Borexino in Nature
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In the core of the sun, at a temperature of about 15 million degrees, hydrogen nuclei fuse together to form Helium nuclei via different chain reactions. In these nuclear processes, along with enormous amounts of energy, neutrinos are emitted as byproducts. In any given second, there are billions of solar neutrinos passing through every square centimeter on the Earth surface, carrying direct and uncompromised information about the Sun’s core. The Borexino detector, located at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, has been studying solar neutrinos since 2007. It has performed a complete neutrino spectroscopy of the so-called "pp-chain", the dominant energy producing process in our star. Now the scientists from the Borexino collaboration have for the first time observed the solar neutrinos from another fusion process, the so-called CNO cycle. This cycle, in which Carbon-Nitrogen-Oxygen catalyze the fusion, has been so far only theoretically hypothesized to contribute less than 1% of the solar energy. This discovery now confirms, with a high statistical significance of more than 5 σ, the real occurrence of this process in nature, expected to be the dominant source of energy in massive stars and thus the dominant hydrogen burning process in the whole universe. CNO neutrinos hold the key to a lot of open questions, including the abundance of heavy metals in the solar core, a burning problem in present-day solar physics.
M. Agostini et al. (Borexino Collaboration), "First Direct Experimental Evidence of CNO neutrinos", Nature 587, p 577–582(2020)