Encyclopedia
The
neutrino is an elementary particle. It has half-integer spin and is therefore a fermion. All neutrinos observed to date have left-handed chirality. Although they had been considered massless for many years, recent experiments have shown their mass to be non-zero. Because 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
gravity.
Because the cross section in weak nuclear interactions is very small, neutrinos can pass through matter almost unhindered. For typical neutrinos produced in the sun , it would take approximately one light year of
lead to block half of them. Detection of neutrinos is therefore challenging, requiring large detection volumes or high intensity artificial neutrino beams.
Types of neutrinos
Neutrinos in the Standard Model
of elementary particles
Fermion | Symbol | Mass | | Generation 1 |
|---|
| Electron neutrino | | < 2.2 eV |
| Electron antineutrino | | < 2.2 eV |
| Generation 2 |
|---|
| Muon neutrino | | < 170 keV |
| Muon antineutrino | | < 170 keV |
| Generation 3 |
|---|
| Tau neutrino | | < 15.5 MeV |
| Tau antineutrino | | < 15.5 MeV |
There are three known types of neutrinos:
electron neutrino ?
e,
muon neutrino ?
µ and tau neutrino ?
t, named after their partner leptons in the
Standard Model . The current best measurement of the number of neutrino types comes from observing the decay of the
Z boson. This particle can decay into any neutrino and its antineutrino, and the more types of neutrinos available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types is 3 . The possibility of
sterile neutrinos — neutrinos which do not participate in the weak interaction but which could be created through flavor oscillation — is unaffected by these Z-boson-based measurements, and the existence of such particles is in fact supported by experimental data from LSND. The correspondence between the six
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.
Flavor Oscillations
Neutrinos are always created or detected with a well defined flavor . 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 the same as the neutrino mass
eigenstates . 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. This effect was first noticed due to the number of electron neutrinos detected from the sun's core failing to match the expected numbers, a discrepancy dubbed the "
solar neutrino problem". The existence of flavor oscillations implies a non-zero neutrino mass, because the amount of mixing between neutrino flavors at a given time depends on the differences in their squared-masses . Despite their massive nature, it is still possible that the neutrino and antineutrino are in fact the same particle, a hypothesis first proposed by the Italian physicist Ettore Majorana.
History
The
neutrino was first postulated in December, 1930 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 , 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.
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 tau, 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.
A practical method for investigating neutrino masses was first suggested by
Bruno Pontecorvo in 1957 using an analogy with the neutral
kaon system; over the subsequent 10 years he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 Stanislav Mikheyev and Alexei Smirnov noted that flavour oscillations can be modified when neutrinos propagate through matter. This so-called MSW effect is important to understand neutrinos emitted by the Sun, which pass through its dense atmosphere on their way to detectors on Earth.
Mass
The
Standard Model of particle physics assumes that neutrinos are massless, although adding massive neutrinos to the basic framework is not difficult. Indeed, the experimentally established phenomenon of
neutrino oscillation requires neutrinos to have non-zero masses.
The strongest upper limit on the masses of neutrinos comes 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 energy of all three types of neutrinos exceeded an average of 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. A much more stringent constraint comes from a careful analysis of cosmological data, such as the cosmic microwave background radiation,
galaxy surveys and the Lyman-alpha forest. These indicate that the sum of the neutrino masses must be less than 0.3 electron volts .
In 1998, research results at the
Super-Kamiokande neutrino detector determined that neutrinos do indeed flavour oscillate, and therefore have mass. The experiment is only sensitive to the difference in the squares of the masses. These differences are known to be very small, less than 0.05 electron volts . Combined, these constraints imply that the heaviest neutrino must be at least 0.05 electron volts, but no more than 0.3 electron volts.
The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by
KamLAND in 2005: ?m
212 = 0.000079 eV
2In 2006, the MINOS experiment measured oscillations from an intense muon neutrino beam, determining the difference in the squares of the masses between neutrino mass eigenstates 2 and 3. The initial results indicate ?m
232 = 0.0031 eV
2, consistent with previous results from
Super-K .
