The realization of that our Sun is powered through nuclear fusion came as a relief to scientists disturbed by the mismatch in the chemical age of the Sun and the geologic and biologic history records of the Earth. The nuclear reactions that power the Sun not only release enormous amounts of radiation, they also release enormous numbers of elusive particles called neutrinos. Recall that a neutrino is produced in each stage of the proton-proton chain that powers the Sun. Unfortunately, the observed products of the fusion didn't match initial theories: the number of observed neutrinos was simply too low.
Neutrinos are particles that aren't theoretically required to possess mass. They interact so weakly with ordinary matter that they mostly pass through the whole Earth as if it were not there. These particles are produced through most radioactive processes, including stellar fusion. After neutrinos are produced in the core of the Sun, they leave the Sun within a few seconds and streak toward the Earth at near the speed of light. Neutrinos therefore offer a great opportunity to "see" material coming directly from the thermonuclear heart of the Sun. The neutrino flux at the Earth's surface is prodigious; about 1014 neutrinos pass through every square meter every second. About 10 trillion neutrinos pass through your body every second, and you don't feel a thing! Neutrinos come in three different types or "flavors" related to the type of reaction the are associated with, specifically whether they come from reactions involving electrons, tau particles, or muons.
Detecting neutrinos is quite a challenge. They interact so weakly that vast detectors must be assembled to catch the rare interactions between a neutrino and an atomic nucleus. Huge vats of ultra-pure special liquids are used; the occasional neutrino collisions create a distinctive flash of light. The detectors must be placed deep underground to shield them from confusing signals due to other particles coming in from interstellar space. The American physicist Ray Davis of Brookhaven National Laboratory was the pioneer of this type of experiment, and his original detector ran from 1970 to 1994 deep in a gold mine in South Dakota. Neutrinos were detected by their rare interactions with chlorine atoms in a 100,000 gallon tank of perchloroethylene, which is basically dry-cleaning fluid. His experiment persistently saw only 1/3 of the neutrinos expected from models of the Sun's core. For a while, people were dubious about his experiment since the implications were dramatic — either solar models were badly wrong or there was new physics associated with neutrinos. But when other detectors came online, in particular the Sudbury Neutrino Observatory in Canada and the Kamiokande experiment in Japan, they confirmed the shortfall. Ray Davis was validated, and in 2002 he shared the Nobel Prize in Physics.
Since new physics is a radical option, researchers concentrated on possible flaws in models of solar energy generation. But the emerging field of helioseismology — looking at the way various sound waves propagate through the Sun's interior & mdash only served to confirm the models. It seemed there was no way they could be badly in error. The finger started to point to new physics. The standard model of particle physics held that neutrinos were massless and that the three types of neutrinos were completely independent. In 1998, evidence from the Super-Kamiokande neutrino detector in Japan showed conclusively that neutrinos can change types, and this result was confirmed in 2001 by the Sudbury Neutrino Observatory. If neutrinos can change type, or "oscillate," then they must have a small mass (they also must travel slightly slower than the speed of light). The puzzle was solved. The Sudbury experiment showed that 1/3 of solar neutrinos are the electron type and the other 2/3 are the tau and muon types. As neutrinos left the Sun and traveled to the Earth they were oscillating between the three types and Ray Davis was only detecting the 1/2 that were electron neutrinos. Solar models are correct, but physics and astronomy are still dealing with the profound implications of neutrinos having mass.