Gotcha! The SNO's huge detector has managed to account for the Sun's errant neutrinos. Credit: SNO

The solar neutrino problem has been solved, according to researchers working at the Sudbury Neutrino Observatory (SNO) in Canada. Their results explain why physicists looking for neutrinos emitted by the Sun have detected between a half and a third as many particles as predicted.

The Sun mainly emits electron neutrinos, but the particles also come in two other 'flavours', known as tau and muon neutrinos. Neutrino detectors are primarily sensitive to electron neutrinos, which means the deficit could be explained if some electron neutrinos were changing into tau or muon neutrinos en route to the Earth.

SNO, a joint venture between Canada, the United States and Britain, is a spherical vat containing 1,000 tonnes of heavy water, located two kilometres below ground in a mine some 300 kilometres north of Toronto (see Nature 411, 10–12; 2001). As currently configured, it can accurately measure the flux of electron neutrinos reaching the Earth. So by comparing these results with those from other detectors — which have a small sensitivity to tau and muon neutrinos — the SNO team has shown in a paper submitted to Physical Review Letters that some electron neutrinos emitted by the Sun are switching flavours.

“It's an enormous achievement,” says John Bahcall of the Institute for Advanced Study in Princeton, New Jersey, who works on the solar neutrino problem. “I'm thrilled.”

The result has important consequences for the standard model, the theory that describes fundamental particles and their interactions. Theorists have assumed that neutrinos have zero mass — but neutrinos must have mass if they are to switch flavours. “These results go beyond the standard model,” says Art McDonald, director of SNO. “Now they have to be explained.”

SNO will now be reconfigured to be sensitive to all three classes of neutrinos, providing further insights into flavour switching.

→ http://owl.phy.queensu.ca/sno/first_resultsFootnote 1