Neutrinos bear first experimental evidence of the CNO energy-production mechanism of the Universe

Borexino detector

Check the interior of the Borexino detector. Credit: Borexino Collaboration

Neutrinos wear first experimental evidence of catalyzed fusion dominant in many stars

An international team of about 100 scientists from the Borexino Collaboration, including particle physicist Andrea Pokar at the University of Massachusetts Amherst, reports in Nature This week detection of neutrinos from the sun, immediately revealed for the first time that the carbon-nitrogen-oxygen (KNO) fusion cycle is at work in our sun.

The CNO cycle is the dominant energy source of stars heavier than the sun, but it has so far never been detected in any star, Pokar explains.

In much of their lives, stars get energy by fusing hydrogen into helium, he adds. In stars like our Sun or Lighter, this happens mostly through the ‘proton-proton’ chains. However, many stars are heavier and hotter than our sun and contain elements heavier than helium in their composition, a quality known as metallicity. The prediction since the 1930s is that the CNO-cycle will be dominant in heavy stars.

Neutrinos emitted as part of these processes provide a spectral signature allowing scientists to distinguish those of the ‘proton-proton chain’ from those of the ‘CNO-cycle.’ Pokar points out, “Confirmation of CNO burning in our sun, where it operates only one percent, reinforces our confidence that we understand how stars work.”

Borexino detector under Apennine Mountains

The Borexino detector lies deep beneath the Apennine Mountains in central Italy at the INFN’s Laboratory national del Gran Sasso. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultra-pure organic scintillator. Credit: Borexino Collaboration

Beyond that, CNO neutrinos can help keep 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 CNO neutrino rate of the heart, is related to metallicity elsewhere in a star. Traditional models have difficulties – surface metallicity measures by spectroscopy do not agree with the surface metallicity measurements of a different method, helioseisology observations.

Pokar says that neutrinos are really the only direct probe that science has for the heart of stars, including the sun, but they are very difficult to measure. As many as 420 billion of them hit every square inch of the earth’s surface per second, but virtually all pass without interacting. Scientists can only detect them with very large detectors with exceptionally low background radiation levels.

The Borexino detector is located deep under the Apennine Mountains in central Italy at the INFN’s Laboratory nacionali del Gran Sasso. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultra-pure organic scintillator. The great depth, size and purity make Borexino a unique detector for this type of science, alone in its class for low-background radiation. 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 at Bell Labs.

Until recent detections, the Borexino collaboration has successfully measured components of the ‘proton-proton’ solar neutrino flux, helped refine neutrino flavor-oscillation parameters, and most impressively, even measured the first step in the cycle: the low energy ‘paste’ Neutrinos, Pocar recalls.

His researchers dreamed of expanding the science scope to also look for the CNO neutrinos – in a narrow spectral region with particularly low background – but the prizes seemed out of reach. However, research groups in Princeton, Virginia Tech and Umas most believed that CNO neutrinos could still be discovered using the additional cleaning steps and methods they developed to realize the new exquisite detector stability.

Over the years and thanks to a sequence of moves to identify and stabilize these backgrounds, the US. It. Scientists and the whole collaboration were successful. Beyond discovering the CNO neutrinos that is the topic of the week Nature Article, there is now even a potential to help solve the metallicity problem, “Pokar says.

Prior to the CNO neutrino discovery, the laboratory had scheduled Borexino to finish operations in the near 2020. But because the data used in the analysis for the Nature Paper is frozen, scientists have continued collecting data, because the central purity has continued to improve, so a new result focused on the metallicity is a real possibility, says Pokar. Data collection may extend into 2021 since the required logistics and authorization are non-existent and time-consuming. “Every extra day helps,” he remarks.

Pokar has been with the project since his graduate school days in Princeton in the group led by Frank Calaprice, where he worked on the design, construction of the nylon ship and the commissioning of the liquid handling system. He later worked with his students at UMass Amherst on data analysis and, more recently, on techniques to characterize the backgrounds for the 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

The work was supported in the US. It. 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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