Handedness
Experimental results show that all produced and observed neutrinos have left-handed helicities , and all antineutrinos have right-handed helicities, within the margin of error. In the massless limit, it means that only one of two possible chiralities is observed for either particle. These are the only chiralities included in the
Standard Model of particle interactions.
It is possible that their counterparts simply do not exist. If they do, their properties are substantially different from observable neutrinos and antineutrinos. It is theorized that they are either very heavy , do not participate in weak interaction , or both.
The existence of nonzero neutrino masses somewhat complicates the situation. Neutrinos are produced in weak interactions as chirality eigenstates. However, chirality of a massive particle is not a constant of motion; helicity is, but the chirality operator does not share eigenstates with the helicity operator. Free neutrinos propagate as mixtures of left- and right-handed helicity states, with mixing amplitudes on the order of . This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small .
Neutrino sources
Artificially produced neutrinos
Nuclear power stations are the major source of human-generated neutrinos. The anti-neutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the anti-neutrino flux are:
uranium-235,
uranium-238,
plutonium-239 and
plutonium-241. An average plant may generate over 10
20 anti-neutrinos per second.
Some
particle accelerators have been used to make neutrino beams. The technique is to smash
protons into a fixed target, producing charged
pions or
kaons. These unstable particles are then magnetically focussed 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.
Geologically produced neutrinos
Neutrinos are produced as a result of natural background radiation. In particular, the decay chains of
uranium-238 and
thorium-232 isotopes, as well as
potassium-40, include
beta decays which emit anti-neutrinos. These so-called geoneutrinos can provide valuable information on the Earth's interior. A first indication for geoneutrinos was found by the
KamLAND experiment in 2005. KamLAND's main background in the geoneutrino measurement are the anti-neutrinos coming from reactors. Several future experiments aim at improving the geoneutrino measurement and these will necessarily have to be far away from reactors.
Atmospheric neutrinos
Atmospheric neutrinos result from the interaction of
cosmic rays with atomic nuclei in the
Earth's atmosphere, creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from Tata Institute of Fundamental Research , Mumbai, Osaka City Univeristy, Japan and Durham University, UK recorded the first cosmic ray neutrino interaction in an underground laboratory in KGF mines in 1965.
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.
Other astrophysical phenomena
Neutrinos are an important product of
supernovae. In such events, the
pressure at the core becomes so high that the degeneracy of electrons is not enough to prevent protons and electrons from combining to form a neutron and an electron neutrino. Most of the energy produced in supernovae is radiated away in the form of an immense burst of neutrinos. The first experimental evidence of this phenomenon came in the year 1987, when neutrinos coming from the
supernova 1987a were detected. 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 . 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.
The energy of supernova neutrinos ranges from few to several 10 of MeV. However, the sites where
cosmic rays are accelerated are expected to produce neutrinos that are one million times more energetic or more, produced from turbulent gasesous environments left over by supernova explosions: the
supernova remnants. The connection between cosmic rays and supernova remants was suggested by Baade and Zwicky, shown to be consistent with the cosmic ray losses of the Milky Way if the efficiency of acceleration is about 10 percent by Ginzburg and Syrovatsky, and it is supported by a specific mechanism called "shock wave acceleration" based on Fermi ideas . The very high energy neutrinos are still to be seen, but this branch of neutrino astronomy is just in its infancy. The main existing or forthcoming experiments that aim at observing very high energy neutrinos from our galaxy are Baikal, AMANDA, ICECUBE, Antares, NEMO and Nestor. A related information is provided by very high energy
gamma ray observatories, such as HESS and MAGIC. Indeed, the collisions of cosmic rays are supposed to produce charged pions, whose decay give the neutrinos, but also neutral pions, whose decay give gamma rays: the environment of a supernova remnant is transparent to both types of radiation.
