Two protons and an electron combine into a deuteron and a neutrino. It is a rare alternative to the fusion of two protons and gives neutrinos of a single, fixed energy.
In the hot, dense centre of the Sun free electrons are everywhere. Instead of emitting a positron, the fusing pair of protons can absorb one of these electrons. This happens in only a small fraction of the fusions. Because only two particles leave the reaction, the neutrino always carries the same energy, which makes these pep neutrinos a clean probe of the solar core and of how neutrinos change flavour inside the Sun.
Facts
- How well known
- Fixed by the pp rate and the solar electron density; the ratio of the pep to the pp rate is calculated with small uncertainty.
- Live evaluation
- No live evaluation yet · Evaluations
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 1.442 MeV
- Gamow window
- 6.1 ± 3.3 keVcentre of the Sun, 15.7 MK (pp-I)
- Kind
- Weak interaction
How it was measured
Details
The rate is not measured but calculated relative to the pp reaction: the nuclear matrix element is the same, and the ratio of pep to pp rates depends on the electron density and temperature at the centre of the Sun through a phase-space and electron-wave-function factor. The predicted pep neutrino flux is therefore tied closely to the pp flux and to the solar luminosity. Its monoenergetic line lies in the energy region where the transition between vacuum and matter-enhanced neutrino oscillations takes place.
Borexino reported the first evidence of pep neutrinos in 2012 (Bellini et al., Phys. Rev. Lett. 108, 051302): an interaction rate of 3.1 ± 0.6 (stat) ± 0.3 (syst) counts per day per 100 t, which corresponds to a flux of (1.6 ± 0.3) × 108 cm−2 s−1 for the MSW-LMA oscillation solution, in agreement with the Standard Solar Model. The absence of a pep signal was disfavoured at 98% C.L.
Peak energy E0 ± half the 1/e width, centre of mass, from the AME2020 masses.
Evaluations
Compilations that combine the data into a recommended rate.
Related reactions
References
- 1.
E. G. Adelberger et al.
Solar fusion cross sections II: the pp chain and CNO cycles
Rev. Mod. Phys. 83, 195 (2011) · 10.1103/RevModPhys.83.195
- 2.
B. Acharya et al.
Solar fusion III: New data and theory for hydrogen-burning stars
Rev. Mod. Phys. 97, 035002 (2025) · 10.1103/8lm7-gs18 · arXiv:2405.06470
How to cite this page
J. Skowronski, 1H(pe−,ν)2H, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/1H-pep-2H.
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