Neutrino
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. The latter is negligible for particle physics, but may play a significant role in cosmology.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.
| 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.
The upper limits for the mass of the neutrinos are shown in the table. Mass is really a coupling between a left handed fermion and a right handed fermion. For example, the mass of an electron is really a coupling between a left handed electron and a right handed electron, which is the antiparticle of a left handed positron.
(In the case of neutrinos, there are large mixings in their mass coupling, so it's not accurate to talk about neutrino masses in the flavor basis or to suggest a left handed electron neutrino and a right handed electron neutrino have the same mass as this table seems to suggest.)
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 C. L. Cowan Jr., F. Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published the article "Detection of the Free Neutrino: a Confirmation" in Science, 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.)
Until 1999 or so, neutrinos were widely believed to be massless. This hypothesis was apparently confirmed by cosmological observations that implied an extremely low upper bound on the neutrino mass (a few electron volts). However we now know that neutrinos have a small but nonzero mass; as a consequence, they will spontaneously mutate between the three flavors, in a phenomenon known as neutrino oscillation (which provides a solution to the solar neutrino problem and the atmospheric neutrino problem at the same time). Raymond Davis Jr and Masatoshi Koshiba were jointly awarded the 2002 Nobel Prize in Physics for their work in the detection of cosmic neutrinos.
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 neutrons.
Solar neutrinos
Solar neutrinos originate from the nuclear fusion powering the Sun and other stars.
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 though 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 origins of the Universe
It has been suggested that neutrinos may have been the main type of matter created during the Big Bang, and that the visible matter now present in the universe may have been created through radioactive neutron decay.
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 νe.
- 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 νe and νμ.
- 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.