Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

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.

Similar content being viewed by others

Data availability

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.


  1. Atkinson, R. & Houtermans, F. Zur Frage der Aufbaumöglichkeit der Elemente in Sternen. Z. Phys. 54, 656 (1929).

    Article  CAS  ADS  Google Scholar 

  2. von Weizsäcker, C. F. Über Elementumwandlungen im Innern der Sterne I. Phys. Z. 38, 176 (1937).

    Google Scholar 

  3. Bethe, H. A. & Critchfield, C. L. The formation of deuterons by proton combination. Phys. Rev. 54, 248 (1938).

    Article  CAS  ADS  Google Scholar 

  4. Bethe, H. Energy production in stars. Phys. Rev. 55, 434 (1939).

    Article  CAS  ADS  Google Scholar 

  5. Bahcall, J. N. How the Sun Shines. (Nobel Media, Stockholm, 2000).

  6. Fowler, W. Experimental and theoretical nuclear astrophysics; the quest for the origin of the elements: Nobel prize lecture. Rev. Mod. Phys. 56, 149 (1984).

    Article  CAS  ADS  Google Scholar 

  7. Davis, R. Nobel lecture: a half-century with solar neutrinos. Rev. Mod. Phys. 75, 985 (2003).

    Article  CAS  ADS  Google Scholar 

  8. Abdurashitov, J. et al. Results from SAGE (the Russian-American gallium solar neutrino experiment). Phys. Lett. B 328, 234 (1994).

    Article  CAS  ADS  Google Scholar 

  9. Anselmann, P. et al. Solar neutrinos observed by GALLEX at Gran Sasso. Phys. Lett. B 285, 376 (1992).

    Article  CAS  ADS  Google Scholar 

  10. Hirata, K. et al. Observation of 8B solar neutrinos in the Kamiokande-II detector. Phys. Rev. Lett. 63, 16 (1989).

    Article  CAS  ADS  Google Scholar 

  11. Ahmad, Q. et al. Direct evidence for neutrino flavor transformation from neutral-current interactions in the Sudbury Neutrino Observatory. Phys. Rev. Lett. 89, 011301 (2002).

    Article  CAS  ADS  Google Scholar 

  12. Pontecorvo, B. Neutrino experiments and the problem of conservation of leptonic charge. Zh. Eksp. Teor. Fiz. 53, 1717 (1967).

    CAS  Google Scholar 

  13. Wolfenstein, L. Neutrino oscillations in matter. Phys. Rev. D 17, 2369 (1978).

    Article  CAS  ADS  Google Scholar 

  14. Mikheyev, S. & Smirnov, A. Resonant amplification of neutrino oscillations in matter and spectroscopy of solar neutrinos. Sov. J. Nucl. Phys. 42, 913 (1985).

    Google Scholar 

  15. Bahcall, J. & Davis, R. The evolution of neutrino astronomy. Publ. Astron. Soc. Pacif. 112, 429 (2000).

    Article  ADS  Google Scholar 

  16. Haxton, W., Hamish Robertson, R. & Serenelli, A. Solar neutrinos: status and prospects. Annu. Rev. Astron. Astrophys. 51, 21 (2013).

    Article  CAS  ADS  Google Scholar 

  17. Bahcall, J. N. Neutrino Astrophysics (Cambridge Univ. Press, Cambridge, 1989).

    Google Scholar 

  18. Vinyoles, N. et al. A new generation of standard solar models. Astrophys. J. 835, 202 (2017).

    Article  ADS  Google Scholar 

  19. Esteban, I. et al. Updated fit to three neutrino mixing: exploring the accelerator-reactor complementarity. J. High Energy Phys. 1701, 087 (2017).

    Article  ADS  Google Scholar 

  20. Arpesella, C. et al. First real time detection of 7Be solar neutrinos by Borexino. Phys. Lett. B 658, 101 (2008).

    Article  CAS  ADS  Google Scholar 

  21. Arpesella, C. et al. Direct measurement of the 7Be solar neutrino flux with 192 days of Borexino data. Phys. Rev. Lett. 101, 091302 (2008).

    Article  CAS  ADS  Google Scholar 

  22. Bellini, G. et al. Precision measurement of the 7Be solar neutrino interaction rate in Borexino. Phys. Rev. Lett. 107, 141302 (2011).

    Article  CAS  ADS  Google Scholar 

  23. Bellini, G. et al. First evidence of pep solar neutrinos by direct detection in Borexino. Phys. Rev. Lett. 108, 051302 (2012).

    Article  CAS  ADS  Google Scholar 

  24. Bellini, G. et al. Measurement of the solar 8B neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector. Phys. Rev. D 82, 033006 (2010).

