solar, atmospheric, reactor, and accelerator neutrinos

Neutrinos come from many places, and physicists have learned to read each source like a different page in a book. Four kinds have been especially powerful. Solar neutrinos pour from the nuclear fire in the heart of the Sun. Atmospheric neutrinos are made high in the sky when cosmic rays smash into the air. Reactor neutrinos stream from nuclear power plants. And accelerator neutrinos are produced deliberately, in dense man-made beams aimed at distant detectors.

Each source is suited to a different question because they differ in energy and in how far the neutrinos travel before you catch them — and oscillation depends on exactly that combination of distance and energy. Solar neutrinos, low in energy and crossing the whole inner solar system, revealed one mixing angle and resolved the solar neutrino problem. Atmospheric neutrinos, arriving from all directions through varying thicknesses of Earth, first revealed oscillation in 1998. Reactor neutrinos, caught a kilometre or so from the core, pinned down the small angle theta-13. Accelerator beams, fired hundreds of kilometres through rock, let experimenters control energy and direction to measure parameters with precision and probe matter-antimatter differences.

Together these complementary sources have turned neutrino oscillation from a stubborn mystery into a precision science within a single generation. They are why we can now quote mixing angles and mass splittings to several significant figures. And the same toolkit reaches outward: detectors built for these neutrinos also stand ready to catch the burst from the next nearby supernova, and feed into the broader enterprise of neutrino astronomy.