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Table of contents 1 Types of neutrinos 2 Flavor Oscillations 2.1 Observations 2.1.1 Solar neutrino oscillation 2.1.2 Atmospheric neutrino oscillation 2.1.3 Reactor neutrino oscillations 2.1.4 Beam neutrino oscillations 2.2 Theory, formally 2.2.1 Maki-Nakagawa-Sakata matrix 2.2.2 Propagation and interference 2.2.3 Two neutrino case 2.3 Theory 2.3.1 Two neutrino probabilities 2.3.2 Three neutrino probabilities 2.4 Observed values of oscillation parameters 2.5 Origins of neutrino mass 2.5.1 See-saw mechanism 2.5.2 Other sources 3 History 4 Mass 5 Handedness 6 Neutrino sources 6.1 Artificially produced neutrinos 6.2 Geologically produced neutrinos 6.3 Atmospheric neutrinos 6.4 Solar neutrinos 6.5 Other astrophysical phenomena 6.6 Cosmic background radiation 7 Neutrino detection 8 Neutrino experiments, neutrino detectors 8.1 General data 8.2 Technical data 9 Motivation for scientific interest in the neutrino 10 Related peopl and concepts 11 References and external articles 1 See also

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 (see Super-Kamiokande, Sudbury Neutrino Observatory, KamLAND and MINOS) 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 (with energies of a few MeV), it would take approximately one light year (~10¹⁶m) 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) Electron neutrino < 2.2 eV Electron antineutrino < 2.2 eV Generation 2 (muon) Muon neutrino < 170 keV Muon antineutrino < 170 keV Generation 3 (tau) Tau neutrino < 15.5 MeV Tau antineutrino < 15.5 MeV
Notes Since neutrino flavor eigenstates are not the same as neutrino mass eigenstates (see neutrino oscillation), the given masses are actually mass expectation values. If the mass of a neutrino could be measured directly, the value would always be that of one of the three mass eigenstates: ν1, ν2, and ν3. In practice, the mass cannot be measured directly. Instead it is measured by looking at the shape of the endpoint of the energy spectrum in particle decays. This sort of measurement directly measures the expectation value of the mass; it is not sensitive to any of the mass eigenstates separately.

There are three known types (flavors) of neutrinos: electron neutrino ν_e, muon neutrino ν_μ and tau neutrino ν_τ, 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 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 (where "light" means having mass less than half the Z mass) is 3 [1] (http://pdg.lbl.gov/2005/listings/lxxx.html). The possibility of sterile neutrinos — neutrinos which do not participate in the weak interaction but which could be created through flavor oscillation (see below) — 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

Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates. Neutrino oscillation is of theoretical and experimental interest as observation of the phenomenon implies that the neutrino has a non-zero mass, which is not part of the original Standard Model of particle physics.

Neutrinos are always created or detected with a well defined flavor (electron, muon, tau). 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 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 (mixing would be zero for massless neutrinos). 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.

Observations

A great deal of evidence for neutrino oscillations has been collected from many sources, over a wide range of neutrino energies and with many different detector technologies.

Solar neutrino oscillation

The first experiment to detect the effects of neutrino oscillations was Ray Davis's Homestake Experiment, in which he observed a deficit in the flux of solar neutrinos using a chlorine-based detector. This gave rise to the Solar neutrino problem. Many subsequent radiochemical and water Cerenkov detectors confirmed the deficit, but neutrino oscillations weren't conclusively identified as the source of the deficit until the Sudbury Neutrino Observatory provided clear evidence of neutrino flavor change.

Solar neutrinos have energies below 20 MeV and travel an astronomical unit between the source and detector. At energies above 5 MeV, solar neutrino oscillation actually takes place in the sun through a resonance known as the MSW effect, a different process from the vacuum oscillations described later in this article.

Atmospheric neutrino oscillation

Large detectors such as IMB, MACRO, and Kamiokande II observed a deficit in the ratio of the flux of muon to electron flavor atmospheric neutrinos. The Super Kamiokande experiment provided a very high precision measurement of neutrino oscillations in an energy range of hundreds of MeV to a few TeV, and with a baseline of the radius of the Earth.

