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 (p-p) chain and the carbon-nitrogen-oxygen (CNO) cycle. Neutrinos that are emitted along such 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 p-p chain, which produces about 99% of the solar energy, has been performed previously. Now, physicists from the Borexino Collaboration report the 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, which is located at the underground Laboratori Nazionali del Gran Sasso in Italy.
“Neutrinos are really the only direct probe science has for the core of stars, including the Sun, but they are exceedingly difficult to measure,” said Professor Andrea Pocar, a particle physicist at the University of Massachusetts Amherst.
“As many as 420 billion of them hit every square inch of the Earth’s surface per second, yet virtually all pass through without interacting.”
“We can only detect them using very large detectors with exceptionally low background radiation levels.”
The Borexino detector lies deep under the Apennine Mountains in central Italy at the 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 latest detections, the Borexino Collaboration had successfully measured components of the ‘proton-proton’ solar neutrino fluxes, helped refine neutrino flavor-oscillation parameters, and most impressively, even measured the first step in the cycle: the very low-energy p-p neutrinos.
The Borexino researchers dreamed of expanding the science scope to also look for the CNO neutrinos – in a narrow spectral region with particularly low background – but that prize seemed out of reach.
However, they believed CNO neutrinos might yet be revealed using the additional purification steps and methods they had developed to realize the exquisite detector stability required.
“Confirmation of CNO burning in our Sun, where it operates at only 1%, reinforces our confidence that we understand how stars work,” Professor Pocar said.
“Beyond this, CNO neutrinos can help resolve an important open question in stellar physics.”
“That is, how the Sun’s central metallicity, as can only be determined by the CNO neutrino rate from the core, is related to metallicity elsewhere in a star.”
“Traditional models have run into a difficulty — surface metallicity measures by spectroscopy do not agree with the sub-surface metallicity measurements inferred from a different method, helioseismology observations.”
“We were able to detect CNO neutrinos using the Borexino experiment’s huge detector located 1,400 m underground,” said Professor Michael Wurm, a neutrino physicist at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz.
“They provide us with clear insights into the processes in the Sun’s core.”
“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, a postdoctoral researcher at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz.
The team’s paper was published in the journal Nature.
M. Agostini et 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