Deep inside the sun pairs of protons fuse to form heavier atoms, releasing mysterious particles called neutrinos in the process. These reactions are thought to be the first step in the chain responsible for 99 percent of the energy the sun radiates, but scientists have never found proof until now. For the first time, physicists have captured the elusive neutrinos produced by the sun’s basic proton fusion reactions.
Earth should be teeming with such neutrinos—calculations suggest about 420 billion of them stream from the sun onto every square inch of our planet’s surface each second—yet they are incredibly hard to find. Neutrinos almost never interact with regular particles and usually fly straight through the empty spaces between the atoms in our bodies and all other normal matter. But occasionally they will collide with an atom and knock an electron loose, creating a quick flash of light visible to extremely sensitive detectors. That is how the Borexino experiment at Italy’s Gran Sasso National Laboratory found them. Its detection of so-called pp neutrinos—neutrinos created by the fusion of two protons in the sun—was a feat far from guaranteed. “Their existence was not in question, but whether some group was capable of building such an exquisitely pristine detector to see these low-energy neutrinos in real time, event by event, was,” says Wick Haxton, a physicist at the University of California, Berkeley, who was not involved in the experiment. “Borexino accomplished this through a long campaign to reduce and understand background events.”
[Slide Show: Giant Experiments Seek Out Tiny Neutrinos]
Borexino uses a vat of liquid scintillator—a material designed to emit light when excited—contained in a large sphere surrounded by 1,000 tons of water, cocooned in layers upon layers of shielding and buried 1.4 kilometers underground. These defenses are meant to keep out everything but neutrinos, thereby excluding all other background radiation that could mimic the signal. “Unfortunately for the pp neutrinos all this is not enough,” says Andrea Pocar of the University of Massachusetts Amherst who is also a member of the Borexino collaboration and lead author of a paper reporting the results in the August 28 Nature (Scientific American is part of Nature Publishing Group).
Some background contamination cannot be shielded because it originates inside the experiment. “The main background is the presence of carbon 14 in the scintillator itself,” Pocar says. Carbon 14 is a radioactive isotope common on Earth. Its predictable decay schedule allows archaeologists to date ancient specimens. When it decays, however, carbon 14 releases an electron that creates a flash of light very similar to that of a pp neutrino. The physicists had to look in a narrow sliver of energies where pp neutrinos can be distinguished from errant carbon 14 decays. Even then, once in a while two carbon 14 atoms in the scintillator will decay simultaneously, and the energies of the electrons they release can “pile up” on top of one another to exactly mimic the pp neutrino flash. “We had to understand these pileup events very precisely and subtract them out,” Pocar explains. The team invented a new way to count the events, and gathered data over multiple years before the researchers were convinced they had isolated a true signal. “This was a very difficult measurement to make,” says Mark Chen of Queen’s University in Ontario, who was not involved in the project. “The campaign by Borexino to purify the liquid scintillator in their detector paid off.”
Borexino’s discovery of pp solar neutrinos is a reassuring confirmation of physicists’ main theoretical models describing the sun. Previous experiments have found higher-energy solar neutrinos created by later stages of the fusion process involving the decay of boron atoms. But the lower-energy pp neutrinos were harder to find; their detection completes the picture of the sun’s fusion chain as well as bolsters plans for next-generation Earthbound neutrino experiments.
A strange quirk of these elementary particles is that they come in three flavors—called electron, muon and tau—and they have the bizarre ability to swap flavors, or “oscillate.” Because of the complex particularities of proton fusion reactions, all of the sun’s neutrinos happen to be born as electron neutrinos. By the time they reach Earth, however, some portion of them have morphed into muon and tau neutrinos.
Each neutrino flavor has a slightly different mass, although physicists do not yet know exactly what those masses are. Determining the masses and how they are ordered among the three flavors is one of the most important goals of current neutrino experiments. The mass differences between flavors are the main factor affecting how neutrinos oscillate.
If neutrinos are traveling through matter, their interactions with it will also alter their oscillation rates. The oscillations of higher-energy neutrinos, it turns out, are more altered by matter, leading to a larger chance they will oscillate—and therefore to fewer of them surviving as electron neutrinos by the time they reach Earth.
The Sudbury Neutrino Observatory in Ontario and Japan’s Super-Kamiokande experiment measured this phenomenon decades ago when they detected the higher-energy solar neutrinos from boron decays. Now, Borexino’s findings confirm the effect: more of the lower-energy neutrinos seen by Borexino persisted as electron flavor than the higher-energy neutrinos measured by those previous experiments. “This is important because matter effects have so far [primarily] been seen in the sun, yet we want to use this effect on Earth in future ‘long-baseline neutrino experiments’ to fully determine the pattern of neutrino masses,” Haxton says.*
These experiments, such as the Fermi National Accelerator Laboratory’s Long-Baseline Neutrino Experiment (LBNE) planned to open in 2022, will probe how neutrinos traveling though matter oscillate. Rather than using solar neutrinos, these projects will create powerful beams of neutrinos in particle accelerators and fine-tune their pathways to make precision measurements. Fermilab’s experiment will generate a stream of neutrinos from its base laboratory near Chicago to the Sanford Underground Research Facility in South Dakota. As the neutrinos fly through about 1,285 kilometers of Earth's mantle on their journey (the so-called “long baseline”), many will oscillate. By studying how the mantle matter interacts with the different flavors to affect their oscillation rates, the researchers hope to reveal which neutrino flavors are lighter and which are heavier.
Solving the neutrino mass puzzle, in turn, could point to a deeper theory of particle physics than the current Standard Model, which does not account for neutrino masses. Borexino’s latest feat of precision neutrino measurement suggests that experiments are finally becoming powerful enough to pry such secrets from the evasive particles.

*Correction (09/02/14): This sentence was edited after posting. It originally stated that matter effects have so far only been seen in the sun. In fact, Earth matter effects were observed earlier this year by the Super-Kamiokande experiment.