Neutrinos from the Sun

In 1931, the neutrino was postulated by the theorist Wolfgang Pauli. Pauli based his prediction on the fact that energy and momentum did not appear to be conserved in certain radioactive decays. Pauli suggested that this missing energy might be carried off by an invisible neutral particle. Later Enrico Fermi takes the neutrino hypothesis and builds his theory of beta decay (weak interaction). Clyde Cowan and Fred Reines announced in 1959 the discovery of a particle fitting the expected characteristics of the neutrino. This neutrino is later determined to be the partner of the electron.

In fact neutrinos are tiny neutral elementary particles which interact with matter via the weak nuclear force. The weakness of the weak force gives neutrinos the property that all matter is nearly transparent to them. The Sun produces neutrinos copiously due to solar nuclear fusion and weak decay processes within its core. Solar neutrinos have an energy between 0 and 20 MeV, depending of the type of solar nuclear reaction they come from.

The deficit of detected neutrinos coming from the Sun compared with our expectations based on laboratory measurements, known as the Solar Neutrino Problem, has remained one of the outstanding problems in basic physics for over thirty years. It appears inescapable that either our understanding of the energy producing processes in the Sun is seriously defective, or neutrinos, one of the fundamental particles of nature, have important properties which have yet to be identified.

To address these physics questions, Canada together with the US and UK have established the Sudbury Neutrino Observatory (SNO) with its unique ability to detect neutrino interactions in heavy water. It is known that neutrinos exist in three different types corresponding to the three known charged leptons, the electron, muon and tau particles. The solar neutrino detectors in operation prior to SNO were sensitive only to the electron neutrino type while the use of heavy water allows the flux of all three to be measured.

Electron neutrinos can interact through a charged-current reaction while all neutrinos can interact through a neutral-current reaction. The determination of these reaction rates is a critical measurement in determining if neutrinos 'oscillate', that is change type in transit between the core of the Sun and the earth. If they do then neutrinos must have mass and the mixing may reveal new physics beyond the scope of current models of elementary particles.

More information about neutrinos can be found in this review by P. Langacker,or in this description of how chemical reactions form neutrinos.