NEUTRINOS, some of the most elusive of subatomic particles, have revealed the clearest picture yet of the reactions happening inside the Sun.
Ever since the model of how fusion reactions power the Sun was first suggested, experimental results have lagged behind theoretical predictions. Now that position has been reversed. Leading solar physicist John Bahcall says recent results from Japan鈥檚 KamLAND experiment finally rule out the existence of 鈥渟terile鈥� neutrinos. This allows the number of neutrinos made in the Sun to be calculated with unprecedented accuracy.
For the past 40 years, Bahcall, of the Institute for Advanced Study in Princeton, New Jersey, has been calculating the rate of fusion in the Sun. When researchers began to detect the electron neutrinos produced by these reactionshe was the first to point out that a lot seemed to have gone missing on their journey to Earth. A possible explanation was that they had changed during their flight into different types of neutrino that could not be detected by the experiments.
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This idea was confirmed in 2001 at the Sudbury Neutrino Observatory in Canada (快猫短视频, 30 June 2001, p 7). SNO found the missing neutrinos by choosing a detection technique that was sensitive not only to electron neutrinos but also to two other types, muon and tau neutrinos.
But some wondered if the problem really had been solved. If neutrinos could oscillate between three different types, why not a fourth type that SNO could not detect? If so, the Sun might be producing many more neutrinos than anyone realised. Because the fourth kind of neutrino would not interact with other particles and would be very hard to detect, researchers dubbed it 鈥渟terile鈥�.
Now Bahcall and his colleagues say the KamLAND experiment at Tohoku University in Sendai, Japan, has banished the idea of the sterile neutrino for good. Instead of detecting neutrinos coming from the Sun, KamLAND looked for antineutrinos, which are thought to behave in just the same way as neutrinos. The antineutrinos were produced by nearby nuclear reactors, so the researchers knew exactly how many of the particles were being produced in the first place. KamLAND鈥檚 detector consists of a giant oil-filled balloon buried in a mine shaft. The vast majority of neutrinos pass straight through undetected, but occasionally, one crashes into a proton in the oil, producing a flash of light.
The researchers compared the number of electron antineutrinos detected with the number that were emitted, and reported last month that the proportion missing was the same proportion that SNO has found to transform into the muon and tau types. Bahcall has now been through KamLAND鈥檚 figures and says the results prove that none of the electron neutrinos changes into a sterile neutrino. 鈥淭his becomes highly implausible,鈥� he says.
Without the spectre of sterile neutrinos haunting them, researchers can say with an uncertainty of only 6 per cent how many high-energy neutrinos the Sun produces: just over 5 million of them flow through each square centimetre of the Earth鈥檚 surface every second. 鈥淭he uncertainty is now less than half of what exists in the theoretical models,鈥� Bahcall told a meeting late last month at the Radcliffe Institute for Advanced Study in Cambridge, Massachusetts. Knowing the number of high-energy neutrinos means physicists can estimate the total number of neutrinos produced 鈥� 65 billion hit each square centimetre of the Earth every second 鈥� as well as work out the rate of fusion in the Sun more accurately than ever before.
The results should spur theorists to refine their ideas. Without the sterile neutrino, a whole class of theories that explain neutrino oscillation by positing large, extra dimensions will bite the dust. But not everyone is mourning the death of sterile neutrinos. 鈥淚 never believed in the things,鈥� says David Wark, a spokesman for SNO.