Reactor neutrino oscillations

Many experiments have searched for oscillations of electron anti-neutrinos produced at nuclear reactors. A high precision observation of reactor neutrino oscillation has been made by the KamLAND experiment. Neutrinos produced in nuclear reactors have energies similar to solar neutrinos, a few MeV. The baselines of these experiments have ranged from tens of meters to over 100km.

Beam neutrino oscillations

Neutrinos beams produced at a particle accelerator offer the greatest control over the neutrinos being studied. Many experiments have taken place which study the same neutrino oscillations which take place in atmospheric neutrino oscillation, using neutrinos with a few GeV of energy and several hundred km baselines. The MINOS experiment recently announced that it observes consistency with the results of the K2K and Super-K experiments. The MINOS result has not yet been published in a peer reviewed journal but it is expected that their results will be published soon.

The controversial observation of beam neutrino oscillation at the LSND experiment is currently being tested by MiniBooNE. Results from MiniBooNE are expected in the fall of 2006.

Theory, formally

Maki-Nakagawa-Sakata matrix

It is generally accepted that neutrino oscillations are due to a mismatch between the flavor and mass eigenstates of neutrinos. The relationship between these eigenstates is given by

,

where

• is a neutrino with definite flavor. α = e (electron), μ (muon) or τ (tau).
• is a neutrino with definite mass. i = 1, 2, 3.
• ^* represents a complex conjugate (for antineutrinos, the complex conjugate should be dropped from the first equation, and added to the second).

U_αi represents the Maki-Nakagawa-Sakata matrix (also called the "MNS matrix", "neutrino mixing matrix", or sometimes "PMNS matrix" to include Pontecorvo). It is the equivalent of the CKM matrix for quarks. If this matrix were the identity matrix, then the flavor eigenstates would be the same as the mass eigenstates. However, experiment shows that it is not. When the standard three neutrino theory is considered, the matrix is 3×3. If only two neutrinos are considered, a 2×2 used. The phase factors α₁ and α₂ are non-zero only if neutrinos are Majorana particles (whether or not they are is unknown), and do not enter into oscillation phenomena regardless. If neutrinoless double beta decay occurs, these factors influence its rate. The phase factor δ is non-zero only if neutrino oscillation violates CP symmetry. This is expected, but not yet observed experimentally. If experiment shows this 3x3 matrix to be not unitary, a sterile neutrino or some other new physics is required.

Propagation and interference

Since are mass eigenstates, their propagation can be described by plane wave solutions of the form

,

where

• quantities are expressed in natural units
• E is the energy of the particle,
• t is the time from the start of the propagation,
• is the 3-dimensional momentum,
• is the current position of the particle relative to its starting position

The energy depends on the mass m where the approximation is appropriate in the ultrarelativistic limit. This limit applies in all practical cases to neutrinos. Their mass is less than 1eV and in all experiments their energies are at least 1MeV, so the Lorentz factor γ is greater than 10⁶ in all cases. Eigenstates with different masses propagate at different speeds. The heavier ones lag behind while the lighter ones pull ahead. Since the mass eigenstates are combinations of flavor eigenstates, this difference in speed causes interference between the corresponding flavor components of each mass eigenstate. Constructive interference causes it to be possible to observe a neutrino created with a given flavor to change its flavor during its propagation. The probability that a neutrino originally of flavor α will later be observed as having flavor β. It is convenient to plug in the oscillation parameters since:

• The mass differences, Δm², are known to be on the order of 1eV²
• Oscillation distances, L, in modern experiments are on the order of kilometers
• Neutrino energies, E, in modern experiments are typically on order of GeV.

If there is no CP-violation (δ is zero), then the second sum is zero.

