About Neutrinos

Neutrinos are tiny, possibly massless, neutral elementary particles which interact with matter via the weak nuclear force. The weakness  of the weak force gives neutrinos the property that matter is almost transparent to them. The sun, and all other stars, produce neutrinos copiously due to nuclear fusion and decay processes within the core. Since they rarely interact, these neutrinos pass through the sun and the earth (and you) unhindered. Other sources of neutrinos include exploding stars (supernovae), relic neutrinos (from the birth of the universe) and nuclear power plants (in fact a lot of the fuel's energy is taken away by neutrinos). For example, the sun produces over two hundred trillion trillion trillion neutrinos every second, and a supernova blast can unleash 1000 times more neutrinos than our sun will produce in its 10-billion year lifetime. Billions of neutrinos stream through your body every second, yet only one or two of the higher energy neutrinos will scatter from you in your lifetime.

In recent years, theoretical models of the sun have permitted detailed calculations of the number (or flux) of neutrinos released from the sun. Several neutrino experiments have detected solar neutrinos and found the flux was too low. It appears that far too few neutrinos are detected than can be consistent with the known energy output of the sun. This is known as the "Solar Neutrino Problem" (SNP).

The neutrino was proposed by Wolfgang Pauli in 1930; but it would be 26 years from then before the neutrino was actually detected. Pauli proposed the existence of the neutrino as a solution to a frustrating problem in a nuclear process called beta decay. It seemed that examination of the reaction products always indicated that some variable amount of energy was missing. Pauli concluded that the products must include a third particle, but one which didn't interact strongly enough for it to be detected. Enrico Fermi called this particle the neutrino which meant "little neutral one". In 1956 Reines and Cowan found evidence of neutrino interactions by monitoring a volume of cadmium chloride with scintillating liquid near to a nuclear reactor. Fred Reines was jointly award the Nobel Prize in physics in 1995 in part for this revolutionary work.

We know that the mass of the neutrino is approximately zero, but we are unsure how large the masses of the three individual neutrino types are because of the difficulty in detecting neutrinos. This is important because neutrinos are by far the most numerous particle in the universe (other than photons of light) and so even a tiny mass for the neutrinos can enable them to have an effect on the evolution of the Universe through their gravitational effects. There are other recent astrophysical measurements that provide information on the evolution of the Universe and it is interesting to seek complementary information by direct determinations of the masses of neutrinos.