Understanding the “Hydrogen Burning” Power of Our Sun – Success After More Than 80 Years

The Borexino collaboration, in which also scientists from TU Dresden are involved, has succeeded after more than 80 years in experimentally confirming the Bethe-Weizsäcker cycle.

Stars produce their energy through nuclear fusion by converting hydrogen into helium — a process known to researchers as “hydrogen burning.” There are two ways of carrying out this fusion reaction: on the one hand, the so-called pp cycle (proton-proton reaction) and the Bethe Weizsäcker cycle (also known as the CNO cycle, derived from the elements carbon (C), nitrogen (N) and oxygen (O)) on the other hand.

The pp cycle is the predominant energy source in our Sun, only about 1.6 per mil of its energy comes from the CNO cycle. However, the Standard Solar Model (SSM) predicts that the CNO cycle is probably the predominant reaction in much larger stars. As early as the 1930s, the cycle was theoretically predicted by the physicists Hans Bethe and Carl Friedrich von Weizsäcker and subsequently named after these two gentlemen. While the pp cycle could already be experimentally proven in 1992 at the GALLEX experiment, also in the Gran Sasso massif, the experimental proof of the CNO cycle has so far not been successful.

Both the pp cycle and the CNO cycle produce countless neutrinos — very light and electrically neutral elementary particles. The fact that neutrinos hardly interact with other matter allows them to leave the interior of the sun at almost the speed of light and to transport the information about their origin to Earth unhindered. Here the ghost particles have no more than to be captured. This is a rather complex undertaking, which is only possible in a few large-scale experiments worldwide, since neutrinos show up as small flashes of light in a huge tank full of a mixture of water, mineral oil, and other substances, also called scintillator. The evaluation of the measured data is complex and resembles looking for a needle in a haystack.

Compared to all previous and ongoing solar neutrino experiments, Borexino is the first and only experiment worldwide that is able to measure these different components individually, in real time and with a high statistical power. This week, the Borexino research collaboration was able to announce a great success: In the renowned scientific journal Nature, they present their results on the first experimental detection of CNO neutrinos — a milestone in neutrino research.

Dresden physicist Professor Kai Zuber is a passionate neutrino hunter.

He is involved in many different experiments worldwide, such as the SNO collaboration in Canada, which was awarded the Nobel Prize for its discovery of a neutrino mass. The fact that with Borexino, he and his colleagues Dr. Mikko Meyer and Jan Thurn have now succeeded in experimentally proving the CNO neutrinos for the first time is another major milestone in Zuber’s scientific career: “Actually, I have now achieved everything I had imagined and hoped for. I (almost) no longer believe in great new discoveries in solar neutrino research for the rest of my lifetime. However, I would like to continue working on the optimization of the experiments, in which the Felsenkeller accelerator here in Dresden plays an extremely important role. For sure, we will be able to have even more precise measurements of the Sun in the future.”

Read Neutrinos Yield First Experimental Evidence of the CNO Energy-Production Mechanism of the Universe for more on this research.

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

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