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 emit during a certain type of [Radioactivity|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 have 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 the what physicists currently know about neutrinos, and to some extent, how they came to that knowledge, its implications regarding the nature of atoms, and its applications for discovery and for human use.

Introduction

The neutrinos appear to constitute by number of species not less than one quarter of the particles which make the world, and even half of the stable ones. By number of particles in the Universe they are perhaps second only to photons.   —Ivan V. Aniĉin, 2005

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.

Of all the things that make the universe, the commonest and weirdest are neutrinos. Able to travel through the earth like a bullet through a bank of fog, they are so shy that half a century after their discovery we still know less about them than all the other varieties of matter that have ever been seen.   —Frank Close, 2010

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' — electron energy too small — 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.