Update on the SNO Measurement
of Neutrino Oscillations and Masses

Last month (April 2002), the Sudbury Neutrino Observatory (SNO) group announced an improvement of its earlier (June 2001) measurement that neutrinos have mass. Previously, SNO had to rely on data from Super Kamiokande to be better than 99 per cent sure that electron neutrinos were changing into muon or tau neutrinos on their way from the Sun to the Earth. Such a metamorphosis, which is known as a neutrino oscillation, is only possible if the masses of neutrinos are non-zero. Now, an additional year's worth of data has allowed SNO to eliminate its dependence on Super Kamiokande. This is significant in that it is sometimes difficult to calibrate and compare results from different experiments. SNO was able to measure the total number of neutrinos being emitted by the Sun and show agreement with standard solar models. If neutrinos were massless, all the Sun's neutrinos should be of the electron type. However, SNO observes that only about one-third are electron neutrinos. This means that two-thirds of the neutrinos are of the muon or tau type and that they changed into non-electron neutrinos sometime after being created in the core of the Sun (most probably as they passed through the Sun). The experimental results are in nice agreement with theoretical considerations and are so well established with data that there can be little doubt that neutrinos have mass.

The original report on the existence of neutrino masses
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