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).
The best initial guess was that its mass - that is, its rest energy - is zero. If so, it ravels at the speed of light, as a photon does, with momentum E/c. It was twenty six years before the neutrino was detected. You may be surprised to learn that billions of neutrinos have passed through you as you read this sentence. But don't worry, they are so weakly interacting that most of them would pass through a lighlt year of lead without hitting anything.