Full Idea
When a neutron decays into a proton and an electron (one example of beta decay), the energy of the two particles adds up to less than the starting energy of the neutron. Pauli and Fermi concluded that a neutrino (an electron antineutrino) is emitted.
Gist of Idea
Neutrinos were proposed as the missing energy in neutron beta decay
Source
New Scientist writers (Why the Universe Exists [2017], 01)
A Reaction
I'm wondering how much they could infer about the nature of the new particle (which was only confirmed 26 years later).
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.7
Related Idea
Idea 21138 Neutrons are slightly heavier than protons, and decay into them by emitting an electron [New Sci.]