The historic Borexino experiment on solar neutrinos and the mechanisms of stars

The collaboration of the National Laboratories of the Gran Sasso of the Infn has intercepted for the first time the solar neutrinos of the carbon-nitrogen-oxygen nuclear fusion cycle: a milestone in the study of the physics of stars (including the Sun)

(photo: Infn) Today we are a little closer to understanding what really moves the Sun and the other stars: in the depths of the Gran Sasso National Laboratories of the National Institute of Nuclear Physics (Infn) the researchers of the Borexino collaboration they managed to identify for the first time solar neutrinos resulting from the fusion reactions of the carbon-nitrogen-oxygen (Cno) cycle. The experiment, described in an article just published in Nature, has been defined a milestone for neutrino physics, paving the way for observations that will allow us to understand the exact composition of the Sun's core and how stars are formed. massive.

Stellar reactions

Our Sun, like other stars, burns hydrogen in its core. More precisely, it fuses it, giving rise to helium atoms, releasing energy and letting out a particular type of subatomic particles: neutrinos.

The fusion reactions that power our star can follow two cycles: one is called a chain proton-proton (pp), the other being the carbon-nitrogen-oxygen (Cno) cycle. In stars of the size of the Sun the pp chain predominates (and in fact the Cno cycle represents only 1% of solar energy), while in the most massive stars it is the Cno cycle that is the protagonist.

The Borexino experiment

Although about 100 billion solar neutrinos pass through it every second, these particles are extremely elusive: they interact very little with matter and are very difficult to detect. To trace them, due to their low energy and the difficulty of separating their signal from the background noise generated by other sources, it took decades of studies and attempts, and extraordinary tools, such as the Borexino detector.

Borexino was developed to detect the light signal produced when charged particles disperse electrons in a tank filled with a special liquid as they pass. Its onion-shaped design, with increasing layers of radiopurity, allowed the energy and time profile of these light signals to be measured with unprecedented precision. Thus, researchers have now succeeded in distinguishing those produced by the passage of solar neutrinos in the Cno cycle.

"The detection of neutrinos produced in the Cno cycle announced by Borexino is the culmination of an incessant effort, which lasted years, which has led to push liquid scintillation technology beyond any previously reached limit, and to make the heart of Borexino the least radioactive place in the world ", commented Marco Pallavicini, professor at the University of Genoa and member of the Infn Executive Board, currently co-spokesperson of the experiment.

Unraveling the mysteries of the stars

These measurements, explain the Infn researchers, represent experimental evidence of what is in fact the dominant channel of universe for the combustion of hydrogen. "Now we finally have the first fundamental experimental confirmation of how the stars heavier than the Sun shine", underlined Gianpaolo Bellini, professor at the University of Milan and Infn researcher, one of the founding fathers of Borexino. "This is the culmination of thirty years of work and more than ten years of Borexino's discoveries in the physics of the sun, neutrinos and finally stars".

The result also represents a great step forward towards solving the mystery of the elemental composition of the core of the Sun and other stars. From the characteristics of the Cno cycle in a star, for example, we might be able to deduce its metal content, its temperature and density profile, its opacity, up to predicting its evolution.