The neutrino
The neutrino was proposed by Wolfgang Pauli in 1930 to account for the conservation of energy and spin angular momentum in beta decay. You can find his original letter to Lise Meitner and others on the Fermilab MicroBooNE database, along with the English translation: Pauli later said “I have done a terrible thing. I have postulated a particle that cannot be detected”. He was wrong about that. He was wrong about some other things too. He talked about a particle that travels slower than light, and…