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# Solar neutrino

Electron neutrinos are produced in the sun as a product of nuclear fusion. This is known as the proton-proton chain reaction.

The net reaction is:

$4 p + 2e = He + 2 \nu_e \!\$,

or in words:

4 protons + 2 electrons = Helium + 2 electron neutrinos.

Neutrinos from the proton-proton process have energy up to 400 keV. There are also several other significant production mechanisms, with energies up to 10 MeV. [1]

The number of neutrinos can be predicted by the standard model. The detected number of electron neutrinos was only a 1/3 of the predicted number, this is known as the solar neutrino problem. This led to the idea of neutrino oscillations and the fact that neutrinos can change flavour. This was confirmed when the total flux of solar neutrinos of all types was measured and it agreed with the earlier predictions of expected electron neutrino flux. This effectively proved that neutrinos have mass as only massive particles can change flavour.