The neutrino is an elementary particle. It has fractional spin and is therefore a fermion. Its mass is very small compared to most other particles, although recent experiments (see Super-Kamiokande and Sudbury Neutrino Observatory) have shown it to be nonzero. Since it is an electrically neutral lepton, the neutrino interacts neither by way of the strong nor the electromagnetic force, but only through the weak force and gravitation.
Due to the fact that the cross section in weak nuclear interactions is very small, neutrinos can pass through ordinary matter almost unhindered. For typical neutrinos produced in the sun (energy of a few MeV), it would take approximately one light year (~1016m) of lead to block half of them. Detection of neutrinos is therefore very challenging as it has to rely either on large detection volumes or artificially produced neutrino beams of high intensity and energy, since the interaction probability is roughly proportional to the energy of the neutrino.
There are three known types (flavors) of neutrinos: electron neutrino, muon neutrino, and tau neutrino, named after their partner leptons in the Standard Model. The current best limit on the number of neutrino types has been set by observing the decay of the Z boson. This particle can decay into any neutrino and its antineutrino, and therefore its lifetime will depend on the number of different neutrinos because it will be the shorter the more possibilities there are to decay.
The latest measurements show that the number of light neutrino types (mass < 1MeV) is 2.984+0.008[. This does not, however, exclude the possibility of a sterile neutrino, i.e. one that does not even interact by way of the weak nuclear force. Such a neutrino could only be created through flavor oscillation. The correspondence between the six - currently known - quarks in the Standard Model and the six leptons, among them the three neutrinos, provides additional evidence that there should be exactly three types. However, conclusive proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
Neutrinos are always created or detected in a well defined flavor (electron, muon, tau). However, in a phenomenon known as neutrino flavor oscillation, neutrinos are able to oscillate between the three available flavors while they propagate through space. Specifically, this occurs because the neutrino flavor eigenstates are not eigenstates of the propagation Hamiltonian. This allows for a neutrino that was produced as an electron neutrino at a given location to have a calculable probability to be detected as either a muon or tau neutrino after it has traveled to another location. The existance of Flavor Oscillations implies a finite neutrino mass, since neutrinos with zero mass could not exhibit observable oscillation. Also, there is some indication that neutrino and antineutrino are in fact the same particle, a hypothesis first proposed by the Italian physicist E. Majorana.
The neutrino was first postulated in 1931 by Wolfgang Pauli to explain the energy spectrum of beta decays, 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 - who developed the first theory describing neutrino interactions - as a word play on neutrone, the Italian name of the neutron. (Neutrone in Italian means big and neutral, and neutrino means small and neutral.)
In 1962 Leon M. Lederman, Melvin Schwartz and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of the muon neutrino. When a third type of lepton, the tauon, was discovered in 1975 at the Stanford Linear Accelerator, it too was expected to have an associated neutrino. First evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay that had led to the discovery of the neutrino in the first place. The first detection of actual tau neutrino interactions was announced in summer of 2000 by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed.
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 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.
Nuclear power stations are the major source of human generated neutrinos. An average plant may generate over 1020 anti-neutrinos per second.
Some particle accelerators have been used to make neutrino beams. Their technique is to smash protons into a fixed target, producing charged pions or kaons. These unstable particles are then beamed into a long tunnel where they decay while in flight. Because of the relativistic boost of the decaying particle the neutrinos are produced as a beam rather than isotropically.
Nuclear bombs also produce very large numbers of neutrinos. Fred Reines and Clyde Cowan thought about trying to detect neutrinos from a bomb before they switched to looking for reactor neutrinos.
Neutrinos are produced as a result of natural background radiation.
Atmospheric neutrinos result from the interaction of cosmic rays with atoms in the Earth's atmosphere, creating showers of particles, many of which are unstable and produce neutrinos when they decay.
Solar neutrinos originate from the nuclear fusion powering the Sun and other stars.
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 become 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 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.
The neutrino is of scientific interest because it can make an exceptional probe for environments that are typically concealed from the standpoint of other observation techniques, such as optical and radio observation.
The first such use of neutrinos was proposed in the early 20th century for observation of the core of the Sun. Direct optical observation of the solar core is impossible due to the diffusion of electromagnetic radiation by the huge amount of matter surrounding the core. On the other hand, neutrinos generated in stellar fusion reactions are very weakly interacting and therefore pass right through the sun with few or no interactions. While photons emitted by the solar core may require 1,000 years to diffuse to the outer layers of the Sun, neutrinos are virtually unimpeded and cross this distance at nearly the speed of light.
Neutrinos are also useful for probing astrophysical sources beyond our solar system. Neutrinos are the only known particles that are not significantly attenuated by their travel through the interstellar medium. Optical photons can be obscured or diffused by dust, gas and background radiation. High-energy cosmic rays, in the form of fast-moving protons and atomic nuclei, are not able to travel more than about 100 megaparsecs due to the GZK cutoff. Neutrinos can travel this distance, and greater distances, with very little attenuation.
The galactic core of the Milky Way is completely obscured by dense gas and numerous bright objects. However, it is likely that neutrinos produced in the galactic core will be measurable by Earth based neutrino telescopes in the next decade.
The most important use of the neutrino is in the observation of supernovae, the explosions that end the lives of highly massive stars. The core collapse phase of a supernova is an almost unimaginably dense and energetic event. It is so dense that no known particles are able to escape the advancing core front except for neutrinos. Consequently, supernovae are known to release approximately 99% of their energy in a rapid (10 second) burst of neutrinos. As a result, the usefulness of neutrinos as a probe for this important event in the death of a star can not be overstated.
Many other important uses of the neutrino may be imagined in the future. It is clear that the astrophysical significance of the neutrino as an observational technique is comparable with all other known techniques, and is therefore a major focus of study in astrophysical communities.
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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 the former Homestake Mine near Lead, South Dakota, containing 520 short tons (470 metric 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 electron neutrinos.
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 electron and muon neutrinos.
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 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 magnetized. 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 ionization 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.
The Sudbury Neutrino Observatory (SNO) is a neutrino observatory located 6,800 feet (about 2 km) underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector was turned on in May 1999, and was turned off on 28 November 2006. While new data is no longer being taken, the SNO collaboration will continue to analyze the data taken during that period for the next several years. The underground laboratory has been enlarged and continues to operate other experiments at SNOLAB. The SNO equipment itself is currently being refurbished for use in the SNO+ experiment.
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