Two neutrino case

The above formula is correct for any number of neutrino generations. Writing it explicitly in terms of mixing angles is extremely cumbersome if there are more than two neutrinos that participate in mixing. Fortunately, there are several cases in which only two neutrinos participate significantly. In this case, it is sufficient to consider a mixing matrix for the probability of a neutrino changing its flavor.

Using SI units, it produces:

This formula is often appropriate for discussing the transition ν_μ ↔ ν_τ in atmospheric mixing, since the electron neutrino plays almost no role in this case. It is also appropriate for the solar case of ν_e ↔ ν_x, where ν_x is a superposition of ν_μ and ν_τ. These approximations are possible because the mixing angle θ₁₃ is very small and because two of the mass states are very close in mass compared to the third.

Theory

It may be easier to understand the process of neutrino oscillation if it is presented with pictures instead of equations. This is easiest to do if only two types of neutrinos are considered. Here is the initial state of the neutrino, a plane wave of a single pure flavor (called "flavor 1" for generality, but it could be, for instance, a muon neutrino). This flavor state is a combination of mass states. However, each mass state is also made up of flavor states. The second flavor state could represent the tau neutrino. Note that if the two flavor 1 curves are added together, the original full wave is reproduced. On the other hand, if the flavor 2 curves are added, they cancel each other completely. Now, each of the mass 1 components travel slower than each of the mass 2 components, so over time they lag behind. If, at this later time, the corresponding flavor states are added together, it is no longer the case that only flavor 1 is non-zero. Now there is less flavor 1 and a non-zero amount of flavor 2. The probability of observing a flavor is equal to the square of the amplitude of its wave. As time goes on, the heights of the resulting flavor waves will change periodically. This is the oscillation. The mixing angle controls how big this oscillation is. If the angle is maximal (sin²2θ = 1), then the probability oscillates from 100% for the first flavor to 100% for the second. If the angle is smaller, then the first flavor's probability never goes to zero, but rather oscillates between 100% and some intermediate value. The oscillation of three or more neutrino flavors can also be visualized this way. However, if there is CP-violation, not all waves will start in phase as is always the case when there are only two neutrinos.

Two neutrino probabilities

In the approximation where only two neutrinos participate in the oscillation, the probability of oscillation follows a simple pattern. One curve shows the probability of the original neutrino retaining its identity. Another curve shows the probability of conversion to the other neutrino. The maximum probability of conversion is equal to sin²2θ. The frequency of the oscillation is controlled by Δm².

Three neutrino probabilities

If three neutrinos are considered, the probability for each neutrino to appear is somewhat complex. Here are shown the probabilties for each initial flavor, with one plot showing a long range to display the slow "solar" oscillation and the other zoomed in to display the fast "atmospheric" oscillation. The oscillation parameters used here are consistent with current measurements, but since some parameters are still quite uncertain, these graphs are only qualitatively correct in some aspects. These values were used:

• sin²θ₁₃ = 0.08. (If it turns out to be much smaller or zero, the small wiggles shown here will be much smaller or non-existent, respectively.)
• sin²θ₂₃ = 0.95. (It may turn out to be exactly one.)
• sin²θ₁₂ = 0.86.
• δ = 0. (If it is actually large, these probabilities will be somewhat distorted and different for neutrinos and antineutrinos.)
• .
• .

Observed values of oscillation parameters

• at 90% confidence level ()
• . This corresponds to ("sol" stands for solar)
• , corresponding to ("atm" for atmospheric)
• 
• 
• δ is unknown

Solar neutrino experiments combined with KamLAND have measured the so-called solar parameters and sin²θ_sol. Atmospheric neutrino experiments such as Super-Kamiokande together with the K2K first long baseline accelerator neutrino experiment have determined the so-called atmospheric parameters and sin²2θ_atm. An additional experiment MINOS is expected to reduce the experimental errors significantly thereby increasing precision. For atmospheric neutrinos (where the relevant difference of masses is about and the typical energies are ), oscillations become visible for neutrinos travelling several hundred km, which means neutrinos that reach the detector from below the horizon. From atmospheric and solar neutrino oscillation experiments, it is known that two mixing angles of the MNS matrix are large and the third is smaller. This is in sharp contrast to the CKM matrix in which all three angles are small and hierarchically decreasing. Nothing is known about the CP-violating phase of the MNS matrix. If the neutrino mass proves to be of Majorana type (making the neutrino its own antiparticle), it is possible that the MNS matrix has more than one phase.

