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Comprehensive measurement of pp-chain solar neutrinos


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 Additional information is available from the Borexino Collaboration spokesperson ( 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).

Reviewer information

Nature thanks A. Serenelli and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

The Borexino detector was designed, constructed, and commissioned by the Borexino Collaboration over the span of more than 15 years. The Borexino Collaboration sets the science goals. Scintillator purification and handling, source calibration campaigns, photomultiplier tube and electronics operations, signal processing and data acquisition, Monte Carlo simulations of the detector, and data analyses were performed by Borexino members, who also discussed and approved the scientific results. This manuscript was prepared by a subgroup of authors appointed by the Collaboration and subjected to an internal collaboration-wide review process. All authors reviewed and approved the final version of the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to B. Caccianiga.

Extended data figures and tables

Extended Data Fig. 1 The Borexino detector.

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.

Extended Data Fig. 2 Frequentist hypothesis test of MSW-LMA versus vacuum-LMA.

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.

Extended Data Fig. 3 Frequentist hypothesis test for LZ and HZ.

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.

Extended Data Table 1 LER analysis systematics
Extended Data Table 2 HER analysis systematics

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Further reading

Fig. 1: Nuclear fusion sequences and neutrino energy spectrum.
Fig. 2: Results of the fit used to extract the neutrino signal.
Fig. 3: Electron neutrino survival probability Pee as a function of neutrino energy.
Fig. 4: Borexino results and analysis in the Φ(7Be)–Φ(8B) space.
Extended Data Fig. 1: The Borexino detector.
Extended Data Fig. 2: Frequentist hypothesis test of MSW-LMA versus vacuum-LMA.
Extended Data Fig. 3: Frequentist hypothesis test for LZ and HZ.


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