Resolving the issue:
It would be Wolfgang Pauli who resolved the issue of the missing energy by proposing the existence of a third particle capable of avoiding detection: the neutrino. Taking the example of bismuth 210 decay, the figure above shows the situation where all three bodies have equal momenta. The recoiling nucleus, some 400,000 times heavier than the electron, takes away a negligible amount of energy. The remaining energy is divided between the electron and the neutrino, shown here as a ghost particle. It would take almost a quarter of a century for the first experimental evidence of a neutrino to come to light.