What Is a Neutrino?
Elusive Very Low Mass Subatomic Fundamental Particle
Neutrinos are among the most elusive of elementary particles. They were not detected until about 25 years after they were first suggested to exist.
Prediction of Neutrinos
The laws of conservation of energy and conservation of momentum state that in a closed system with no outside influences the total energy and momentum must remain constant. These conservation laws have no known exceptions. Physicists consider these laws sacrosanct.
In the 1920s beta decay experiments seemed to violate these laws. Beta decay is a nuclear reaction in which neutrons emit electrons, which are sometimes called beta particles, and become protons. Physicists measured the total momentum, energy, and mass of the initial neutrons and of the protons and electrons after the beta decay reactions. The total mass of the protons and electrons after the decay equaled the mass of the neutrons before the decay, to within the experimental errors. Physicists therefore thought that there were no missing fundamental particles.
There was however a problem. The total momentum and energy after the decay were both less than before the beta decay reactions. What happened to the momentum and energy?
Physicists were faced with a hard choice. Either the laws of conservation of energy and momentum were wrong, or there was a massless elementary particle that had the missing momentum and energy.
Wolfgang Pauli timidly suggested the latter in 1930 and finally dared publish the idea in 1933. Pauli called these fundamental particles neutrinos, meaning little neutral ones. These neutrinos apparently had zero mass, yet plenty of energy and momentum. They were also very small in what physicists call "cross section" and did not react with anything easily.
Enrico Fermi, in 1934, theoretically described beta decay using Pauli's neutrino idea. Fermi's theory worked so well that physicists adopted the neutrino hypothesis. Physicists preferred these unlikely sounding elementary particles to thinking that the conservation laws had been violated. However at the time no experiments to discover neutrinos were possible.
Discovery of Neutrinos
Finding these little neutral ones proved very difficult. To understand why, consider an analogy.
Throwing a volleyball through a wire fence with ping-pong ball size holes will not work. A ping-pong ball might get through if it is aimed exactly right; BBs will almost always get through. The key is the cross-sectional area of the ball. The smaller it is relative to the holes, the less likely it is to interact with the fence.
While it's not exactly the same, subatomic fundamental particles also have an effective cross-sectional area. The smaller it is the less likely the particle is to interact with matter. The effective cross-section of a neutrino is about 10E-40 square centimeters. Only about one in a trillion neutrinos is absorbed by passing through the Earth. About one in three will be absorbed by a 100 light years of lead. So few neutrinos will interact with any particle detector that they are very difficult to detect.
Frederick Reines and Clyde L. Cowan finally proved that neutrinos really do exist in 1956. They detected neutrinos produced by the nuclear reactions at the Savannah River Reactor. Reines shared the 1995 Nobel prize in physics for their discovery, but Cowan died before the prize was awarded.
Neutrino Mass
Physicists originally thought that neutrinos were massless fundamental particles. Most physicists now think that neutrinos have a nonzero mass that is still too small to measure. The best current experiments suggest that if neutrinos do have mass it is less that about 1/250,000 times the mass of the electron.
Further Reading
Bilenky, S.M. et al., "Absolute Values of Neutrino Masses: Status and Prospects" Physical Review vol 379, p 69-148, 2003.
Heckert, P.A., "Neutrino" Gale Encyclopedia of Science, Gale Research, 1995.
