Our star’s second-most critical fusion reaction was observed by the Borexino experiment

Physicists have now accounted for neutrinos emitted by the key mechanisms that power the sun.

In the sun’s core, two types of nuclear fusion events prevail, both of which produce an abundance of lightweight subatomic particles. Scientists had already discovered neutrinos as a result of the most common procedure. For the first time, neutrinos from the second set of reactions have been detected, according to researchers from the Borexino experiment, who spoke at the Neutrino 2020 virtual meeting on June 23.

Physicist Gioacchino Ranucci of Italy’s National Institute for Nuclear Physics in Milan said, “With this result, Borexino has totally unwrapped the two processes that fuel the sun.”

Hydrogen fuses into helium in two processes at the sun’s core. The proton-proton chain, for example, is the source of nearly all of the star’s energy. The CNO cycle, which stands for carbon, nitrogen, and oxygen — the elements that allow the reactions to happen — is the other category of fusion reactions. Borexino has earlier discovered neutrinos from the proton-proton chain (SN: 9/1/14) (SN: 9/1/14). However, neutrinos from the CNO cycle had been missing until recently.

“They’re at the top of everyone’s list to try and identify and spot,” says King’s College London physicist Malcolm Fairbairn. “Now they think they’ve found them, which is a significant accomplishment and a truly difficult measurement to make.”

Borexino is a deep underground search facility at the Gran Sasso National Laboratory in Italy that looks for flashes of light caused by neutrinos knocking into electrons in a big liquid vat. The experiment has been fine-tuned for years in order to identify the elusive neutrinos that signal the start of the CNO cycle. Borexino confirmed that the particles are numerous, although being difficult to see. According to the researchers, roughly 700 million neutrinos from the sun’s CNO cycle pass through a square centimeter on Earth every second.

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Before becoming fully official, the result, which was presented for the first time at the virtual meeting, must first pass peer review in a scientific publication.

The study of these particles may indicate how much of the sun is made up of heavier elements than hydrogen and helium, a trait known as metallicity. This is because the pace at which CNO cycle neutrinos are created is determined by the amount of carbon, nitrogen, and oxygen present in the sun. Currently, several sorts of measurements dispute about the sun’s metallicity, with one technique implying a higher metallicity than the other. More sensitive measurements of CNO neutrinos in the future could aid scientists in unraveling the mystery.

In stars heavier than the sun, the CNO cycle is considerably more significant, as it is the primary fusion mechanism. According to Zara Bagdasarian, a physicist at the University of California, Berkeley and a member of the Borexino Collaboration, studying this cycle in the sun will help scientists better comprehend the inner workings of other stars. “Understanding how the sun operates is critical for us.”

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