Neutrino

**Neutrinos** are a type of elementary particle that exist all across the universe. The word _neutrino_ means a small neutral particle. Physicists study these particles, but they are hard to find because they have a very small chance of interacting with regular matter. (For example, they usually pass through the whole earth without touching any other particles). Neutrinos travel near the speed of light.

Neutrinos are very difficult to detect. They are very unlikely to collide (interact) with other particles as they travel through space or through matter. Electrons have an electric charge, but neutrinos do not.  This means that neutrinos are unaffected by the electromagnetic force. The first detectors built to find neutrinos observed only 10 or 15 each year. Recent detectors use tanks that contain about a thousand tons of water or other liquid, which lets them detect about 10 to 100 neutrinos per day.

Neutrinos are generated in particle accelerators, in the sun, in other stars, and in nuclear reactions such as in nuclear reactors. They are generated whenever there is a nuclear reaction in the form of beta decay. This process starts off with one neutron, and ends with one electron, one proton, and one neutrino.

We used to think that neutrinos have no mass, but a few years ago physicists found that they have a very small mass, much lighter than electrons. By finding neutrinos, we can learn about the structure and the history of the universe. Since most of them pass easily through sun, physicists have learned about the reactions in the center of the sun that make the sun's heat, by detecting the neutrinos that come out from the sun's center.

The three types of neutrinos are named after the three leptons that have electric charge. There is the electron neutrino (v<sub>e</sub>), the muon neutrino (v<sub>µ</sub>), and the tau neutrino (v<sub>τ</sub>). Each neutrino has an antiparticle, called an antineutrino. Therefore, there is an electron antineutrino, a muon antineutrino, and tau antineutrino.

The three types of neutrinos change into each other over time, so an electron neutrino could turn into a muon or tau neutrino and then back again. This is called neutrino oscillation. This oscillation was suggested when then number of electron neutrinos from the Sun was measured in the 1960s, and the amount was only about a third of what theories at the time said there should be. The result of neutrino oscillation is that most of the electron neutrinos made in the Sun change to another type of neutrino.

Most neutrinos passing through the planet Earth come from the Sun. About 65 billion () solar neutrinos per second pass through every square centimeter of area, including our own bodies.
This is true even on the night side of Earth, because nearly all the neutrinos from the sun pass easily through the Earth.

## Related pages

- Antares Telescope - uses neutrinos to make images

## Infobox (particle)

- **name**: Neutrino/Antineutrino
- **image**:
- **caption**: The first use of a hydrogen bubble chamber to detect neutrinos, on November 13, 1970. A neutrino hit a proton in a hydrogen atom. The collision occurred at the point where three tracks emanate on the right of the photograph.
- **num types**: 3 – electron neutrino, muon neutrino and tau neutrino
- **composition**: Elementary particle
- **statistics**: Fermion
- **group**: Lepton, antilepton
- **generation**: First, second and third
- **interaction**: Weak interaction and gravitation
- **antiparticle**: Antineutrinos are possibly identical to the neutrino (see _Majorana fermion_).
- **theorized**: (Electron neutrino): Wolfgang Pauli (1930)
 (Muon neutrino): Late 1940s
 (Tau neutrino): Mid 1970s
- **discovered**: : Clyde Cowan, Frederick Reines (1956)
: Leon Lederman, Melvin Schwartz and Jack Steinberger (1962)
: DONUT, DONUT collaboration (2000)
- **symbol**: , ,
- **mass**: Small, but non-zero.
- **electric charge**: 0 elementary charge, e
- **x charge**: −3
- **weak hypercharge**: −1
- **b-l**: −1