Neutrinos provide the first experimental proof of the CNO energy production mechanism of the universe



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Borexino detector

Inside view of the Borexino detector. Credits: Borexino Collaboration

Neutrinos provide first experimental evidence for dominant catalyzed fusion in many stars

An international team of around 100 scientists from the Borexino collaboration, including particle physicist Andrea Pocar from the University of Massachusetts Amherst, reports in Nature this week, detection of neutrinos from the sun, directly revealing for the first time that the carbon-nitrogen-oxygen (CNO) fusion cycle is at work in our sun.

The CNO cycle is the dominant energy source powering stars heavier than the sun, but it has never been directly detected in any star until now, Pocar explains.

For much of their life, stars obtain energy by fusing hydrogen into helium, he adds. In stars like our sun or lighter, this mainly occurs through “proton-proton” chains. However, many stars are heavier and hotter than our sun and include elements heavier than helium in their makeup, a quality known as metallicity. The prediction since the 1930s is that the CNO cycle will be dominant in heavy stars.

The neutrinos emitted as part of these processes provide a spectral signature that allows scientists to distinguish those in the “proton-proton chain” from those in the “CNO cycle”. Pocar points out: “Confirmation of the combustion of CNO in our sun, where it operates at only 1%, strengthens our confidence in our understanding of how stars work.”

Borexino detector under the Apennine mountains

The Borexino detector sits deep beneath the Apennine mountains in central Italy at INFN’s Laboratori Nazionali del Gran Sasso. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultra-pure organic scintillator. Credits: Collaboration Borexino

Beyond that, CNO neutrinos can help solve an important open question in stellar physics, he adds. That is, how the central metallicity of the sun, which can only be determined by the CNO neutrino level of the core, relates to the metallicity elsewhere in a star. Traditional models have encountered a difficulty – surface metallicity measurements by spectroscopy do not agree with underground metallicity measurements deduced from a different method, helioseismology observations.

Pocar says neutrinos really are the only direct probe science has for the nucleus of stars, including the sun, but they are extremely difficult to measure. Up to 420 billion of them reach every square inch of the earth’s surface per second, but virtually all of them pass through without interacting. Scientists can only detect them using very large detectors with exceptionally low background radiation levels.

The Borexino detector sits deep beneath the Apennine mountains in central Italy at INFN’s Laboratori Nazionali del Gran Sasso. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultra-pure organic scintillator. Its great depth, size and purity make Borexino a unique detector for this type of science, alone in its class for low background radiation, Pocar says. The project was started in the early 1990s by a group of physicists led by Gianpaolo Bellini at the University of Milan, Frank Calaprice at Princeton, and the late Raju Raghavan at Bell Labs.

Until its last detections, the Borexino collaboration had successfully measured the components of solar “ proton-proton ” neutrino fluxes, helped refine the oscillation parameters of the neutrino flavor and, more impressively, even measured the first stage of the cycle: the very low energy ” pp ‘neutrinos, recalls Pocar.

Its researchers dreamed of broadening the scientific field to also search for CNO neutrinos – in a narrow spectral region with a particularly low background – but that price seemed out of reach. However, research groups at Princeton, Virginia Tech, and UMass Amherst believed CNO neutrinos could still be revealed using the additional purification steps and methods they had developed to achieve the required exquisite detector stability.

Over the years and thanks to a sequence of steps to identify and stabilize the origins, the American scientists and the whole collaboration have succeeded. “Beyond the revelation of CNO neutrinos which is the subject of this week Nature article, there is even now potential to help solve the metallicity problem too, ”says Pocar.

Before the discovery of CNO neutrinos, the lab had scheduled Borexino to shut down at the end of 2020. But because the data used in the analysis for the Nature the paper was frozen, the scientists continued to collect data, as the core purity continued to improve, making a new metallicity-focused result a real possibility, Pocar says. Data collection could extend to 2021, as the logistics and permits required, when underway, are not trivial and take time. “Every extra day helps,” he remarks.

Pocar has been involved in the project since his graduate studies at Princeton in the group led by Frank Calaprice, where he worked on the design, construction of the nylon vessel and commissioning of the fluid handling system. He then worked with his students at UMass Amherst on data analysis and more recently on techniques for characterizing backgrounds for measuring CNO neutrinos.

Reference: “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun” by The Borexino Collaboration, November 25, 2020, Nature.
DOI: 10.1038 / s41586-020-2934-0

This work was supported in the United States by the National Science Foundation. Borexino is an international collaboration also funded by the Italian National Institute of Nuclear Physics (INFN) and funding agencies in Germany, Russia and Poland.



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