Still higher energy neutrinos, resulting from the interactions of extragalactic cosmic rays, could be observed with the cosmic ray observatory Auger or with the dedicated experiment named ANITA.
Cosmic background radiation
It is thought that the
cosmic microwave 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 are very light. This means that they move at speeds close to the
speed of light except when they have extremely low kinetic energy. Thus, dark matter made from neutrinos is termed "hot dark matter". The problem is that being fast moving, the neutrinos would tend to have spread out evenly in the
universe before cosmological expansion made them cold enough to congregate in clumps. This would cause the part of
dark matter made of neutrinos to be smeared out and unable to cause the large
galactic structures that we see.
Further, these same galaxies and
groups of galaxies appear to be surrounded by dark matter which is not fast enough to escape from those galaxies. Presumably this matter provided the gravitational nucleus for
formation. This implies that neutrinos make up only a small part of the total amount of dark matter.
From cosmological arguments, relic background neutrinos are estimated to have density of ~56 and temperature . Although their density is quite high , due to extremely low neutrino cross-sections at sub-eV energies, relic neutrino background has not yet been observed in the laboratory.
Neutrino detection
Neutrinos can interact via the neutral current or charged current
weak interactions.
- In a neutral current interaction, the neutrino leaves the detector after having transferred some of its energy and momentum to a target particle. All three neutrino flavors can participate regardless of the neutrino energy. However, no neutrino flavor information is left behind.
- In a charged current interaction, the neutrino transforms into its partner lepton . However, if the neutrino does not have sufficient energy to create its heavier partner's mass, the charged current interaction is unavailable to it. Solar and reactor neutrinos have enough energy to create electrons. Most accelerator-based neutrino beams can also create muons, and a few can create taus. A detector which can distinguish among these leptons can reveal the flavor of the incident neutrino in a charged current interaction. Because the interaction involves the exchange of a charged boson, the target particle also changes character .
Antineutrinos were first detected in 1953 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. Antineutrino charged current interactions with the protons in the water produced positrons and neutrons. 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. Today, the much larger
KamLAND detector uses similar techniques and 53 Japanese nuclear power plants to study neutrino oscillation.
Chlorine detectors consist of a tank filled with a chlorine containing fluid such as
Tetrachloroethylene. A neutrino converts a chlorine atom into one of
argon via the charged current interaction. The fluid is periodically purged with
helium gas which would remove the argon. The helium is then cooled to separate out the argon. A chlorine detector in the former Homestake Mine near
Lead, South Dakota, containing 520 short tons of fluid, made the first measurement of the deficit of electron neutrinos from the sun . A similar detector design uses a
gallium ?
germanium transformation which is sensitive to lower energy neutrinos. This latter method is nicknamed the "
Alsace-Lorraine" technique because of the reaction sequence involved. These chemical detection methods are useful only for counting neutrinos; no neutrino direction or energy information is available.
"Ring-imaging" detectors take advantage of the
Cherenkov light produced by charged particles moving through a medium faster than the
speed of light in that medium. In these detectors, a large volume of clear material is surrounded by light-sensitive
photomultiplier tubes. A charged lepton produced with sufficient energy creates Cherenkov light which leaves a characteristic ring-like pattern of activity on the array of photomultiplier tubes. This pattern can be used to infer direction, energy, and flavor information about the incident neutrino. Two water-filled detectors of this type recorded the neutrino burst from
supernova 1987a. The largest such detector is the water-filled Super-Kamiokande.
The
Sudbury Neutrino Observatory uses heavy water. In addition to the neutrino interactions available in a regular water detector, the deuterium in the heavy water can be broken up by a neutrino. The resulting free neutron is subsequently captured, releasing a burst of gamma rays which are detected. All three neutrino flavors participate equally in this dissociation reaction.
The MiniBooNE detector employs pure mineral oil as its detection medium. Mineral oil is a natural scintillator, so charged particles without sufficient energy to produce Cherenkov light can still produce scintillation light. This allows low energy muons and protons, invisible in water, to be detected.
