neutrino

The Columbia Encyclopedia, 6th ed.
Save to active project

neutrino

neutrino (nōōtrē´nō) [Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy during beta decay (see conservation laws; radioactivity). When a radioactive nucleus emits a beta particle (electron), the electron may have any energy from zero up to a certain maximum. Pauli suggested that when the electron has less than the maximum possible value, the remaining energy is carried away by an undetected particle, the neutrino. Its charge must be zero because a charged particle would easily be detected. Moreover, if it were charged, the law of conservation of charge would be violated during beta decay. The neutrino was named by Enrico Fermi. Further studies showed that the neutrino was also necessary to maintain the conservation laws of momentum and spin. Like the electron, the neutrino is a lepton; it participates only in the weak decay of nuclear particles and has no role in the strong force binding nuclei together. Neutrinos are also emitted when a pion decays into a muon and in the decays of a number of other elementary particles. Neutrinos are stable and can be absorbed only by the same weak interactions through which they are created; an energetic neutrino can induce the reverse of the decay that produced it.

The neutrino was not detected directly until 1956, when American physicists Frederick Reines and Clyde L. Cowan recognized them by their impact with subnuclear particles in mineral water. In 1962 it was found that the neutrino associated with the muon (the muon neutrino) is distinct from that associated with the electron (the electron neutrino). A third type, the tau neutrino, associated with the tau particle, was identiified in the mid-1970s but not detected until 2000. Each type of neutrino has its own antiparticle.

According to the so-called oscillation theory, neutrinos can change from one type to another as they travel through space; in order to make these transformations, neutrinos have to have a tiny amount of mass and not be massless, as was originally theorized. Beginning in the late 1960s a number of experiments designed to detect neutrinos failed to produce the expected results when fewer than expected neutrinos were detected, a result that could be explained by the conversion of the type (or flavor) of neutrino the experiments were trying to detect into another type, a process known as flavor oscillation. In 1995 and again in 1996 a team at the Los Alamos National Laboratory claimed to have detected the oscillation of muon antineutrinos into electron antineutrinos, and in 1998 the participants in the Super-Kamiokande experiment in Japan, which examined neutrinos produced by the interaction of cosmic rays with the upper atmosphere, announced that they had discovered evidence that neutrinos oscillate and must have mass. In 2001 researchers at the Sudbury Neutron Observatory in Ontario, Canada, found evidence that the electron neutrinos produced by fusion reactions within the sun can change into tau and muon neutrinos as they travel to the earth. Additional work by Fermilab in Illinois and Minnesota confirmed (2006) that neutrinos have mass. This is significant because of its implications for the composition and evolution of the universe, including the rate of the universe's expansion. Neutrinos would exert gravitational effects and thus could account for some of the dark matter in the universe.

See also neutrino astronomy.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
Loading One moment ...
Project items
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

Cited article

neutrino
Settings

Settings

Typeface
Text size Smaller Larger
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

While we understand printed pages are helpful to our users, this limitation is necessary to help protect our publishers' copyrighted material and prevent its unlawful distribution. We are sorry for any inconvenience.
Full screen

matching results for page

Cited passage

Style
Citations are available only to our active members.
Sign up now to cite pages or passages in MLA, APA and Chicago citation styles.

Cited passage

Welcome to the new Questia Reader

The Questia Reader has been updated to provide you with an even better online reading experience.  It is now 100% Responsive, which means you can read our books and articles on any sized device you wish.  All of your favorite tools like notes, highlights, and citations are still here, but the way you select text has been updated to be easier to use, especially on touchscreen devices.  Here's how:

1. Click or tap the first word you want to select.
2. Click or tap the last word you want to select.

OK, got it!

Thanks for trying Questia!

Please continue trying out our research tools, but please note, full functionality is available only to our active members.

Your work will be lost once you leave this Web page.

For full access in an ad-free environment, sign up now for a FREE, 1-day trial.

Already a member? Log in now.

Are you sure you want to delete this highlight?