Physicists spot a new class of neutrinos from the sun

Neutrinos spit out by the main processes that power the sun are finally accounted for, physicists report.

Two sets of nuclear fusion reactions predominate in the sun’s core and both produce the lightweight subatomic particles in abundance. Scientists had previously detected neutrinos from the most prevalent process. Now, for the first time, neutrinos from the second set of reactions have been spotted, researchers with the Borexino experiment said June 23 in a talk at the Neutrino 2020 virtual meeting.

“With this outcome, Borexino has completely unraveled the two processes powering the sun,” said physicist Gioacchino Ranucci of Italy’s National Institute for Nuclear Physics in Milan.

In the sun’s core, hydrogen fuses into helium in two ways. One, known as the proton-proton chain, is the source of about 99 percent of the star’s energy. The other group of fusion reactions is the CNO cycle, for carbon, nitrogen and oxygen — elements that allow the reactions to proceed. Borexino had previously spotted neutrinos from the proton-proton chain (SN: 9/1/14). But until now, neutrinos from the CNO cycle were MIA.

“They’re top of everybody’s list to try and identify and to spot,” says physicist Malcolm Fairbairn of King’s College London. “Now they think they’ve spotted them, which is a major achievement, really an extremely difficult measurement to make.”

Located deep underground at the Gran Sasso National Laboratory in Italy, Borexino searches for flashes of light produced as neutrinos knock into electrons in a large vat of liquid. Researchers have spent years fine-tuning the experiment to detect the elusive neutrinos that herald the CNO cycle. Although difficult to observe, the particles are plentiful, Borexino confirmed. On Earth, around 700 million neutrinos from the sun’s CNO cycle pass through a square centimeter each second, the researchers report.

The result, presented for the first time at the virtual meeting, must still clear the hurdle of peer review in a scientific journal before it is fully official.

Studying these particles could help reveal how much of the sun is composed of elements heavier than hydrogen and helium, a property known as metallicity. That’s because the rate at which CNO cycle neutrinos are produced depends on the sun’s content of carbon, nitrogen and oxygen. Different types of measurements currently disagree about the sun’s metallicity, with one technique suggesting higher metallicity than another. In the future, more sensitive measurements of CNO neutrinos could help scientists disentangle the problem.

The CNO cycle is even more important in stars heavier than the sun, where it is the main fusion process. Studying this cycle in the sun can help physicists understand the inner workings of other stars, says Zara Bagdasarian, a physicist at the University of California, Berkeley and a member of the Borexino Collaboration. “It’s very important for us to understand how the sun works.”

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