We really do know why the sun shines
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Oxford physicists began the millennium by sharing in one of the most important discoveries in particle physics for decades: the solution of the solar neutrino problem.
Solar neutrinos stream from the sun as a consequence of the fusion reactions that power it. According to the standard model of particle physics, they have no mass and interact exceedingly weakly with other particles. In 1967 the American scientist Ray Davis first succeeded in detecting them, but he found only a third of the number expected. Either our understanding of how the sun worked, or the ‘standard model’ of particle physics, was wrong.
Professor Nick Jelley of Oxford’s Department of Physics is the UK Group Leader of a joint US/UK/Canadian project that set out to solve the problem. The instrument they built was the Sudbury Neutrino Observatory (SNO), a tank of heavy water 2 km below ground in a nickel mine in Canada surrounded by a bank of detectors to pick up the tiny flashes of light emitted when neutrinos interact with the heavy water nuclei. SNO has been able to make a very precise measurement of the total number of solar neutrinos. It reveals that no neutrinos are missing: they have just changed from one kind to another, more difficult to detect, on their way to Earth. This transformation, or ‘oscillation’, implies that while still extremely small even in relation to an electron, neutrinos do have mass and so the ‘standard model’ needs to be revised.
Oxford’s physicists played a crucial role both in the analysis of the data and in developing complex water purification systems to establish that the radioactivity in the heavy water, which would otherwise swamp the tiny neutrino signals, was at extremely low levels. ‘It’s been tremendously exciting to be part of this project’, says Professor Nick Jelley. ‘We’ve at last proved that neutrinos change from one kind to another – now the challenge is to understand why.’
