Journal
NATURE
Volume 587, Issue 7835, Pages 577-+Publisher
NATURE PORTFOLIO
DOI: 10.1038/s41586-020-2934-0
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Funding
- Laboratori Nazionali del Gran Sasso (Italy)
- Istituto Nazionale di Fisica Nucleare (INFN) (Italy)
- National Science Foundation (NSF) (USA)
- Deutsche Forschungsgemeinschaft (DFG)
- Helmholtz-Gemeinschaft (HGF) (Germany)
- Russian Foundation for Basic Research (RFBR) [16-29-13014ofi-m, 17-02-00305A, 19-02-00097A]
- Russian Science Foundation (RSF) [17-12-01009]
- Ministry of Science and Higher Education of the Russian Federation (Russia) [075-15-2020-778]
- Narodowe Centrum Nauki (NCN) (Poland) [UMO 2017/26/M/ST2/00915]
- PLGrid Infrastructure (Poland)
- Russian Science Foundation [17-12-01009] Funding Source: Russian Science Foundation
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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 (pp) chain and the carbon-nitrogen-oxygen (CNO) cycle(1,2). Neutrinos that are emitted along such fusion 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 pp chain, which produces about 99 per cent of the solar energy, has been performed previously(3); however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal-only a few counts per day above the background per 100 tonnes of target-that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate-as well as the flux of emitted CNO neutrinos-depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe.
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