Borexino spots solar neutrinos from elusive fusion cycle


Neutrinos produced by the elusive carbon–nitrogen–oxygen (CNO) cycle in the Sun have been observed for the first time – confirming a theory first proposed over 80 years ago. The observation was made by physicists working on Italy’s Borexino detector and provides an important insight into how stars power themselves by converting hydrogen into helium. Now that the CNO neutrinos have been detected, future studies could help resolve the mystery surrounding the “metallicity” of the Sun – the abundance of carbon, nitrogen and oxygen in the star.

Astrophysicists believe that stars convert hydrogen to helium via two processes of nuclear fusion. One is called the pp chain and accounts for 99% of fusion energy in the Sun. It involves a pair of protons fusing to create deuterium, which then fuses with a third proton to create helium-3. Finally, two helium-3 nuclei fuse to create a helium-4. There are two other branches of the pp chain that also produce helium-4 via the intermediary production of lithium, beryllium and boron.

The second process is the CNO cycle, which was proposed independently in 1938 by Hans Bethe and Carl Friedrich von Weizsacker. It is believed to account for about 1% of fusion energy in Sun-sized stars — but is thought to dominate the energy output of larger stars. The cycle is driven by the fusion of protons with carbon, nitrogen and oxygen nuclei in a six-step process that creates one helium-4 nucleus before repeating itself.

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Both the pp and CNO processes involve the emission of distinct spectra of solar neutrinos. In 2018 the Borexino collaboration made a comprehensive measurement of the solar neutrinos produced by the pp chain.

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Detection challenges

Now physicists working on the detector have measured the much weaker neutrino signal from the CNO cycle. To do so, the physicists had to overcome detection challenges posed by the relatively low energy and flux of the CNO neutrinos.

Borexino comprises 278 tonne of ultrapure liquid scintillator and detects solar neutrinos when they collide with electrons in the scintillator. As the electron recoils it produces light, which is captured  by an array of photomultiplier tubes.  Despite the huge flux of solar neutrinos that passes through Borexino, collisions rarely happen and only tens of neutrinos are detected daily. As a result, the detector is located deep under Gran Sasso mountain to shield it from cosmic rays, which would completely overwhelm the neutrino signal. Furthermore, the scintillator contains very low levels of radioactive impurities, which also contribute to the background signal.

The data in this study were acquired during phase-III of the Borexino experiment, which ran for over 1000 h in July 2016–February 2020. Because the CNO signal is very weak, the researchers had to account for background from two low-level impurities – bismuth-210 and carbon-11 – that can mimic the signal expected from CNO neutrinos. The team also had to account for neutrinos created by the proton–electron–proton process in the Sun, which can also be mistaken for CNO neutrinos.

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Painstaking characterization

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By painstakingly characterizing these background signals, the team was able detect neutrinos from the CNO process with a statistical significance of 5.1σ – above the 5σ level is considered a discovery in particle physics. As well as confirming the longstanding ideas of Bethe and von Weizsacker, the measurement also backs the current belief that about 1% of solar fusion energy is created by the CNO cycle.

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While the result does provide a measure of the abundance of carbon, nitrogen and oxygen in the Sun, it is not precise enough to resolve the “metallicity puzzle” of the Sun. This mystery has emerged recently as spectrographic measurements of the opacity of the Sun and helioseismological measurements of the speed of sound in the Sun suggest conflicting values for metallicity. Following Borexino’s success, future improvements to neutrino detectors could address this mystery.

The result also provides an important confirmation of how the CNO cycle should dominate fusion within stars larger than the Sun.

The research is described in Nature.


Physics World

Journal Reference

Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun

For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton–proton (pp) chain and the carbon–nitrogen–oxygen (CNO) cycle. Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously; however, there has been no reported experimental evidence of the CNO cycle.

Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal—only a few counts per day above the background per 100 tonnes of target—that is attributed to interactions of the CNO neutrinos.

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Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate—as well as the flux of emitted CNO neutrinos—depends directly on the abundance of these elements in the solar core.

This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe.

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