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For most of their existence, stars are fueled by the fusion of hydrogen into helium. Fusion takes place according to two theoretically well understood processes: the proton-proton chain (pp) and the carbon-nitrogen-oxygen (CNO) cycle. The neutrinos emitted along these processes in the solar nucleus are the only direct probe from the deep interior of the Sun. A comprehensive spectroscopic study of the pp chain neutrinos, which produce about 99% of solar energy, has been performed previously. Now, physicists from the Borexino Collaboration report direct observation of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using a large volume neutrino detector called Borexino, located at the Laboratori Nazionali del Gran Sasso in Italy.
On August 31, 2012, a long filament of solar material that hovered in the Sun’s atmosphere, the corona, burst into space at 4:36 p.m. EDT. The CME traveled over 900 miles per second. It did not travel directly to Earth, but connected to Earth’s magnetic environment, or magnetosphere, at a glance, causing auroras to appear on the night of September 3. Image credit: NASA’s Goddard Space Flight Center.
“Neutrinos are really the only direct probe that science has for the nucleus of stars, including the Sun, but they are extremely difficult to measure,” said Professor Andrea Pocar, particle physicist at the University of Massachusetts at Amherst.
“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.”
“We 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.
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, most impressively, even measured the first stage of the cycle: the pp at very low neutrino energy.
Borexino researchers dreamed of expanding 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, they believed that CNO neutrinos could still be revealed using the additional purification steps and methods they had developed to achieve the required exquisite detector stability.
The Borexino detector after thermal stabilization. Image credit: Borexino Collaboration.
“Confirmation that CNO is burning in our Sun, where it operates at just 1%, strengthens our confidence in our understanding of how stars work,” said Prof Pocar.
“Beyond that, CNO neutrinos can help solve an important open question in stellar physics.”
“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 subsurface metallicity measurements derived from a different method, helioseismological observations.
“We were able to detect CNO neutrinos using the huge detector from the Borexino experiment located 1,400 m underground,” said Professor Michael Wurm, neutrino physicist at the PRISMA + cluster of excellence at the Johannes Gutenberg University in Mainz.
“They provide us with clear information about the processes at the heart of the Sun.”
“This is consistent with the theoretical expectations that the CNO cycle in the Sun is responsible for about 1% of the energy it produces,” said Dr Daniele Guffanti, postdoctoral researcher at the PRISMA + Cluster of Excellence at Johannes Gutenberg University in Mainz.
The team’s article was published in the journal Nature.
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Mr. Agostini and al. (The Borexino collaboration). 2020. Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun. Nature 587, 577-582; doi: 10.1038 / s41586-020-2934-0
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