Tracking calorimeters such as the MINOS detectors use alternating planes of absorber material and detector material. The absorber planes provide detector mass while the detector planes provide the tracking information. Steel is a popular absorber choice, being relatively dense and inexpensive and having the advantage that it can be magnetised. The NO?A proposal suggests eliminating the absorber planes in favor of using a very large active detector volume. The active detector is often liquid or plastic scintillator, read out with photomultiplier tubes, although various kinds of ionisation chambers have also been used. Tracking calorimeters are only useful for high energy neutrinos. At these energies, neutral current interactions appear as a shower of hadronic debris and charged current interactions are identified by the presence of the charged lepton's track A muon produced in a charged current interaction leaves a long penetrating track and is easy to spot. The length of this muon track and its curvature in the magnetic field provide energy and charge information. An electron in the detector produces an electromagnetic shower which can be distinguished from hadronic showers if the granularity of the active detector is small compared to the physical extent 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.
Most neutrino experiments must address the flux of
cosmic rays that bombard the earth's surface. The higher energy neutrino experiments often cover or surround the primary detector with a "veto" detector which reveals when a cosmic ray passes into the primary detector, allowing the corresponding activity in the primary detector to be ignored . For lower energy experiments, the cosmic rays are not directly the problem. Instead, the spallation neutrons and radioisotopes produced by the cosmic rays may mimic the desired physics signals. For these experiments, the solution is to locate the detector deep underground so that the earth above can reduce the cosmic ray rate to tolerable levels.
Neutrino experiments, neutrino detectors
General data
| General data | |
BOREXINO | BORon EXperiment | Gran Sasso, Italy | | LNGS, INFN | | | CLEAN | Cryogenic Low-Energy Astrophysics with Neon | | | LANL | future
experiment |
| GALLEX | GALLium EXperiment | Gran Sasso, Italy | | LNGS, INFN | 1991 - 1997 |
| GNO | Gallium Neutrino Observatory | Gran Sasso, Italy | | LNGS, INFN | 1998 - |
| HERON | Helium Roton Observation of Neutrinos | | | LBNL | |
| HOMESTAKE–CHLORINE | Homestake chlorine experiment | Homestake mine, South Dakota, USA | | BNL | 1967 - 1998 |
| HOMESTAKE–IODINE | Homestake iodine experiment | Homestake mine, South Dakota, USA | | BNL | 1996 - |
| ICARUS | Imaging Cosmic And Rare Underground Signal | Gran Sasso, Italy | | CERN to CNGS | |
| Kamiokande | Kamioka Nucleon Decay Experiment | Kamioka, Japan | | | 1986 - 1995 |
| LENS | Low Energy Neutrino Spectroscopy | | | LANL | |
| MOON | Molybdenum Observatory Of Neutrinos | Washington, USA | | | |
| SAGE | Soviet–American Gallium Experiment | Baksan valley, Russia | | | 1990 - 2006 |
| SNO | Sudbury Neutrino Observatory | Sudbury mine, North Ontario, Canada | | SNOLAB, LBNL | 1999 |
| SK | Super-Kamiokande | Kamioka, Japan | | | 1996 - 2001 |
| UNO | Underground Nucleon decay and neutrino Observatory | Henderson mine, Colorado | | NUSL | future
experiment |
| IceCube | IceCube Neutrino Detector | South Pole, Antarctica | | | future
experiment |
Technical data
| Technical data | |
BOREXINO | lS | E | vx + e- ? vx + e- | ES | H2O + PC+PPO
PC=C6H33
