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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 ³ He–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.

The yields and production rates of the radioisotopes $^{9}$ Li and $^{8}$ He created by cosmic muon spallation on $^{12}$ C, have been measured by the two detectors of the Double Chooz experiment. The identical detectors are located at separate sites and depths, which means that they are subject to different muon spectra. The near (far) detector has an overburden of ∼120 m.w.e. (∼300 m.w.e.) corresponding to a mean muon energy of 32.1 ± 2.0 GeV (63.7 ± 5.5 GeV). Comparing the data to a detailed simulation of the $^{9}$ Li and $^{8}$ He decays, the contribution of the $^{8}$ He radioisotope at both detectors is found to be compatible with zero. The observed $^{9}$ Li yields in the near and far detectors are 5.51 ± 0.51 and 7.90 ± 0.51, respectively, in units of 10 $^{−8}$ μ $^{−1}$ g $^{−1}$ cm $^{2}$ . The shallow overburdens of the near and far detectors give a unique insight when combined with measurements by KamLAND and Borexino to give the first multi-experiment, data driven relationship between the $^{9}$ Li yield and the mean muon energy according to the power law$ Y = {Y}_0{\\left(\\left\\langle {E}_{\\mu}\\right\\rangle /1\\ GeV\\right)}^{\\overline{\\alpha}} $, giving$ \\overline{\\alpha} = 0.72 \\pm 0.06 $and Y $_{0}$= (0.43 ± 0.11) × 10 $^{−8}$ μ $^{−1}$ g $^{−1}$ cm $^{2}$ . This relationship gives future liquid scintillator based experiments the ability to predict their cosmogenic $^{9}$ Li background rates.