Neutrino

Based on theoretical considerations concerning events that occur spontaneously in the nuclei of atoms, in 1930, the physicist, Wolfgang Pauli (1900-1958), postulated a new fundamental particle of matter, subsequently (1935) named by the physicist, Enrico Fermi (1901-1954), the neutrino ("little neutral one", in Fermi's Italian), an electrically uncharged particle associated with the negatively electrically charged particle, the electron, specifically the electron that certain unstable atoms' nuclei create and emit during a certain type of radioactive emission called beta decay, which occurs spontaneously in certain varieties (isotopes) of certain of Earth's naturally-occurring chemical elements. Initially the electron-accompanying neutrino was presumed to have no mass.

Physicists subsequently confirmed the existence of the 'electron neutrino' and provided evidence that it has a "vanishingly small" but finite mass. Physicists have also provided evidence of two additional types, or 'flavors', of neutrinos, the muon neutrino and the tau neutrino, and evidence that an antineutrino exists for each of the three flavors of neutrinos.

This article will discuss what physicists currently know about neutrinos, and to some extent, how they came to that knowledge, its implications regarding the nature of atoms and the universe, and its applications for discovery and for human use.

Introduction
At the time of Pauli's postulated existence of the neutrino, physicists had already discovered that atoms, previously thought of as homogeneous and indivisible, consisted of sub-particles, called subatomic particles, specifically, protons and neutrons localized in a center-of-the-atom nucleus, the major location of the atoms' mass, and electrons, by comparison tiny, surrounding the nucleus. The protons each carried a unit of positive electrical charge and the electrons, equal in number to the number of protons, each carried a unit of negative electrical charge, rendering the atom as a whole electrically neutral, inasmuch as the neutron itself carried no electrical charge. They had also discovered that the some atoms were unstable, in that their nuclei might emit one or more subatomic particles, a process called radioactivity...

Basing his thinking on the widely accepted law of conservation of energy, Pauli postulated the existence of an electron-associated, presumably massless, neutrino, postulated in order to reconcile the observed discrepancy between the energy carried by an electron emitted from the nucleus during the radioactive process called 'beta decay' &mdash; electron energy too small &mdash; and the energy change of the nucleus itself, the missing energy, the energy lost by the nucleus during the beta decay process, carried off by the postulated chargeless neutrino particle.