PPO=C15H11NO] | liquid scintillation | 250–665 keV | | CLEAN | lS, SN, WIMP | E | vx + e- ? vx + e-
ve + 20Ne ? ve + 20Ne | ES
ES | 10 t liquid Ne | scintillation | ??? |
| GALLEX | S | E | ve+71Ga ? 71Ge+e- | CC | GaCl3 | radiochemical | 233.2 keV |
| GNO | lS | E | ve+71Ga ? 71Ge+e- | CC | GaCl3 | radiochemical | 233.2 keV |
| HERON | lS | mainly E | ve + e- ? ve + e- | NC | superfluid
He | scintillation | 1000 keV |
| HOMESTAKE–CHLORINE | S | E | 37Cl+ve ? 37Ar*+e- 37Ar* ? 37Cl + e+ + ve | CC | C2Cl4 | radiochemical | 814 keV |
| HOMESTAKE–IODINE | S | E | ve + e- ? ve + e-
ve + 127I ? 127Xe + e- | ES
CC | NaI | radiochemical | 789 keV |
| ICARUS | S, ATM, GSN | E, M, T | ve + e- ? ve + e- | ES | liquid Ar | Cherenkov | 5900 keV |
| Kamiokande | S, ATM | E | ve + e- ? ve + e- | ES | H2O | Cherenkov | 7500 keV |
| LENS | lS | E | ve + 176Yb ? 176Lu+e- | CC | In3 | scintillation | 120 keV |
| MOON | lS, lSN | E | ve+100Mo ? 100Tc+e- | CC | 100Mo + MoF6 | scintillation | 168 keV |
| SAGE | lS | E | ve+71Ga ? 71Ge+e- | CC | GaCl3 | radiochemical | 233.2 keV |
| SNO | S, ATM, GSN | E, M, T | ve + 21D ?p++p++e-
vx + 21D ?vx+no+p+
ve + e- ? ve + e- | CC
NC
ES | 1000 t D2O | heavy water Cherenkov | 6.75 MeV |
| Super Kamiokande | S, ATM, GSN | E, M, T | ve + e- ? ve + e-
ve + no ? e- + p+
ve + p+ ? e+ + no
| ES
CC | H2O | water Cherenkov
| |
| UNO | S, ATM, GSN, RSN | E, M, T | ve + e- ? ve + e- | ES | 440 kt H2O | water Cherenkov | ??? |
| IceCube | S, ATM, CR, ? | E, M, T | ve + e- ? ve + e-
etc. | ES | 1 km3 H2O | ice Cherenkov | ~10 MeV |
NotationSensitivity - solar neutrinos
- low-energy solar neutrinos
- reactor neutrino experiment
- terrestrial neutrinos
- atmospheric neutrinos
- accelerator experiment
- cosmic ray
- supernova neutrinos
- low-energy supernova neutrinos
- Active Galactic Nuclei
- neutrinos from pulsars
Sensitivity - electron neutrino
- muon neutrino
- tau neutrino
Type of process- elastic scattering
- neutral current
- charged current
Research InstitutionMotivation for scientific interest in the neutrino
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 burst of neutrinos. As a result, the usefulness of neutrinos as a probe for this important event in the death of a star cannot be overstated.
Determining the mass of the neutrino is also an important test of cosmology . 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.
In
particle physics the main virtue of studying neutrinos is that they are typically the lowest mass, and hence lowest energy examples of particles theorized in extensions of the
Standard Model of particle physics. For example, one would expect that if there is a fourth class of fermions beyond the electron, muon, and tau generations of particles, that a fourth generation neutrino would be the easiest to generate in a particle accelerator.
Neutrinos are also obvious candidates for use in studying quantum gravity effects. Because they are not affected by either the strong interaction or electromagnetism, and because they are not normally found in composite particles or prone to near instantaneous decay it is easier to isolate and measure gravitational effects on neutrinos at a quantum level.
See also
neutrino physicistsNotes
References
External links
- : An index of experiments and subjects related to neutrino mass and oscilations
- : On-line review and e-archive on Neutrino Physics and Astrophysics
- : Documentary on US public television from WGBH
- : Using neutrino detectors to receive early warning of supernovae
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