The Neutrino reference article from the English Wikipedia on 24-Jul-2004
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Neutrino

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The neutrino is an elementary particle. It has spin 1/2 and so it is a fermion. Its mass is very small, although recent experiments (see Super-Kamiokande) have shown it to be above zero. It feels neither the strong nor the electromagnetic force, so it only interacts through the weak force and gravitation.

Because the neutrino only interacts weakly, when moving through ordinary matter its chance of interacting with it is very small. It would take a light year of lead to block half the neutrinos flowing through it. Neutrino detectors therefore typically contain hundreds of tons of a material constructed so that a few atoms per day would interact with the incoming neutrinos.

Table of contents
1 Types of neutrinos
2 History
3 Mass
4 Neutrino Sources
5 Neutrino detectors
6 See also
7 External link

Types of neutrinos

   
   

   
   

   
   

   
   

   
   

   
   

   
   

Left handed neutrinos
in the Standard Model
Fermion Symbol Mass**
Generation 1 (electron)
Electron neutrino < 50 eV
Electron antineutrino < 50 eV
Generation 2 (muon)
Muon neutrino < 0.5 MeV
Muon antineutrino < 0.5 MeV
Generation 3 (tau)
Tau neutrino < 70 MeV
Tau antineutrino < 70 MeV

There are three different kinds, or flavors, of neutrinos: the electron neutrino νe, the muon neutrino νμ and the tau neutrino ντ, named after their partner lepton in the Standard Model (see table at right). In a phenomenon known as neutrino oscillation neutrinos spontaneously mutate between the three flavors.

History

The neutrino was first postulated in 1931 by Wolfgang Pauli to explain the continuous spectrum of beta decay, the decay of a neutron into a proton and an electron. Pauli theorized that an undetected particle was carrying away the observed difference between the energy and angular momentum of the initial and final particles. Because of their "ghostly" properties, the first experimental detection of neutrinos had to wait until about 25 years after they were first discussed. In 1956 Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published the article "Detection of the Free Neutrino: a Confirmation" in Science (see neutrino experiment), a result that was rewarded with the 1995 Nobel Prize. The name neutrino was coined by Enrico Fermi as a word play on neutrone, the Italian name of the neutron particle. (Neutrone in Italian also means big and neutral, and neutrino means small and neutral.) In 1962 Leon Max Lederman, Melvin Schwartz and Jack Steinberger find out, that not only one types of neutrino exists.

Mass

The basic standard model of particle physics assumes that the neutrino is massless, although adding massive neutrinos to the basic framework is not difficult, and recent experiments suggest that the neutrino has a small although non-zero mass.

The strongest upper limits on the mass of the neutrino come from cosmology. The big bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total mass of all three types of neutrinos exceeded 50 electron volts, there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable, however there are limits within the standard model that make this difficult.

However, it is now widely believed that the mass of the neutrino is non-zero. When one extends the standard model to include neutrino masses, one finds that the prediction that massive neutrinos can change type whereas massless neutrinos cannot. This phenonemnon known as neutrino oscillation explains why there are many fewer electron neutrinos observed from the sun and the upper atmosphere than expected, and has also been directly observed.


Neutrino Sources

Human generated

Nuclear power stations are the major source of human generated neutrinos. An average plant may generate over 50,000 neutrinos per second. Particle accelerators are another source.

The Earth

Neutrinos are produced as a result of the natural background radiation from radioactive atomic nuclei within the Earth.

Atmospheric neutrinos

Atmospheric neutrinos result from the interaction of cosmic rays with atoms withn the Earth's atmosphere, creating showers of a particles including neutrinos.

Solar neutrinos

Solar neutrinos originate from the nuclear fusion powering the Sun and other stars.


Raymond Davis Jr and Masatoshi Koshiba were jointly awarded the 2002 Nobel Prize in Physics for their work in the detection of cosmic neutrinos.

Cosmological phemomena

Neutrinos are an important product of supernovas. Most of the energy produced in supernovas is radiated away in the form of an inmense burst of neutrinos, which are produced when protons and electrons in the core combine to form neutrons. The first experimental evidence of this phenomenon came in the year 1987, when neutrinos coming from the supernova 1987a were detected. In such events, the densities at the core becomes so high (1014 gram/cm3) that interaction between the produced neutrinos and surrounding stellar matter becomes significant. It's thought that neutrinos would also be produced from other events such as the collision of neutron stars.

Cosmic background radiation

It is thought that the cosmic background radiation left over from the Big Bang includes a background of low energy neutrinos. In the 1980s it was proposed that these may be the explanation for the dark matter thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: we know they exist. However, they also have serious problems. From particle experiments, it is known that neutrinos tend to be hot, i.e. move at speeds close to the speed of light—hence this scenario was also known as hot dark matter. The problem is that being hot and fast moving, the neutrinos would tend to spread out evenly in the universe. This would tend to cause matter to be smeared out and prevent the large galactic structures that we see.

Neutrino detectors

There are several types of neutrino detectors. Those used to detect stellar neutrinos consist of a large amount of material in an underground cave designed to shield it from cosmic radiation.

See also

External link

Particles in Physics - Elementary particles - Leptons Edit
Particles: Electron | Muon | Tauon | Electron neutrino | Muon neutrino | Tau neutrino
Antiparticles : Positron | Antimuon | Antitauon | Electron antineutrino | Muon antineutrino | Tau antineutrino