**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 (ν _{e}), the muon neutrino (ν_{μ}), and the tau neutrino (ν_{τ}), as well as their antimatter counterparts (ν_{e}, ν_{μ}, and ν_{τ}). 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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