Beta Decay and the Neutrino

During Beta decay experiments it was found that momentum and energy were not conserved as they should be. In Beta decay, a Neutron from the nucleus decays into a Proton and a high speed Electron (beta particle).

For example:-

Or more generally we can just consider the Neutron decaying to form a Proton and an Electron.

This decay should give an Electron of known Kinetic Energy, however, experimentally this is not the case:-

Pauli explained the range of energies by suggesting that another particle was also being emitted, sharing the Kinetic Energy.

In order to match experimental observations, the particle must:-

1. Have a very small mass

2. Have a neutral charge 

3. Be highly penetrating

The particle became known as the Neutrino.

Therefore the formula should actually be written as:-

The video below gives a more detailed explanation of Beta decay for both and Electron and a Positron.