NEUTRINO, DISCOVERY OF

The particle called the neutrino was conceived in 1930 by the Austrian-Swiss theoretical physicist Wolfgang Pauli (1900–1958) as a possible solution to two vexing problems confronting a widely accepted model of the structure of the atomic nucleus, which used the two elementary constituents of matter then known: the electron and the proton. The neutral atom of mass number A and atomic number Z was supposed to contain in its nucleus A protons and A Z electrons; Z electrons made up the shells of the atom. This picture seemed reasonable because protons were knocked out of light nuclei by alpha particles from radioactive decay, and, while in the beta-decay form of radioactivity, electrons emerged from the nucleus.

However, there were puzzles concerning the nuclear electrons. Beta decay causes the positive nuclear charge Z to increase by one unit and decreases the energy of the nucleus by a definite amount; but, the electron emerges with varying (lesser) amounts of energy, so that a part of the energy loss of the nucleus is unaccounted for. Another puzzle was related to a property known as spin angular momentum (by analogy with a spinning top). Electrons and protons each have spin of ½ (in units h /2π, where h is Planck's quantum of action). In those nuclei where the total number of nuclear particles is odd, such as the common element nitrogen (₇N¹⁴), the total nuclear angular momentum should be half an odd integer. However, in the case of ₇N¹⁴, it was shown to be the integer one. Nevertheless, most physicists accepted the electron-proton model, even though it contradicted the well-known laws of conservation of energy and angular momentum, believing that different physical laws might hold within the tiny space of the nucleus. Indeed, physicists had recently learned that the new puzzling laws of quantum mechanics ruled within the atom. The influential atomic physicist Niels Bohr believed that the law of conservation of energy held only in a statistical sense, like the law of increase of entropy in statistical mechanics.

The Idea of the Neutrino

Pauli, who was unwilling to give up the conservation laws, conjectured the existence of a new particle in order to solve the two difficulties mentioned. This was a neutral particle of spin ½ with a mass "not larger than 0.01 proton mass," as Pauli suggested in a famous letter sent on December 4, 1930, to nuclear physicists who were holding a meeting in Tübingen, Germany. He proposed that each electron in the nucleus was accompanied by one of the new particles, which he provisionally named neutrons. This solved the problem of ₇N¹⁴ and analogous cases. When a nucleus underwent beta decay, a neutron would emerge with each electron, carrying away the energy that appeared to be lost. Pauli's particle would have been almost undetectable.

There Pauli let the matter rest, presenting his idea publicly in October 1933 at an international conference held in Brussels. He renamed his particle the neutrino, following a suggestion by the Italian Enrico Fermi. The nuclear particle that we now call the neutron was discovered in 1932 by the Englishman James Chadwick.

Fermi's Theory of Beta Decay

A Russian, Dmitri Iwanenko, suggested earlier that Chadwick's neutron was a kind of neutral proton— that is, a massive elementary particle of spin ½—and that the nucleus contains no electrons, only neutrons and protons. He also proposed that an electron and a neutrino are created together in the process of beta decay, much as a photon is created in an ordinary atomic transition.

Shortly after the Brussels conference, in 1933, Fermi put forth a quantum field theory of beta decay. It used the relativistic theory of Paul Dirac, which provides the possibility of creation and annihilation of particles in pairs. In Dirac's theory, the electron is accompanied by a matching positive particle of the same mass and spin, called the positron. In Fermi's theory, the spin-half neutrino also has an analogous partner called the antineutrino. The beta decay of neutrino-rich nuclei produces an electron-antineutrino pair, while the beta decay of proton-rich nuclei produces a positron-neutrino pair. With some important later modifications, Fermi's theory (when generalized) forms a part of the modern electroweak theory that unifies electromagnetism with the weak nuclear interaction, of which beta decay is one example.

As a result of Chadwick's discovery of the neutron and the success of Fermi's theory of beta decay, nuclear electrons were soon rejected and other nuclear models took their place. By 1936, Bohr also agreed that the conservation laws were valid in each individual nuclear event.

