In a technical tour de force, physicists have spotted long-sought low-energy neutrinos zipping from the sun. The discovery confirms one of the first possible steps in the fusion cycle that helps power the star, says Cristiano Galbiati, a physicist at Princeton University and member of the large international team that reports the discovery February 2 in Physical Review Letters.
The newfound particles are produced when two protons and an electron interact to make deuterium, a heavy form of hydrogen that helps feed the sun’s fusion. About 1 in 400 deuterium atoms in the sun are made in this proton-electron-proton, or pep, reaction.
Scientists can probe the sun’s inner workings by studying the particles produced in its thermonuclear reactions — in particular, the neutrinos that flood through Earth in great numbers but hardly interact with matter here. Researchers must build detectors underground to screen out these elusive solar neutrinos from other particle chatter.
In 2007, an Italian-led collaboration known as Borexino started trying to do just that in the Gran Sasso National Laboratory, buried in a mountain in central Italy. Borexino consists of a giant vat of liquid, which sets off tiny sparkles when neutrinos interact with it.
Team scientists knew they could spot neutrinos from the more common and higher-energy proton-proton, or pp, reaction. “We didn’t expect to be able to see the pep neutrino when we started,” says Frank Calaprice, a team member also at Princeton. “We knew it might be possible, but there were huge barriers.”
For instance, even though the detector is buried to protect it from stray particles, some cosmic rays do manage to get through the mountain and into the experiment. There they can produce radioactive carbon-11, which sets off the detector in the energy range expected for pep neutrinos. But new ways to remove carbon-11 from the analysis allowed the Borexino scientists to filter out the unwanted signal, Galbiati says.
Once the background interference was gone, the scientists could spot the telltale signs of pep neutrinos at the expected energy of around 1.4 million electron volts. (For comparison, a typical particle of light, or photon, has an energy of around 1 electron volt.) About three such flashes happened each day per 100 tons of detector liquid.
Borexino’s technical accomplishments in cleaning up the background signal are already helping other experiments, such as those that hunt for other neutrino events or even dark matter, says Galbiati. “This result opens up the avenue for future solar neutrino detectors to make much more precise measurements,” he says.
Further studies of pep neutrinos should help scientists fine-tune their understanding of the sun, says physicist Mark Chen of Queen’s University in Canada. The SNO+ detector, at an underground laboratory near Sudbury in the Canadian province of Ontario, will be filling its own detector this summer with an eye to collecting data in 2013. That experiment aims to better measure the rate of pep neutrinos and also detect another kind of low-energy solar neutrino called the CNO, Chen says.