp-chain.

The thing with neutrinos is, they have a very small cross-section, and therefore are very difficult to detect (they can go all the way through Earth without interacting with anything).

The Homestake experiment, Lead, South Dakota, had the collection and counting of neutrinos coming from the Sun as a purpose. It consisted on a tank with 100,000 gallons (~380,000 L) of C2Cl4.

Cl-37 can interact with neutrinos of sufficient energy to produce radioactive Ar-37, with a half-life of 35 days.

The counting rate of neutrinos was measured in solar neutrino units (SNU, 1 SNU = 1e-36 reactions per target atom per second). The standard solar model predicts a rate of 7.9 SNU, while the outcome of the experiment was 2.23 ± 0.26 SNU. This discrepancy is the solar neutrino problem (SNP).

The Super-Kamiokande: Consisted on a tank of 3,000 metric tons of water. Neutrinos are able to scatter electrons, and the Kamiokande detector is able to detect the Cherenkov radiation produced when this happens. Cherenkov radiation is produced when an electron travelling in a medium (e.g. in water) travels faster than the speed of light in that medium (which is not physically impossible, since the speed of light in any medium is always slower than in vacuum). This is somewhat analogous to breaking the sound barrier.

The Kamiokande experiment was the first able to experimentally verify that the neutrinos have a non-zero mass.

The explanation for the SNP is that, since neutrinos have non-zero mass, they can transform between different types or flavors. Thus, there are 3 types of neutrinos:

• Electron neutrinosνe
• Muon neutrinos, νµ
• Tau neutrinosντ

The transformation between one flavor and the others (called neutrino oscillation) takes place when neutrinos interact with electrons. The experiments only detected the electron neutrinos, explaining why a smaller number of neutrinos than that predicted by the models was detected.

EDIT: The transformation between flavors seems to be of a more complex nature than simply “interact with electrons”, which I don’t fully understand. To get some clue, read the paragraph under the “Theory” heading in this wikipedia article.

Neutrino oscillation was evidenced by the Sudbury Neutrino Observatory.

See the text under the heading “Resolution” in the wikipedia article. The explanation is quite neat.