About 99 per cent of solar energy is produced through sequences of nuclear reactions that convert hydrogen into helium, starting from the fusion of two protons (the pp chain). The neutrinos emitted by five of these reactions represent a unique probe of the Sun’s internal working and, at the same time, offer an intense natural neutrino beam for fundamental physics. Here we report a complete study of the pp chain. We measure the neutrino–electron elastic-scattering rates for neutrinos produced by four reactions of the chain: the initial proton–proton fusion, the electron-capture decay of beryllium-7, the three-body proton–electron–proton (pep) fusion, here measured with the highest precision so far achieved, and the boron-8 beta decay, measured with the lowest energy threshold. We also set a limit on the neutrino flux produced by the 3He–proton fusion (hep). These measurements provide a direct determination of the relative intensity of the two primary terminations of the pp chain (pp-I and pp-II) and an indication that the temperature profile in the Sun is more compatible with solar models that assume high surface metallicity. We also determine the survival probability of solar electron neutrinos at different energies, thus probing simultaneously and with high precision the neutrino flavour-conversion paradigm, both in vacuum and in matter-dominated regimes.
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The datasets generated during the current study are freely available in the repository https://bxopen.lngs.infn.it/. Additional information is available from the Borexino Collaboration spokesperson (firstname.lastname@example.org) upon reasonable request.
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The Borexino programme is made possible by funding from INFN (Italy), NSF (USA), BMBF, DFG, HGF and MPG (Germany), RFBR (grants 16-29-13014ofi-m and 17-02-00305A), RSF (grant 17-12-01009) (Russia), and NCN (grant number UMO 2017/26/M/ST2/00915) (Poland). We acknowledge also the computing services of the Bologna INFN-CNAF data centre and LNGS Computing and Network Service (Italy), of Jülich Supercomputing Centre at FZJ (Germany), and of ACK Cyfronet AGH Cracow (Poland). We acknowledge the hospitality and support of the Laboratori Nazionali del Gran Sasso (Italy).
Nature thanks A. Serenelli and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Schematic view of the ‘onion-like’ structure of the Borexino apparatus. From outside to inside: the external water tank; the Stainless Steel Sphere, where about 2,200 photomultiplier tubes (PMTs) are mounted; the outermost nylon vessel, which serves as a barrier against radon; the innermost nylon vessel, which contains 300 t of liquid scintillator, the active detection medium.
The probability distribution of the test statistics t is obtained by simulating thousands of sets of Pee values (at the pp, 7Be, pep and 8B energies) in the MSW-LMA hypothesis (red curve on the left) and in the vacuum-LMA hypothesis (blue curve on the right). The dotted black line corresponds to the results of Borexino discussed in the main text.
The probability distribution of the test statistics t is obtained by simulating thousands of fake sets of 8B–7Be values in the HZ hypothesis (red curve on the left) and in the LZ hypothesis (blue curve on the right). The dotted black line corresponds to the results of Borexino discussed in the main text.
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Hyperfine Interactions (2019)