Neutrinos as Elementary Particles

Neutrinos are created in nuclear processes and in various elementary particle interactions. The most familiar process is nuclear beta-decay, in which an unstable nucleus simultaneously emits an electron (beta-ray) and a neutrino. This process may be visualized as an unstable nucleus radiating its energy by creating a pair of leptons a neutrino and an electron. It is referred to as beta-minus decay when an electron (e ) is emitted with an antineutrino Ve) or beta-plus decay when a positron (e+) is emitted with a neutrino (Vg). In another beta-decay process, called electron capture, one of the orbital electrons in an atom is absorbed by the nucleus and a neutrino is emitted. Examples of these processes are 

In these examples, the guiding principle is the principle of lepton number conservation. In any process, the total number of leptons and antileptons does not change the number before and after is conserved. Table I lists the assigned lepton numbers for each lepton family. The lepton number is positive for a lepton and negative for an antilepton. By applying the lepton numbers for the electron family, the principle of lepton conservation is exhibited in the three examples given. 

The principle of lepton conservation applies to each of the three families, and in addition, the leptons of each family appear to be separately conserved. It has been shown, for example, that when a neutrino of the muon type (v ) is absorbed in a nucleus, a muon is emitted, never an electron, for example, 

It was by this process that the muon neutrino was discovered and shown to be a different particle from the electron neutrino. The principle of lepton conservation for each lepton type (or flavor) has been tested experimentally in many different ways and appears to be valid, but it is possible that separate lepton conservation is not rigorously true and additional tests are still needed. A superior way of testing these lepton properties is by studying the neutrino spectrum from the sun and cosmic rays. This topic will be discussed further in Section n. 

Neutrinos may be absorbed in nuclei with the emission of an electron, a muon, or a tauon, depending on the incident neutrino type. These are called inverse-beta-decay processes because they are, in the case of the electron neutrino, the inverse of normal radioactive beta-decay. Neu- 

---