Neutrino Nomads
About Neutrinos
The neutrino is a ghostly particle that leaves hardly a trace of its passing. Most neutrinos go right through Earth without any deviation or interaction, and trillions harmlessly pierce your body each second. The table shows some neutrino properties.
Neutrinos were hypothesized in 1931 by Wolfgang Pauli to resolve a crisis in physics that threatened the bedrock principle of the conservation of energy. In a spontaneous nuclear reaction called beta-decay, one nucleus was observed to decay into an electron and another nucleus with one more unit of charge, as shown in the diagram. In this kind of reaction, conservation of energy demands that each decay product be created with a unique energy. But much to their dismay, physicists found the electron had a wide range of energies that could not be explained if the electron was the only particle emitted. Some physicists were so distressed at this result that they were willing to abandon the conservation of energy in this case. To save the day, Pauli hypothesized that the nucleus emitted a second particle that could carry away this unaccounted-for energy. Since the charges of the extra proton and the emitted electron cancel, he concluded the neutrino was neutral. Since no second particle had been detected, he concluded that the particle had no mass and no charge, since it could hardly interact with anything at all. Pauli had replaced one dilemma with another, by explaining a paradoxical experiment result with a particle that could not be observed.
There it stood for 25 years. But advances in nuclear physics and the development of nuclear reactors made possible new experiments, and two physicists, Frederick Reines and Clyde Cowan, set out to find neutrinos emitted by a reactor core. They succeeded in 1956, by observing a nuclear reaction initiated by the absorption of a neutrino, essentially the reverse of beta-decay. Their work established neutrino physics as a legitimate field, and Reines won the Nobel prize in 1995. (Cowan died in 1974.)
Where do neutrinos come from? Most neutrinos that reach Earth—a huge number, some tens of billions per square centimeter—come from nuclear reactions in the sun. Once physicists had detected solar neutrinos, a second great neutrino mystery developed in the 1970s, because the number observed was only about a third of what was expected. Only around the turn of the 21st century did physicists begin to make progress in solving this problem. (See Neutrino Astrophysics)
