Leptons

We now know that neutrinos are emitted in the beta decay process because of another conservation law, the conservation of leptons. The leptons are a family of light particles that include the electron and the neutrino. When an electron is created, an anti neutrino is also created so that the number of leptons does not change.
Actually there are three distinct conservation laws for leptons. The lepton family consists of six particles, the electron, two more particles with rest mass and three different kinds of neutrino. The other massive particles are the muon which is 207 times as massive as the electron, and the recently discovered tau particle which is 3490 times heavier. The three kinds of neutrino are the electron type neutrino, the muon type neutrino and the tau type neutrino. The names come from the fact that each type of particle is separately conserved. For example when a neutron decays into a proton and an electron is created, it is an anti electron type neutrino that is created at the same time to conserve electron type particles.
In the other common beta decay process, where a proton turns into a neutron, a positron is created to conserve electric charge. Since the positron is the anti particle of the electron, its opposite, the electron type neutrino, must be created to conserve leptons.