... electrons and electron-neutrinos can be created and destroyed, the sum of the number of electrons and electron-neutrinos is conserved. This fact leads to dividing the leptons into three families, each with a charged lepton and its accompanying neutrino. To detect neutrinos, very large and very sensitive detectors are required. Typically, a low-energy neutrino will travel through many light-years of normal matter before interacting with anything. Consequently, all terrestrial neutrino experiments rely on measuring the tiny fraction of neutrinos that interact in reasonably sized detectors. For example, in the Sudbury Neutrino Observatory, a 1000 ton heavy water solar-neutrino detector picks up about 1012 neutrinos each second. About 30 neutrinos per day are detected. Wolfgang Pauli first postulated the existance of the neutrino in 1930. At that time, a problem arose because it seemed that both energy and angular momentum were not conserved in beta-decay. But Pauli pointed out that if a ...

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