Origins of neutrino mass

The question of how neutrino masses arise has not been answered conclusively. In the Standard Model of particle physics, fermions only have mass because of interactions with the Higgs field (see Higgs boson). These interactions involve both left- and right-handed versions of the fermion (see chirality). However, only left-handed neutrinos have been observed so far.

Neutrinos may have another source of mass through the Majorana equation. This mechanism only applies for electrically-neutral particles since otherwise it would allow particles to turn into anti-particles, which would violate conservation of electric charge.

The smallest modification to the Standard Model, which only has left-handed neutrinos, is to allow these left-handed neutrinos to have Majorana masses. The problem with this is that the neutrino masses are implausibly smaller than the rest of the known particles (at least 500,000 times smaller than the mass of an electron), which, while it does not invalidate the theory, is not very satisfactory.

The next simplest addition would be to add right-handed neutrinos into the Standard Model, which interact with the left-handed neutrinos and the Higgs field in an analogous way to the rest of the fermions. These new neutrinos would interact with the other fermions solely in this way, so are not phenomenologically excluded. Still, the problem of the disparity of the mass scales remains.

See-saw mechanism

The most popular solution currently is the seesaw mechanism, where right-handed neutrinos with very large Majorana masses are added. If the right-handed neutrinos are very heavy, they induce a very small mass for the left-handed neutrinos, which is proportional to the inverse of the heavy mass.

If it is assumed that the neutrinos interact with the Higgs field with approximately the same strength as electrons do (which is quite reasonable as neutrinos and electrons/muons/tau leptons are associated with each other in the same way as up and down quarks are associated with each other), the heavy mass should be close to the GUT scale. Note that, in the Standard Model there is just one fundamental mass scale (which can be taken as the scale of breaking) and all masses (such as the electron or the mass of the Z boson) have to originate from this one.

The apparently innocent addition of right handed neutrinos has the effect of adding new mass scales, completely unrelated to the mass scale of the Standard Model. Thus, heavy right handed neutrinos look to be the first real glimpse of physics beyond the Standard Model. It is interesting to note that right handed neutrinos can help to explain the origin of matter through a mechanism known as leptogenesis.

Other sources

There are other ideas for the origin of neutrino mass, such as R-parity violating supersymmetry, which proposes that the masses for the neutrinos come from interactions with squarks and sleptons, rather than the Higgs field. However, these interactions are normally excluded from theories as they come from a class of interactions that lead to unacceptably rapid proton decay (if they are all included), do not help to understand why neutrinos are so light and are not able to provide a cold dark matter candidate. Still, these theories have not been ruled out yet.

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 (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 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 (that is, flavour 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 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 (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. These differences are known to be very small, less than 0.05 electron volts (Mohapatra, 2005). 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: 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 = 0.0031 eV², consistent with previous results from Super-K . [2] (http://www.fnal.gov/pub/presspass/press_releases/minos_3-30-06.html)

Handedness

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_ν / 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 can't exceed 10 ^{− 10} ). [3] (http://pdg.lbl.gov/2006/reviews/numixrpp.pdf) [4] (http://www.nu.to.infn.it/pap/0102320/)

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 10²⁰ 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 (TIFR), 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 (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. 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 (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.

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 (that is still in development). 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 cm ^{− 3} and temperature . Although their density is quite high (boron-8 solar neutrinos have been detected definitively despite having density that is lower by some 6 orders of magnitude), 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 (involving the exchange of a Z boson) or charged current (involving the exchange of a W boson) 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 (electron, muon, or tau). 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 (e.g., neutron → proton).

