What neutrino flavor is primarily produced in the Sun?

The neutrino flavor primarily produced in the Sun is the electron neutrino.

The Sun generates energy mainly through the proton–proton chain, in which hydrogen nuclei ultimately fuse into helium. Several steps in this chain produce electron neutrinos, especially the initial proton–proton reaction. These neutrinos escape from the solar core almost immediately because they interact only weakly with matter, providing a direct way to study nuclear fusion deep inside the Sun.

The wording “primarily produced” matters because the Sun makes neutrinos through several reactions, including branches involving beryllium-7, boron-8, and the CNO cycle. These reactions produce electron neutrinos at their point of creation. Muon and tau neutrinos are not the flavors directly generated by ordinary solar fusion reactions.

However, a solar neutrino may not remain an electron neutrino while traveling to Earth. Because neutrinos have mass and their flavor states are mixtures of mass states, they can oscillate into muon and tau flavors. This explained the historic solar-neutrino problem: early detectors saw fewer electron neutrinos than solar models predicted. The Sudbury Neutrino Observatory’s measurements confirmed that many had changed flavor rather than disappeared.