The conservation law in fundamental particles of matter is the idea that which every decay of a nucleus, the charge, energy, lepton number and nucleon number is always conserved (a bit like momentum). With alpha decay, the energy, lepton number and charge stays the same before the decay and after because scientists were able to measure the properties of the alpha particle and calculate that everything was conserved. This led scientists to believe that the same must apply to beta decay.
- It must be neutral (to conserve electric charge).
- It must be an anti lepton with lepton number -1.
- It must carry away energy.
- It must carry away linear momentum to conserve momentum.
- It must interact extremely weakly with matter (otherwise it would be detected).