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.

Types of neutrinos

There are three different kinds, or flavors, of neutrinos: the electron neutrino ν_e, the muon neutrino ν_μ and the tau neutrino ν_τ, named after their partner lepton in the Standard Model (see table at right). In a phenomenon known as neutrino oscillation neutrinos spontaneously mutate among the three flavors. Whereas the more common view holds, that there are separate neutrinos and antineutrinos, it cannot be ruled out that these particles are actually the same.

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 M. Lederman, Melvin Schwartz and Jack Steinberger found out that more than one type 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 (per neutrino), 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 massive neutrinos can change type whereas massless neutrinos cannot. This phenomenon, 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

Atmospheric neutrinos

Atmospheric neutrinos result from the interaction of cosmic rays with atoms within Earth's atmosphere, creating showers of 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 phenomena

Neutrinos are an important product of supernovas. Most of the energy produced in supernovas is radiated away in the form of an immense 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 g/cm³) that interaction between the produced neutrinos and surrounding stellar matter becomes significant. It is thought that neutrinos would also be produced from other events such as the collision of neutron stars.

Because neutrinos interact so little with matter, it is thought that a supernova's neutrino emissions carry information about the innermost regions of the explosion. Much of the visible light comes from the decay of radioactive elements produced by the supernova shock wave, and even light from the explosion itself is scattered by dense and turbulent gases. Neutrinos, on the other hand, pass through these gases, providing information about the supernova core (where the densities were large enough to influence the neutrino signal). Furthermore, the neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays or radio waves. The exact time delay is unknown, but for a Type II supernova, astronomers expect the neutrino flood to be released seconds after the stellar core collapse, while the first electromagnetic signal may be hours or days later. The SNEWS (http://snews.bnl.gov) project uses a network of neutrino detectors to monitor the sky for candidate supernova events; it is hoped that the neutrino signal will provide a useful advance warning of an exploding star.

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.

• In 1953 the first neutrino detection device was used to detect neutrinos near a nuclear reactor. Reines and Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Neutrino interactions with protons of the water produced positrons. The resulting positron annihilations with electrons created photons with an energy of about 0.5 MeV. Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about 8 MeV that were detected a few microseconds after the photons from a positron annihilation event.
• Chlorine detectors consist of a tank filled with carbon tetrachloride. In these detectors a neutrino would convert a chlorine atom into one of argon. The fluid would periodically be purged with helium gas which would remove the argon. The helium would then be cooled to separate out the argon. These detectors had the failing that it was impossible to determine the direction of the incoming neutrino. It was the chlorine detector in Homestake, South Dakota, containing 520 tons of fluid, which first detected the deficit of neutrinos from the sun that led to the solar neutrino problem. This type of detector is only sensitive to ν_e.
• Gallium detectors are similar to chlorine detectors but more sensitive to low-energy neutrinos. A neutrino would convert gallium to germanium which could then be chemically detected. Again, this type of detector provides no information on the direction of the neutrino.
• Pure water detectors such as Super-Kamiokande contain a large area of pure water surrounded by sensitive light detectors known as photomultiplier tubes. In this detector, the neutrino transfers its energy to an electron which then travels faster than the speed of light in the medium (though slower than the speed of light in a vacuum). This generates an "optical shockwave" known as Cherenkov radiation which can be detected by the photomultiplier tubes. This detector has the advantage that the neutrino is recorded as soon as it enters the detector, and information about the direction of the neutrino can be gathered. It was this type of detector that recorded the neutrino burst from Supernova 1987a. This type of detector is sensitive to ν_e and ν_μ.
• Heavy water detectors use three types of reactions to detect the neutrino. The first is the same reaction as pure water detectors. The second involves the neutrino striking the deuterium atom releasing an electron. The third involves the neutrino breaking the deuterium atom into two. The results of these reactions can be detected by photomultiplier tubes. This type of detector is in operation in the Sudbury Neutrino Observatory (SNO). This type of detector is sensitive to all three neutrino flavors.
• Tracking calorimeters such as the MINOS detectors - see the NuMI-MINOS (http://www-numi.fnal.gov) project page - use alternating planes of a passive absorber material to provide detector mass and active detector planes to detect the charged particles produced by a neutrino interaction. Steel is a popular choice, being relatively dense and inexpensive, and having the advantage that it can be magnetised. The Nova proposal suggests the use of particle board as a cheap way of getting a large amount of less dense mass. The active detector is often liquid or plastic scintillator, read out with photomultiplier tubes, although various kinds of ionisation chamber have also been used. Neutrinos interact in the passive absorber either via the Neutral Current interaction, producing a hadronic shower in the detector, or via the Charged Current interaction, producing their partner charged lepton. A muon produces a long penetrating track, and is easy to spot; measurement of its range or curvature in the magnetic field will give its momentum. Electrons produce an electromagnetic shower, which is different in shape from a hadronic shower; the two kinds of showers can be separated if the granularity of the active detector is small compared to the size of the shower. Tau leptons decay essentially immediately to either pions or another charged lepton, and can't be observed directly in this kind of detector. To directly observe taus, one typically looks for a kink in tracks in photographic emulsion.

Neutrino Detectors

• Antarctic Muon And Neutrino Detector Array
• Project DUMAND
• Super-Kamiokande

See also

• neutrino astronomy
• solar neutrino problem
• particle physics
• list of particles

External links

• OMNIS: (http://www.wipp.ws/science/SNovae/SNovae.htm) Observatory for Multi-flavor NeutrInos from Supernovae (Suggested experiment at WIPP[1] (http://www.wipp.ws/))
• Ultimate neutrino page (http://cupp.oulu.fi/neutrino)
• Super-Kamiokande neutrino detector finds neutrino mass (http://physicsweb.org/articles/news/2/6/2/1)