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 (470 metric tons) of fluid, made the first measurement of the deficit of electron neutrinos from the sun (see solar neutrino problem). 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 (gallium-germanium-gallium) 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 (e.g., water) 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 (sometimes) flavor information about the incident neutrino. Two water-filled detectors of this type (Kamiokande and IMB) recorded the neutrino burst from supernova 1987a. The largest such detector is the water-filled Super-Kamiokande.

The Sudbury Neutrino Observatory (SNO) 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 NOνA 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 (GeV range) 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 (possibly alongside some form of hadronic debris.) 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 (μ ⁺ versus μ ⁻ ) 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. (To directly observe taus, one typically looks for a kink in tracks in photographic emulsion.)

Most neutrino experiments must address the flux of cosmic rays that bombard the earth's surface. The higher energy (>50 MeV or so) 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 ("vetoed"). 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<center>
Abbreviation	Experiment	Place	homepage	Cooperation	scheduled to start
BOREXINO	BORon EXperiment	Gran Sasso, Italy	[5] (http://www.ge.infn.it/borexino/) [6] (http://borex.lngs.infn.it/)	LNGS, INFN
CLEAN	Cryogenic Low-Energy Astrophysics with Neon		([7] (http://mckinseygroup.physics.yale.edu/publications/CLEAN.pdf), PDF)	LANL	future experiment
GALLEX	GALLium EXperiment	Gran Sasso, Italy	[8] (http://www.mpi-hd.mpg.de/nuastro/gallex.html)	LNGS, INFN	1991 - 1997
GNO	Gallium Neutrino Observatory	Gran Sasso, Italy	[9] (http://www.lngs.infn.it/site/exppro/gno/Gno_home.htm)	LNGS, INFN	1998 -
HERON	Helium Roton Observation of Neutrinos		[10] (http://www.physics.brown.edu/physics/researchpages/cme/heron/LTD_home.html)	LBNL
HOMESTAKE–CHLORINE	Homestake chlorine experiment	Homestake mine, South Dakota, USA	[11] (http://www-spires.dur.ac.uk/cgi-bin/spiface/find/experiments/www2?rawcmd=fin+expt+homestake)	BNL	1967 - 1998
HOMESTAKE–IODINE	Homestake iodine experiment	Homestake mine, South Dakota, USA	[12] (http://www-spires.dur.ac.uk/cgi-bin/spiface/find/experiments/www2?rawcmd=fin+expt+homestake)	BNL	1996 -
ICARUS	Imaging Cosmic And Rare Underground Signal	Gran Sasso, Italy	[13] (http://www.aquila.infn.it/icarus/)	CERN to CNGS
Kamiokande	Kamioka Nucleon Decay Experiment	Kamioka, Japan	[14] (http://www-sk.icrr.u-tokyo.ac.jp/doc/kam/index.html)		1986 - 1995
LENS	Low Energy Neutrino Spectroscopy		[15] (http://wwwphys.vt.edu/~kimballton/) [16] (http://lens.in2p3.fr/) [17] (http://laser.physics.sunysb.edu/~thomas/report1/lens_report.html)	LANL
MOON	Molybdenum Observatory Of Neutrinos	Washington, USA	[18] (http://ewi.npl.washington.edu/moon/)
SAGE	Soviet–American Gallium Experiment	Baksan valley, Russia	[19] (http://ewi.npl.washington.edu/SAGE/sage.html)		1990 - 2006
SNO	Sudbury Neutrino Observatory	Sudbury mine, North Ontario, Canada	[20] (http://www.sno.phy.queensu.ca/)	SNOLAB, LBNL	1999 (- 2006)
SK	Super-Kamiokande	Kamioka, Japan	[21] (http://neutrino.phys.washington.edu/~superk/) [22] (http://www-sk.icrr.u-tokyo.ac.jp/sk/index_e.html)		1996 - 2001
UNO	Underground Nucleon decay and neutrino Observatory	Henderson mine, Colorado	[23] (http://ale.physics.sunysb.edu/uno/)	NUSL	future experiment
IceCube	IceCube Neutrino Detector	South Pole, Antarctica	[24] (http://icecube.wisc.edu/)		future experiment

