© Michelle Leber, 2009.
In the early days of beta decay research, physicists knew that a neutron in the nucleus of an atom turned into a proton and an electron went flying away. It was expected that the electron's energy would be equal to the difference in energy between the initial and final nuclear states. However, when experimenters measured the electron's energy, they found that it varied over a wide range, as shown in the plot above. This led Pauli to posit the existence of the neutrino—another particle to share the energy of the decay process with the electron. (Unit: 1)