Of all the particles that we know of, the elusive neutrino is by far the most difficult to explain. We know there are three types of neutrino: the electron neutrino (νe), the muon neutrino (νμ), and the tau neutrino (ντ), as well as their antimatter counterparts (νe, νμ, and ντ). We know that they have extremely tiny but non-zero masses: the heaviest they can be means it would take over 4 million of them to add up to an electron, the next-lightest particle.
We know that they oscillate — or transform — from one type into another as they travel through space. We know that when we calculate the number of neutrinos produced by the Sun from nuclear fusion, only about a third of the expected number arrive on Earth. We know that they're generated in the atmosphere from cosmic rays, and from accelerators and reactors when particles decay. According to the Standard Model, there should be only three.
But that story doesn't add up.
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