Scientists who are members of the Borexino Collaboration have provided the first experimental proof of the occurrence of the so-called CNO cycle in the Sun: They have managed to directly detect the distinctive neutrinos generated during this fusion process. This is an important milestone on the route to better understanding the fusion processes that occur in the Sun. At the same time, although the CNO cycle plays a minor role in our Sun, it is most likely the predominant way of producing energy in other more massive and hotter stars. The Borexino Collaboration’s findings have been published in the latest issue of the journal Nature.

How does the Sun generate energy? As a gigantic fusion reactor, it continuously converts hydrogen into helium — a process also referred to as ‘hydrogen burning’. Essentially, this involves two types of processes. On the one hand, there is the proton-proton reaction (pp reaction). This begins with the direct fusion of two hydrogen nuclei to create the intermediate hydrogen isotope deuterium from which helium is subsequently formed. On the other hand, the heavier elements carbon (C), nitrogen (N) and oxygen (O) are involved in the second type of reaction chain, known as the CNO cycle or Bethe-Weizs├Ącker cycle. While the pp reaction is predominant in smaller stars such as our Sun, the CNO cycle is the main process for generating energy in more massive and hotter stars.

As is the case with all fusion processes that occur within the Sun, countless neutrinos are produced in addition to helium and the enormous amounts of energy which cause the Sun and its sister stars to shine. The neutrinos reach the Earth in their billions and normally pass through it unhindered. “However, we are able to detect these neutrinos using the Borexino experiment’s huge detector located 1400 meters underground,” points out Prof. Michael Wurm, a neutrino physicist at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) and a member of the Borexino Collaboration. “They provide us with clear insights into the processes in the Sun’s core.”

While the Borexino Collaboration has been able to detect neutrinos originating from several reactions along the pp chain in recent years, their current achievement has been to explicitly identify neutrinos released in the CNO cycle, which are significantly less abundant in comparison. “Although on the basis of model calculations we expected the CNO cycle also to occur in the Sun, direct evidence of this has never been obtained before. Only a characteristic neutrino signal can provide conclusive proof that this actually happens — now we have that conclusive proof without a shadow of a doubt.”

In addition, the research team was also able to estimate the total flow of CNO neutrinos reaching the Earth. About 700 million of them fly through a square centimeter of our planet each second, but this accounts for only one hundredth of the total number of solar neutrinos. “This is consistent with the theoretical expectations that the CNO cycle in the Sun is responsible for about one percent of the energy it produces,” adds Dr. Daniele Guffanti, a postdoc in Michael Wurm’s team and also a member of the Borexino Collaboration.

The two neutrino physicists from Mainz consider the new results to be an important milestone along the route to obtaining a complete understanding of the fusion processes which not only drive our Sun but also massive stars, and make these latter light up our night sky. It also paves the way for a better insight into the elements that compose the solar core, particularly with regard to how frequently heavier elements such as carbon, nitrogen and oxygen can be found in the solar plasma in addition to hydrogen and helium — researchers call this metallicity. Neutrinos might once again be our only guides to help us discover this.

“Experimental Evidence of Neutrinos Produced in the CNO Fusion Cycle in the Sun,” The Borexino Collaboration, 2020 November 25, Nature [].

About the Borexino detector:
The Borexino detector has been collecting data on solar neutrinos since 2007. It is located in the largest underground laboratory in the world, the Laboratori Nazionali del Gran Sasso in Italy. At the heart of the Borexino detector is an extremely thin-walled, spherical nylon balloon that contains 280 tons of special scintillator fluid. A neutrino interacts with the detector material just a few hundred times a day. This then generates tiny flashes of light which are detected by around 2.000 extremely sensitive sensors.

In order to make sure that the detected signals actually come from neutrinos, the scientists have to switch off other potential signal sources or filter them out during data analysis — this includes natural background radioactivity and interference caused by cosmic radiation, especially that associated with muons. But even though the tank is shielded under a 1.400-meter thick layer of rock in the Gran Sasso massif near Rome, some muons are still able to reach it, while radioactive decay can produce signals that at first glance cannot be distinguished from a real neutrino signal. The Mainz team is specialized in developing sophisticated analysis techniques that help suppress such background events, so that the rare neutrino signals can be reliably identified.