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Neutrinos galore - an old problem, a new challenge - The Physics Congress 2002

April 02, 2002

Neutrinos - probably the most baffling forms of known matter in the Universe - are setting new challenges for physicists, just as one long-running mystery seems close to being resolved. On Wednesday 10 April, John Bahcall from the Institute for Advanced Study in Princeton, USA, will tell the Institute of Physics Congress in Brighton how we are at last coming close to understanding the properties of neutrinos reaching Earth from the largest nearby source - the Sun. And David Wark, from the University of Sussex, will describe proposals to find out more about neutrinos in experiments at a veritable "neutrino factory", here on Earth.

John Bahcall has been calculating the expected number of neutrinos from the Sun for about 40 years. But several experiments, one running for more than 30 years, have consistently failed to detect as many neutrinos as he expected to arrive here at Earth. Last year Bahcall began to see his stance vindicated with results from the Sudbury Neutrino Observatory in Canada. This experiment indicated that some of the solar neutrinos have been evading detection here on Earth because they have changed their nature on their way from the Sun. The Sun emits electron-neutrinos, but by the time they reach the detectors here, a fraction seem to have changed into a different type of neutrino.

"There is a real sense of excitement in the field of solar neutrinos," says Bahcall, "with the promise that we are on the right track after all these years, and that neutrinos themselves hold the key to new physics."

Bahcall will describe for Congress how our understanding of the interior of the sun, and of nuclear fusion as the source of solar energy, leads to the predicted number of neutrinos. He will explain how the results of several experiments designed to test these ideas indicate that neutrinos must change - or oscillate - from one type to another. And he will look to the future and describe how these ideas might be tested using neutrinos both from the nuclear reactions in the Sun, and also from nuclear reactors and particle accelerators here on Earth.

To improve our knowledge of neutrinos - as it now seems that we do indeed understand a lot about those that come from the Sun - requires closer sources of large numbers of neutrinos. Neutrinos react very, very weakly with other matter, which makes them extremely difficult to detect - they can easily pass right through the Earth! This has made it difficult to unlock their secrets. In particular, we do not know how much they weigh, although the evidence from the solar neutrinos suggests they do at least have a very small weight, or mass.
If this mass can be measured it will cast light on several open questions in particle physics and cosmology, from the nature of the dark matter in the Universe, to the mystery of the missing antimatter.

David Wark will tell The Physics Congress about efforts to demonstrate the same kind of changes seen in solar neutrinos using sources of neutrinos right here on Earth - culminating in plans for a new kind of intense source of neutrinos - a "neutrino factory". The neutrino factory will allow experimenters to produce very intense beams of neutrinos, which will make it easier to detect their very feeble interactions with other matter. To produce the neutrinos will require a powerful but relatively small proton accelerator. Several groups of physicists world-wide, including groups at many UK universities and laboratories, are working on proposals for such a neutrino factory. Wark, a member of the UK?s Neutrino Factory and Muon Collider Group, will describe how these studies are progressing, and the prospects for a future machine in the UK.

"Building a neutrino factory will be an enormous challenge," says Wark, "but it will provide the opportunity to pin down some of the properties of neutrinos once and for all."

Institute of Physics