Technical data

<center>Technical data
Abbreviation	Sensitivity (1)	Sensitivity (2)	Induced reaction*	Type of reaction	Detector	Type of detector	threshold energy
BOREXINO	lS	E	vx + e− → vx + e−	ES	H2O + PC+PPO PC=C6H3(CH3)3 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 (30 t Ga)	radiochemical	233.2 keV
GNO	lS	E	ve+71Ga → 71Ge+e−	CC	GaCl3 (30 t Ga)	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 (615 t)	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	In(acc)3[25] (http://www.mpi-hd.mpg.de/nubis/lens/images/inacac.html)	scintillation	120 keV
MOON	lS, lSN	E	ve+100Mo → 100Tc+e−	CC	100Mo (1 t) + MoF6 (gas)	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)	ice Cherenkov	~10 MeV

Notation

Sensitivity (1)

• solar neutrinos (S)
• low-energy solar neutrinos (ls)
• reactor neutrino experiment (R)
• terrestrial neutrinos (T)
• atmospheric neutrinos (ATM)
• accelerator experiment (AC)
• cosmic ray (CR)
• supernova neutrinos (S)
• low-energy supernova neutrinos (lSN)
• Active Galactic Nuclei (AGN)
• neutrinos from pulsars (PUL)

Sensitivity (2)

• electron neutrino (E)
• muon neutrino (M)
• tau neutrino (T)

Type of process

• elastic scattering (ES)
• neutral current (NC)
• charged current (CC)

Research Institution

• Brookhaven National Laboratory (BNL)
• Conseil Européen pour la Recherche Nucleaire (CERN)
• CERN Neutrino to Gran Sasso (CNGS)
• Istituto Nazionale di Fisica Nucleare (INFN)
• Los Alamos National Laboratory (LANL)
• Laboratori Nazionali del Gran Sasso (LNGS)
• National Underground Science Laboratory (NUSL)

Motivation 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 (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 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 (unlike quarks) or prone to near instantaneous decay (like many other standard model particles) it is easier to isolate and measure gravitational effects on neutrinos at a quantum level.

Related peopl and concepts

• MSW effect
• Neutral kaon mixing
• Kamioka Observatory
• List of particles
• Particle physics
• Neutrino astronomy
• Neutrino Factory
• Neutrino oscillation
• Solar neutrino problem

neutrino physicists

• John N. Bahcall
• Clyde Cowan
• Raymond Davis Jr.
• Enrico Fermi
• Masatoshi Koshiba
• Leon Lederman
• Wolfgang Pauli
• Frederick Reines
• Melvin Schwartz
• Jack Steinberger