    Article  ADS  Google Scholar 

  25. Borexino Collaboration. Neutrinos from the primary proton-proton fusion process in the Sun. Nature 512, 383 (2014).

    Article  ADS  Google Scholar 

  26. Alimonti, G. et al. The Borexino detector at the Laboratori Nazionali del Gran Sasso. Nucl. Instrum. Meth. A 600, 568 (2009).

    Article  CAS  ADS  Google Scholar 

  27. Bellini, G. et al. Final results of Borexino Phase I on low-energy solar neutrino spectroscopy. Phys. Rev. D 89, 112007 (2014).

    Article  ADS  Google Scholar 

  28. Back, H. et al. Borexino calibrations: hardware, methods and results. J. Instrum. 7, P10018 (2012).

    Article  Google Scholar 

  29. Agostini, M. et al. The Monte Carlo simulation of the Borexino detector. Astropart. Phys. 97, 136 (2018).

    Article  ADS  Google Scholar 

  30. Bellini, G. et al. Muon and cosmogenic neutron detection in Borexino. J. Instrum. 6, P05005 (2012).

    Google Scholar 

  31. Abe, K. et al. Solar neutrino measurements in Super-Kamiokande-IV. Phys. Rev. D 94, 052010 (2016).

    Article  ADS  Google Scholar 

  32. Aharmim, B. et al. Combined analysis of all three phases of solar neutrino data from the Sudbury Neutrino Observatory. Phys. Rev. C 88, 025501 (2013).

    Article  ADS  Google Scholar 

  33. Bergström, J. et al. Updated determination of the solar neutrino fluxes from solar neutrino data. J. High Energy Phys. 2016, 132 (2016).

    Article  Google Scholar 

  34. Chapman, G. A. in Encyclopedia of Planetary Science and Encyclopedia of Earth Science 748 (Springer, 1997).

  35. Fröhlich, C. & Lean, J. The Sun’s total irradiance: cycles, trends and related climate change uncertainties since 1976. Geophys. Res. Lett. 25, 4377 (1998).

    Article  ADS  Google Scholar 

  36. Bahcall, J. & Pena-Garay, C. A road map to solar neutrino fluxes, neutrino oscillation parameters and tests for new physics. J. High Energy Phys. 2003, 4 (2003).

    Article  Google Scholar 

  37. Caldwell, A., Kollar, D., Kroninger, K. BAT—the Bayesian Analysis Toolkit. Comput. Phys. Commun. 180, 2197 (2009).

    Article  CAS  ADS  Google Scholar 

  38. Feng Pen An et al. Measurement of electron antineutrinon oscillation based on 1230 days of operation of the Daya Bay experiment. Phys. Rev. D 95, 072006 (2017).

    Article  ADS  Google Scholar 

  39. Gando, A. et al. Reactor on-off antineutrino measurement with KamLAND. Phys. Rev. D 88, 033001 (2013).

    Article  ADS  Google Scholar 

  40. Holmgren, H. & Johnston, R. He3(α,γ)Li7 and He3(α,γ)Be7 reactions. Phys. Rev. 113, 1556 (1959).

    Article  CAS  ADS  Google Scholar 

  41. Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481 (2009).

    Article  CAS  ADS  Google Scholar 

  42. Caffau, E., Ludwig, H. G., Steffen, M., Freytag, B. & Bonifacio, P. Solar chemical abundances determined with a CO5BOLD 3D model atmosphere. Sol. Phys. 268, 255 (2011).

    Article  CAS  ADS  Google Scholar 

  43. Asplund, M., Grevesse, N. & Sauval, A. J. The Solar Chemical Composition. (eds Barnes, T. G. & Bash, F. N.) Astronomical Society of the Pacific Conference Series 336, 25 (ASP, 2005).

  44. Grevesse, N. & Sauval, A. J. Standard solar compositon. Space Sci. Rev. 85, 161 (1998).

    Article  CAS  ADS  Google Scholar 

  45. Grevesse, N. & Noels, A. in Origin and Evolution of the Elements (eds Prantzos, N., Vangioni-Flam, E. & Casse, M.) 15 (Cambrige Univ. Press, Cambridge, 1993).

  46. Franco, D., Consolati, G. & Trezzi, D. Positronium signature in organic liquid scintillators for neutrino experiments. Phys. Rev. C 83, 015504 (2011).

    Article  ADS  Google Scholar 

  47. Geant4. A simulation toolkit. (2018).

  48. Bahcall, J. N. & Pena-Garay, C. Solar models and solar neutrino oscillations. New J. Phys. 6, 63 (2004).

    Article  ADS  Google Scholar 

  49. Blennow, M. & Coloma, P. Quantifying the sensitivity of oscillation experiments to the neutrino mass ordering. J. High Energy Phys. 03, 028 (2013).

    ADS  Google Scholar 

Download references


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

Authors and Affiliations



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.

Corresponding author

Correspondence to B. Caccianiga.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

The Borexino Collaboration. Comprehensive measurement of pp-chain solar neutrinos. Nature 562, 505–510 (2018).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing