Atmospheric neutrinos are generated about 15 kilometers above the Earth’s surface. When a cosmic ray, an energetic particle from space, collides with Earth’s atmosphere, they develop. Protons are the most common type of particle, however they can also be helium or heavier nuclei. There is a cascade of particles when they hit an atomic nucleus in our environment. Mesons, which are mostly pions, are short-lived particles (disappointingly, they are not made of pie). These are unstable two-quark particles that decay quickly into muons and muon antineutrinos (or antimuons and muon neutrinos). Because muons are inherently unstable, they frequently decay into electrons, electron antineutrinos, and muon neutrinos. Muon neutrinos and antineutrinos make up around two-thirds of atmospheric neutrinos, while electron neutrinos and antineutrinos make up the rest.
In fact, deviations from this ratio produced part of the evidence that neutrinos oscillate or change flavor as they travel. Scientists working on a variety of experiments, including Japan’s Super-Kamiokande experiment, discovered that muon neutrinos were coming in much lower numbers than expected.
What is the explanation? Muon neutrinos oscillated into various forms of neutrinos, resulting in an excess of electron neutrinos and a deficiency of muon neutrinos. It took the massive Super-Kamiokande detector, which was filled with 50,000 tons of ultrapure water, 5,400 atmospheric neutrino encounters to collect enough data.