References and external articles

• Super-Kamiokande Super-Kamiokande at UC Irvine (http://www.ps.uci.edu/~superk/)
• G. A. Tammann, F. K. Thielemann, D. Trautmann Opening new windows in observing the Universe (http://www.europhysicsnews.com/full/20/article8/article8.html) Europhysics News
• Bahcall, John N., "Neutrino Astrophysics" Cambridge University Press, 1989 ISBN 0-521-35113-8
• Griffiths, David J., "Introduction to Elementary Particle" Wiley, John & Sons, Inc., 1987 ISBN 0-471-60386-4
• Perkins, Donald H., "Introduction to High Energy Physic", publisher=Cambridge University Press, =1999 ISBN 0-521-62196-8
• Povh, Bogdan, "Particles and Nuclei: An Introduction to the Physical Concepts" Springer-Verlag, year=1995 ISBN 0-387-59439-6
• Tipler, Paul; Llewellyn, Ralph, "Modern Physics"(4th ed.), W. H. Freeman, 2002 ISBN 0-7167-4345-0
• R. N. Mohapatra et al. (APS neutrino theory working group), "Theory of neutrinos: a white paper" arxiv:hep-ph|id=0510213
• A. Goobar, S. Hannestad, E. Mörtsell and H. Tu "A new bound on the neutrino mass from the SDSS baryon acoustic peak"
• Wikipedia contributors (http://en.wikipedia.org/wiki/Special:Recentchanges), Wikipedia: The Free Encyclopedia. Wikimedia Foundation. <http://en.wikipedia.org>.
• C. Bemporad [Chooz Collaboration], Nucl. Phys. Proc. Suppl. 77 (1999) 159.
• Limit from the Solar and KamLAND Experiments, Phys. Rev. C 72, 055502 (2005)
• Evidence for an oscillatory signature in atmospheric neutrino oscillation. Apr 2004. Published in Phys. Rev. Lett. 93, 101801 (http://arxiv.org/pdf/hep-ex/0404034)
• Limit from the Solar and KamLAND Experiments, Phys. Rev. C 72, 055502 (2005)
• Double Chooz Letter of Intent (http://doublechooz.in2p3.fr/0405032.pdf)
• M.C. Gonzalez-Garcia, Y. Nir, "A review of evidence of neutrino masses and the implications (http://arxiv.org/abs/hep-ph/0202058)", Reviews of Modern Physics 75 (2003) p.345-402.
• The Neutrino Oscillation Industry page (http://www.hep.anl.gov/ndk/hypertext/nuindustry.html): An index of experiments and subjects related to neutrino mass and oscilations
• NEUTRINO UNBOUND  (http://www.nu.to.infn.it): On-line review and e-archive on Neutrino Physics and Astrophysics
• Maury Goodman, "The Neutrino Oscillation Industry (http://neutrinooscillation.org/)" (2005). (Provides links to many other neutrino oscillation websites.)
• Nova: The Ghost Particle (http://www.pbs.org/wgbh/nova/neutrino/): Documentary on US public television from WGBH
• SNEWS (http://snews.bnl.gov): Using neutrino detectors to receive early warning of supernovae
• Ultimate neutrino page (http://cupp.oulu.fi/neutrino)
• Super-Kamiokande neutrino detector finds neutrino mass (http://physicsweb.org/articles/news/2/6/2/1)
• Measuring the density of the earth's core with neutrinos (http://www.newscientist.com/channel/fundamentals/mg18524885.900)
• The IceCube Neutrino Observatory Web site (http://icecube.wisc.edu)
• GENIE Neutrino MC Generator (http://www.genie-mc.org)
• Fermilab's MINOS experiment (http://www.fnal.gov/pub/inquiring/physics/neutrino/)
• Ray Davis Press release (http://www.bnl.gov/bnlweb/raydavis/1967PR.pdf)
• Bulletin Board - "Solar neutrinos are counted at Brookhaven" (http://www.bnl.gov/bnlweb/raydavis/BB_sept1967.pdf)
• Laboratori Nazionali del Gran Sasso (http://www.lngs.infn.it)
• GALLEX (http://www.mpi-hd.mpg.de/nuastro/gallex.html)
• John Bahcall Website (http://www.sns.ias.edu/~jnb/)
• SNO Home page (http://www.sno.phy.queensu.ca/)
• Super-Kamiokande neutrino detector finds neutrino mass (http://physicsweb.org/articles/news/2/6/2/1)
• Measuring the density of the earth's core with neutrinos (http://www.newscientist.com/channel/fundamentals/mg18524885.900)
• The IceCube Neutrino Observatory Web site (http://icecube.wisc.edu)
• MiniBooNE (http://www-boone.fnal.gov)
• Images of Super-Kamiokande events from tscan (http://www.ps.uci.edu/~tomba/sk/tscan/pictures.html)
• Evidence for oscillation of atmospheric neutrinos (http://xxx.lanl.gov/abs/hep-ex/9807003)
• Super-Kamiokande (http://www.phys.washington.edu/~superk/)
• All neutrino data in one plot (http://hitoshi.berkeley.edu/neutrino/alltan2006.eps)

See also

- PowerPedia main index
- PESWiki home page