AskDefine | Define neutrino

Dictionary Definition

neutrino n : an elementary particle with zero charge and zero mass

User Contributed Dictionary



  • (UK) , /njuːˈtɹiːnəʊ/, /nju:"tri:n@U/
  • (US) , /njuːˈtɹiːnoʊ/, /nju:"tri:noU/
  • Rhymes: -iːnəʊ
  • Hyphenation: neu·tri·no


  1. An elementary particle that is classified as a lepton, and has an extremely small but nonzero mass and no electric charge. It interacts with the surroundings only via the weak force or gravitation, making it very difficult to detect.




  1. neutrino

Extensive Definition

Neutrinos are elementary particles that travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. As of 1999, it is believed neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter \nu_^ (nu).
Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos; each type also has an antimatter partner, called an antineutrino. Electron neutrinos or antineutrinos are generated whenever neutrons change into protons or vice versa, the two forms of beta decay. Interactions involving neutrinos are generally mediated by the weak force.
Most neutrinos passing through the Earth emanate from the sun, and more than 50 trillion solar electron neutrinos pass through the human body every second.


The neutrino was first postulated in December 1930 by Wolfgang Pauli to preserve conservation of energy, conservation of momentum, and conservation of angular momentum in beta decay, the decay of a neutron into a proton, an electron and an antineutrino. Pauli theorized that an undetected particle was carrying away the observed difference between the energy, momentum, and angular momentum of the initial and final particles.
The current name neutrino was coined by Enrico Fermi, who developed the first theory describing neutrino interactions, as a pun on neutrone, the Italian name of the neutron: neutrone seems to use the -one suffix (even though it is a complete word, not a compound), which in Italian indicates a large object, whereas -ino indicates a small one.
In 1942 Kan-Chang Wang first proposed to use beta-capture to experimentally detect neutrinos. 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. In this experiment, now known as the neutrino experiment, neutrinos created in a nuclear reactor by beta decay were shot into protons producing neutrons and positrons both of which could be detected. It is now known that both the proposed and the observed particles were antineutrinos.
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, which earned them the 1988 Nobel Prize. 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 leading to the discovery of the neutrino. The first detection of 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; its existence had already inferred both by theoretical consistency, as well as by experimental data from LEP.
Starting in the late 1960s, several experiments found that the number of electron neutrinos arriving from the sun was between one third and one half the number predicted by the Standard Solar Model, a discrepancy which became known as the solar neutrino problem and remained unresolved for some thirty years.
The Standard Model of particle physics assumes massless neutrinos that don't change flavor. However, nonzero neutrino mass and accompanying flavor oscillation remained a possibility.
A practical method for investigating neutrino masses (that is, flavor oscillation) 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 (expanding on 1978 work by Lincoln Wolfenstein) noted that flavor 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.
Starting in 1998, experiments began to show that solar and atmospheric neutrinos change flavors (see Super-Kamiokande, Sudbury Neutrino Observatory). This resolved the solar neutrino problem: the electron neutrinos produced in the sun had partly changed into other flavors which the experiments could not detect.
Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations. Especially relevant in this context are the reactor experiment KamLAND and the accelerator experiments such as MINOS. The KamLAND experiment has indeed identified oscillations as the neutrino flavor conversion mechanism involved in the solar electron neutrinos. Similarly MINOS confirms the oscillation of atmospheric neutrinos and gives a better determination of the mass squared splitting (Maltoni, 2004).
Raymond Davis Jr. and Masatoshi Koshiba were jointly awarded the 2002 Nobel Prize in Physics. Ray Davis for his pioneer work on solar neutrinos and Koshiba for the first real time observation of supernova neutrinos. The detection of solar neutrinos, and of neutrinos of SN 1987A supernova in 1987 marked the beginning of neutrino astronomy.


The neutrino has half-integer spin (\begin\frac\hbar\end) and is therefore a fermion. 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.
An experiment done by C. S. Wu at Columbia University showed that neutrinos always have left-handed chirality.
Detection of neutrinos is challenging and often requires large detection volumes or high intensity artificial neutrino beams.
To date, not very much is experimentally established concerning the interactions of neutrinos with matter.

