Update on the SNO Measurement
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.
of Neutrino Oscillations and Masses
The original report on the existence of neutrino masses
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