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.
The problem with beta decay is that if you take the same approach to beta decay you did for alpha decay,it simply would not work. If after the decay, one beta particle is released, scientists saw that the energy before the decay was in fact more than the energy after the decay. There must therefore be another fundamental particle that is emitted from beta decay.
In 1930 Austrian theoretical physicist Wolfgang Pauli came up with a brilliant solution. He suggested that a third, unseen and almost undetectable particle is emitted in beta decay. The particle is called an anti neutrino (v) (neutrino in beta-plus decay). Energy is conserved for all beta decays because the 0.546 MeV is shared between the electron and the anti neutrino.
Pauli’s idea also solved other problems with beta decay. The creation of the anti neutrino balances the creation of an electron, in the same way that an anti-electron balances an electron in pair creation. Neutrinos and electrons are assigned to the same particle family, called leptons, and all interactions (as far as is known) conserve lepton number. This works by giving electrons and neutrinos lepton number 1 and the antiparticles (positrons and anti neutrinos) lepton number -1. Perhaps you begin to see more clearly why anti matter is a theoretical necessity. There is more to conserve than just electric charge.
Consideration like these define the properties of an anti neutrino:
- 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.