Detection of the Neutrino

The neutrino was theoretically indispensable, but it was necessary to detect it directly. This formidable task took two decades to accomplish, since the neutrino can pass through light-years of matter without interacting. It was first observed in 1956 by a group led by Clyde L. Cowan and Frederick Reines of Los Alamos National Laboratory, who used the enormous flux of antineutrinos from a nuclear re-actor at the Savannah River Plant in South Carolina, using a "target" consisting of cadmium chloride dissolved in water, surrounded by large detectors filled with a liquid scintillator. They detected the nuclear reaction known as inverse beta decay, in which a proton captures an antineutrino. In this process, a neutron and a positron result; the capture of these particles produces characteristic flashes of light in the scintillator. In 1995, Frederick Reines was awarded the Nobel Prize in Physics for the discovery of the neutrino. (Clyde Cowan died in 1974.)

Additional Neutrinos

In 1962 at Brookhaven National Laboratory on Long Island, New York, Leon M. Lederman, Melvin Schwartz, and Jack Steinberger used a very large apparatus, consisting of spark chambers and scintillators, to detect a second type of neutrino, whose existence theory had suggested. This new neutrino is produced together with another elementary particle called the muon in the decay of an elementary particle, the pion. The decay process, first observed in the cosmic rays in 1947, takes one-hundred millionth of a second. The pion belongs to the class of particles with strong nuclear interaction called hadrons, and they were produced copiously at Brookhaven in a proton accelerator called the Cosmotron. The muon is a lepton, the general name for particles whose nuclear interaction is weak (such as the electron and the neutrino). The muon also decays, in about a microsecond, into an electron and two different neutrinos, the electron neutrino (Pauli's original neutrino) and the muon neutrino. A third kind, the tau neutrino, corresponds to a third charged lepton, the tau, discovered in 1975 by a group at the Stanford Linear Accelerator Center led by Martin Perl.

The mass of the electron neutrino is very small and was for a long time believed to be zero. It is now thought that neutrinos have mass, but the masses are not well determined. Upper limits are listed as about0.01, 0.5, and 40 electron masses, respectively, for the electron, muon, and tau types of neutrinos.

Neutrinos in High-Energy Physics and Astrophysics

Beta decay and its inverse play an essential role in the nuclear reaction cycles that produce energy in stars; and neutrinos, with their high degree of penetration, carry information to scientists on earth about processes occurring deep in stellar interiors. Neutrinos from the sun have been monitored by Raymond Davis Jr. of Brookhaven National Laboratory and his collaborators for more than a quarter-century. They used a large tank of cleaning fluid (C₂Cl₄) located 1,500 meters underground to reduce charged cosmic ray background. Neutrinos absorbed by chlorine nuclei convert chlorine to argon at a measurable rate (about one atom per day). The number of solar neutrinos detected this way has been smaller than theoretically expected; the reason for this is an open question. Other large underground installations are used for detecting very high-energy neutrinos from outer space (neutrino astronomy). In 1987, two of these detectors observed neutrinos produced by a supernova in the skies of the Southern Hemisphere.

The number of solar neutrinos observed in underground detectors is about one-third to one-half the number expected on theoretical grounds for massless neutrinos. However, if neutrinos have nonzero mass, a phenomenon known as neutrino oscillation is expected, in which one type of neutrino transforms into another. This process could account for the missing electron neutrinos from the sun. Some experiments performed at the Japanese underground neutrino detector Super-Kamiokonde look at muon neutrinos produced in the earth's atmosphere by cosmic rays. These experiments suggest that oscillation occurs and thus that neutrinos have mass.

After it was shown in the Brookhaven experiment that found the second neutrino that high-energy neutrinos from large particle accelerators can be detected, neutrino beams were used as effective probes of the proton and neutron, supplementing the use of high-energy electron beams. Both types of beams, produce simpler, more easily interpretable interactions than beams of hadrons. Neutrino experiments carried out at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, and at Fermilab in Batavia, Illinois, beginning in 1972, formed the basis for the electroweak theory. Other neutrino experiments have helped to establish the color quark model, the other sector of the Standard Model of elementary particle interactions which has dominated the theory for the past two decades. Accelerator experiments are in preparation for testing neutrino oscillation.

See also:Fermi, Enrico; Pauli, Wolfgang; Reines, Frederick

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Laurie M. Brown