Chargeless, nearly massless and rarely seen interacting with matter, the subatomic particles known as neutrinos have proved exasperatingly difficult to study. For more than three decades, efforts to count the elusive particles yielded less than half the number the sun was thought to manufacture. Physicists finally solved that so-called solar neutrino problem last year, revealing that during their journey from the sun to Earth, some of the neutrinos change from one "flavor" to another. The sum total of the varietals, it turns out, amounts to approximately the predicted number. With that conundrum behind them, researchers turned their attention to sharpening some of their calculations. Findings reported in the current edition of the journal Physical Review Letters represent progress on that front.

Scientists have estimated the rate at which the sun manufactures neutrinos based on the sun's temperature; the amounts of beryllium, hydrogen, helium and other elements used for fuel; and the rates at which these materials combine with one another. The most uncertain of these measured reaction rates, Kurt Snover of the University of Washington and colleagues note, is that describing fusion between beryllium-7 and hydrogen. To reduce that uncertaintyand hence fine-tune the predicted rate of solar neutrino productionthe team conducted an experiment. Using a particle accelerator, they bombarded a piece of beryllium-7 with protons (the nuclei of hydrogen atoms). This resulted in the transformation of the beryllium into boron-8, and neutrino emission quickly followed. (The experimental setup is pictured at the right.)

Based on the new results, the team concludes that the fusion rate producing the solar neutrinos is 17 percent greater than previously thought. "It helps us understand better the differences in mass, as well as other properties, among these character-changing neutrinos," Snover says of the finding. "It's factors like these that go into the soup pot when you're trying to figure out what are the properties of a neutrino."