Neutrinos betrayed their existence through their absence. In 1914 physicist James Chadwick was looking at beta decay – a from of radiactive decay in which a nucleus emits an electron and transforms a neutron into a proton. Conservation of energy appears to be an accepted Law of physics (although that might be providing it with a bigger status than warranted). Suffice to say the mainstream position supports the idea so that electrons from a particular nucleus, such as lead-214, should always emerge with the same energy. Indeed, Chadwick showed that they emerge with a range of energies extending down to zero – as if energy was disappearing.
Where was the energy going? Neils Bohr, theoretical physicist, suggested energy might not be conserved on the atomic scale. Wolfgang Pauli, in 1930, has speculated that beta decay emits an unseen particle that emerges alongside the electron – and it absconds with a random fraction of the energy. Three years late Enrico Fermi gave this unseen particle a name – the neutrino. The link was also forwarded by Jovan.