Popular Misconception

It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. For this reason, in early experiments a special reaction channel was chosen to facilitate the identification: The interaction of an antineutrino with a hydrogen nucleus, which is a single proton. It turns out that an antineutrino would travel about 30 light years through water, before it undergoes this specific reaction. This doesn't prove in any way that an antineutrino cannot undergo other reactions with matter. But however it led to popular misconceptions like this one: ''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 (with energies of a few MeV), it would take approximately one light year (~1016 m) of lead to block half of them.
For example, in later theories, like the one describing a so called MSW effect, it is thought that most solar neutrinos are interacting with matter inside the sun''.

Types of neutrinos

There are three known types (flavours) of neutrinos: electron neutrino νe, muon neutrino νμ and tau neutrino ντ, named after their partner leptons in the Standard Model (see table at right). 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 light neutrino and its antineutrino, and the more types of light neutrinos available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types (with "light" meaning of less than half the Z mass) is 3. The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicists' intuition that there should be exactly three types of neutrino. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
The possibility of sterile neutrinos — relatively light neutrinos which do not participate in the weak interaction but which could be created through flavour oscillation (see below) — is unaffected by these Z-boson-based measurements, and the existence of such particles is in fact hinted by experimental data from the LSND experiment. However, the currently running MiniBooNE experiment suggested, until recently, that sterile neutrinos are not required to explain the experimental data, although the latest research into this area is on-going and anomalies in the MiniBooNE data may allow for exotic neutrino types, including sterile neutrinos.

Flavor oscillations

Neutrinos are most often created or detected with a well defined flavour (electron, muon, tau). However, in a phenomenon known as neutrino flavour 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 (simply called 1, 2, 3). 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 quantum mechanical effect was first hinted by the discrepancy between the number of electron neutrinos detected from the sun's core failing to match the expected numbers, dubbed as the "solar neutrino problem". In the Standard Model the existence of flavor oscillations implies a nonzero neutrino mass, because the amount of mixing between neutrino flavors at a given time depends on the differences in their squared-masses (although it is not generally so, on the Standard Model mixing would be zero for massless neutrinos). In keeping with 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. The reason for the need for mass to make neutrinos equivalent to antineutrinos, is that only with a massive particle (which therefore cannot move at the speed of light) is it possible to postulate an inertial frame which moves faster than the particle, and thereby converts its spin from one type of "handedness" to the other (for example, right to left-handed spin), thus making any type of neutrino in the new frame, appear as its own antiparticle.


Before the idea of neutrino oscillations came up, it was generally assumed that neutrinos travel at the speed of light. The question of neutrino velocity is closely related to their mass. According to relativity, if neutrinos are massless, they must travel at the speed of light. However, if they carry a mass, they cannot reach the speed of light.
In the early 1980s, first measurements of neutrino speed were done using pulsed pion beams (produced by pulsed proton beams hitting a target). The pions decayed producing neutrinos, and the neutrino interactions observed within a time window in a detector at a distance were consistent with the speed of light. This measurement has been repeated using the MINOS detectors, which found the speed of 3 GeV neutrinos to be 1-((5.1±2.9)×10−5) times the speed of light. While the central value is lower than the speed of light, the uncertainty is great enough that it is very likely that the true velocity is too close to the speed of light to see the difference. This measurement set an upper bound on the mass of the muon neutrino of 50 MeV at 99% confidence.
The same observation was made, on a somewhat larger scale, with supernova 1987a. The neutrinos from the supernova were detected within a time window that was consistent with a speed of light for the neutrinos. So far, the question of neutrino masses cannot be decided based on measurements of the neutrino speed.


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 nonzero 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 electronvolts 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 electronvolt (Goobar, 2006).
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 (Mohapatra, 2005).
The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by KamLAND in 2005: Δm212 = 0.000079 eV2
In 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 Δm232 = 0.003 eV2, consistent with previous results from Super-K.
Currently a number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments. The methods applied involve nuclear beta decay (KATRIN and MARE) or neutrinoless double beta decay (e.g. GERDA, CUORE/Cuoricino, NEMO 3 and others).


Experimental results show that (nearly) all produced and observed neutrinos have left-handed helicities (spins antiparallel to momenta), 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 (right-handed neutrinos and left-handed antineutrinos) 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 (on the order of GUT scale—see Seesaw mechanism), do not participate in weak interaction (so-called sterile neutrinos), 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 m_/E. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small (for example, most solar neutrinos have energies on the order of 100 keV–1 MeV, so the fraction of neutrinos with "wrong" helicity among them cannot exceed 10-10).


