The flux of neutrinos at Earth is several tens of billions per square centimetre per second, mostly from the Sun's core.
Particle physicists knew that a mechanism, discussed in 1957 by Bruno Pontecorvo, could explain the deficit in electron neutrinos.
Today it is accepted that the neutrinos produced in the Sun are not massless particles as predicted by the Standard Model but rather mixed quantum states made up of defined-mass eigenstates in different (complex) proportions.
The Sun performs nuclear fusion via the proton–proton chain reaction, which converts four protons into alpha particles, neutrinos, positrons, and energy.
In the late 1960s, Ray Davis and John N. Bahcall's Homestake Experiment was the first to measure the flux of neutrinos from the Sun and detect a deficit.
Since it takes thousands of years for heat energy to move from the core to the surface of the Sun, this would not immediately be apparent.
Advances in helioseismology observations made it possible to infer the interior temperatures of the Sun; these results agreed with the well established standard solar model.
That experiment aimed at the 8B solar neutrinos, which at around 10 MeV are not much affected by oscillation in both the Sun and the Earth.
SNO's unique design employing a large quantity of heavy water as the detection medium was proposed by Herb Chen, also in 1985.
[12] After extensive statistical analysis, the SNO collaboration determined that fraction to be about 34%,[13] in perfect agreement with prediction.
The total number of detected 8B neutrinos also agrees with the then rough predictions from the solar model.