The neutrino is an elementary particle, which was postulated in 1930 by Wolfgang Pauli, 1945 Nobel Laureate in Physics, in order to solve an energy crisis in nuclear physics. Scientists had difficulty finding energy in radioactive decays and Pauli suggested the existence of a particle which he believed was carrying away the missing energy. But it took some years before the neutrino was discovered. It was Clyde Cowan and Frederick Reines who first detected and identified this particle in 1965. For his contribution, Reines was awarded the 1995 Nobel Prize in Physics.
The neutrino is an obscure particle with no electric charge and which only interacts with matter via the weak nuclear force. In recent years it has been discovered that neutrinos have a small mass, debunking the earlier assumption that it was massless. In the sun, an enormous number of neutrinos are produced in the fusion process when four hydrogen atoms transform into one helium atom. Despite the large number of neutrinos, an average of only about one of these will interact with a person’s body during a lifetime. The flux of neutrinos from the sun at the surface of the earth is 6×1010 neutrinos per square centimeter and second. The neutrinos from the fusion process in the sun can pass through several light years of solid lead before being absorbed by matter. The probability for a neutrino to interact with matter increases, however, with the energy of the neutrino.
Three different kinds of neutrinos have been observed: the electron neutrino , the muon neutrino and the tau neutrino . These neutrinos are related with three electrically charged particles, the electron, the muon and the tau. All six particles are called leptons. When a neutrino interacts with matter, it can either continue as a neutrino after the interaction (“neutral current interaction”) or create the corresponding charged particle (“charge current interaction”). The electron neutrino creates an electron, the muon neutrino a muon, and the tau neutrino a tau lepton.
During a high energy neutrino interaction the charged lepton will continue in almost the same direction as the incoming neutrino. In matter, an electron that is produced during interaction will be stopped by a few meters whilst a muon, with its larger mass, might continue for several kilometers depending on its energy. Determining the direction of the created muon will give the direction of the muon neutrino within a few degrees. This is the key to understanding high energy neutrino astronomy.