Neutrinos provide the first experimental evidence of the universe’s CNO energy production mechanism

Neutrinos give the first experimental evidence of dominant catalyzed fusion in many stars

An international team of about 100 Borexino Collaboration scientists, including particle physicist Andrea Pocar of the University of Massachusetts Amherst, reports in Nature neutrino detection from the sun this week, 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 that powers stars heavier than the sun, but has never been detected directly in any star so far, 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 occurs mainly through the “proton-proton” chains. However, many stars are heavier and hotter than our sun and include elements heavier than helium in their composition, a quality known as metallicity. The prediction from 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 of the “proton-proton chain” from those of the “CNO cycle”. Pocar points out: “The confirmation of the CNO burning in our sun, where it operates at only 1%, strengthens our confidence in understanding how the stars work.”

In addition to this, CNO neutrinos can help solve an important open question in stellar physics, he adds. That is, how the central metallicity of the sun, as can only be determined by the speed of the CNO neutrinos from the nucleus, is related to the metallicity elsewhere in a star. Traditional models have encountered difficulties: measurements of surface metallicity by spectroscopy do not agree with measurements of sub-surface metallicity inferred by a different method, helioseismological observations.

Pocar says neutrinos are actually the only direct probe science has for the core of stars, including the sun, but they are extremely difficult to measure. Up to 420 billion of them hit every square inch of the earth’s surface per second, yet 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 is located in the depths of the Apennines in central Italy at the INFN Gran Sasso National Laboratories. Detect 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, unique in its class for low background radiation, says Pocar. The project was initiated 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 of Bell Labs.

Up until its latest detections, the Borexino collaboration had successfully measured the components of ‘proton-proton’ solar neutrino fluxes, helped refine the neutrino flavor oscillation parameters, and most impressively, even measured the first. step of the cycle: very low energy “pp neutrinos, remembers Pocar.

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

Over the years and thanks to a sequence of moves to identify and stabilize the backgrounds, the US scientists and the entire collaboration have been successful. “In addition to revealing the CNO neutrinos, the subject of this week Nature article, there is now the potential to help solve the metallicity problem as well, ”says Pocar.

Prior to the CNO’s discovery of the neutrino, the lab had scheduled Borexino to finish operations at the end of 2020. But since the data used in the analysis for the Nature The paper was frozen, scientists continued to collect data, as central purity continued to improve, making a new metallicity-centric result a real possibility, Pocar says. Data collection could extend into 2021 as the necessary logistics and authorizations, while underway, are non-trivial and time-consuming. “Each additional day helps,” he notes.

Pocar has collaborated on the project since graduate school at Princeton in the group led by Frank Calaprice, where he worked on the design, construction of the nylon ship and commissioning of the fluid management system. He subsequently worked with his students at UMass Amherst on data analysis and, more recently, on techniques for characterizing backgrounds for CNO neutrino measurement.

Reference: “Experimental Evidence of Neutrinos Produced in the CNO Fusion Cycle in the Sun” by The Borexino Collaboration, 25 November 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 by funding agencies in Germany, Russia and Poland.