(This refers to neutrinos associated with electrons, see neutrino flavor.)
In theory, you can construct all isotopes from neutrons and neutrinos, using this method with A neutrons and B neutrinos (B isotope(mass number A, atomic number B)
The resulting isotope would be a neutral atom (i.e. with equal number of protons and electrons). This is true because of the reaction:
neutron + neutrino => proton(el. charge +) + electron(el. charge -)
Everything else about this reaction is just a matter of energy and chance (how often it happens).
In a reverse process, which is also just a matter of energy, you can disintegrate all isotopes into a cloud of neutrons by shooting antineutrinos at them, like this:
isotope(mass number A, atomic number B) + B antineutrinos => A neutrons
This is listed as trivia here because these types of reactions are expected to happen extremely rarely in nature.

Neutrino sources

Artificially produced neutrinos

Nuclear reactors are the major source of human-generated neutrinos. 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, plutonium-241 (e.g. the anti-neutrinos emitted during beta-minus decay of their respective fission fragments). The average nuclear fission releases about 200 MeV of energy, of which roughly 6% (or 9 MeV, depending on quoted reference) are radiated away as anti-neutrinos. For a typical nuclear reactor with a thermal power of 4,000 MW and an electrical power generation of 1,300 MW, this corresponds to a total power production of 4,250 MW (Mega-Watts), of which 250 MW is radiated away, and disappears, as anti-neutrino radiation. This is to say, 250 MW of fission energy is lost from this reactor and does not appear as heat, since the anti-neutrinos penetrate all normal building materials essentially tracelessly. The exact energy spectrum is mostly uncertain and depends, for example, on the degree to which the fuel is burned.
There is no established experimental method to measure the flux of low energy anti-neutrinos. Only anti-neutrinos with an energy above threshold of 1.8 MeV can be uniquely identified (see "Neutrino Detection" below). An estimated 3% of all anti-neutrinos from a nuclear reactor carry an energy above this threshold. An average nuclear power plant may generate over 1020 anti-neutrinos per second above this threshold, and a much larger number which cannot be seen with present detector technology.
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 focused 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 quantities of neutrinos. Fred Reines and Clyde Cowan considered the detection neutrinos from a bomb prior to their search 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 (TIFR), India, Osaka City University, Japan and Durham University, UK recorded the first cosmic ray neutrino interaction in an underground laboratory in KGF gold mines in India in 1965.

Solar neutrinos

Solar neutrinos originate from the nuclear fusion powering the sun and other stars. The details of the operation of the sun are explained by the Standard Solar Model. In short: when four protons fuse to become one helium nucleus, two of them have to convert into neutrons, and each such conversion releases one electron neutrino.
The sun sends enormous numbers of neutrinos in all directions. Every second, about 70 billion (7×1010) solar neutrinos pass through every square centimeter on Earth that faces the sun. Since neutrinos are insignificantly absorbed by the mass of the Earth, the surface area on the side of the Earth opposite the Sun receives about the same number of neutrinos as the side facing the Sun.


Neutrinos are an important product of Types Ib, Ic and II (core-collapse) supernovae. In such events, the pressure at the core becomes so high (1014 g/cm³) that the degeneracy of electrons is not enough to prevent protons and electrons from combining to form a neutron and an electron neutrino. A second and more important neutrino source is the thermal energy (100 billion kelvins) of the newly formed neutron core, which is dissipated via the formation of neutrino-antineutrino pairs of all flavors. Most of the energy produced in supernovas is thus 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 from supernova 1987A were detected. The water-based detectors Kamiokande II and IMB detected 11 and 8 antineutrinos of thermal origin, 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 cannot be overstated.
Determining the mass of the neutrino (see above) is also an important test of cosmology (see Dark matter). 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 could also be used for 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 (unlike quarks) or prone to near instantaneous decay (like many other standard model particles) it might be possible to isolate and measure gravitational effects on neutrinos at a quantum level.



  • Neutrino Astrophysics
  • Introduction to Elementary Particles
  • Introduction to High Energy Physics
  • Particles and Nuclei: An Introduction to the Physical Concepts
  • Modern Physics (4th ed.)
  • Neutrino Oscillations, Masses And Mixing: W.M.Alberico, Torino University&S.M. Bilenky, Dubna NRI; 2003;

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