Neutrino Oscillations

There is an alternative explanation for the Solar Neutrino Puzzle; perhaps the Sun is understood but maybe there is a problem with our understanding of neutrinos. This is a fairly exciting possibility because it is currently believed that neutrinos are among the most basic of the sub-atomic particles in the Universe (the basic particles are quarks and leptons; neutrinos are a type of lepton).

There are three types (families) of neutrinos, electron, muon and tau neutrinos. The various kinds of neutrinos are only produced in interactions which produce electrons, muons, and tau's, respectively (and hence their names). The Proton-Proton chain produces electron neutrinos. Under the old (and currently accepted ) idea that neutrinos are massless like photons, these are the fundamental types of neutrinos in that once you are born as an electron neutrino, you spend your life as an electron neutrino. Because of this, most Solar neutrino experiments were designed to detect electron neutrinos, other types of neutrinos were invisible to the detectors. They would simply pass straight through the detectors without interacting.

Neutrino Masses

A recent suggestion has been made that neutrinos may not be massless, they may have mass. This has some interesting consequences;

• The mass allows mixing between the various neutrino types.

• Say, the initial state is an electron neutrino formed by the addition of the two (differnt types of) waves.
• The waves may propagate at different speeds and thus will get out of phase.
• The out-of-phase addition (mixing) of the waves leads to a different amplitude for the resultant wave ---> a different type of neutrino.

  The suggestion is that electron type neutrinos are produced in the core of the Sun, but then transform into, say, muon type neutrinos as they move through the Sun ---> the reaction rates are going on at the correct rates, it is just that the detectors cannot see the bulk of the neutrinos!!

  Other Evidence for this Suggestion?

  Need to show that neutrinos have mass.

  • Direct experiments to detect the mass of neutrinos, inconclusive. Interesting points
    • massless particles travel at the speed of light, particles with mass cannot.
    • Supernova explosions produce neutrinos and photons. If neutrinos are massless, they should reach the Earth at the same time as the photons reach the Earth from Supernova explosions.
    • This experiment was tried for SN1987A, but the theory of Supernova eplosions wasn't good enough and there weren't enough neutrinos to get a reliable answer.

      This situation will definitely improve however.

  • Direct experiments to look for neutrino oscillations in terrestrial accelerators and using the products of cosmic rays interacting with the atmosphere of the Earth.

    Two tentative results which suggest that neutrinos oscillate ---> neutrinos have mass. Work needs to be followed up upon.

• Sudbury Neutrino Observatory will be sensitive to all types of neutrinos and thus could also answer this question. Planned for later this decade.