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## PowerPedia:Neutrino

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: See also Directory:Neutrinos

The neutrino is an There was an error working with the wiki: Code[1] and is therefore a There was an error working with the wiki: Code[2]. Although they had been considered massless for many years, recent experiments (see There was an error working with the wiki: Code[3] nor the Electromagnetism force, but only through the There was an error working with the wiki: Code[4] and Gravity.

Because the There was an error working with the wiki: Code[5] 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 There was an error working with the wiki: Code[6]), it would take approximately one There was an error working with the wiki: Code[70] (~1016m) of There was an error working with the wiki: Code[71] to block half of them. Detection of neutrinos is therefore challenging, requiring large detection volumes or high intensity artificial neutrino beams.

#### Types of neutrinos

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|+Neutrinos in the Standard Model of elementary particles

|-style="background:#efefef"

!Fermion

!Symbol

!Mass

|-

!colspan="3" style="background:#ffdead"|Generation 1 (electron)

|-

|style="background:#efefef"| Neutrino

| \nu_e\,

| &lt 2.2 eV

|-

|style="background:#efefef"| There was an error working with the wiki: Code[7]

| \bar{\nu}_e\,

| &lt 2.2 eV

|-

!colspan="3" style="background:#ffdead"|Generation 2 (muon)

|-

|style="background:#efefef"| Neutrino

| \nu_\mu\,

| &lt 170 keV

|-

|style="background:#efefef"| There was an error working with the wiki: Code[8]

| \bar{\nu}_\mu\,

| &lt 170 keV

|-

!colspan="3" style="background:#ffdead"|Generation 3 (tau)

|-

|style="background:#efefef"| Neutrino

| \nu_{\tau}\,

| &lt 15.5 MeV

|-

|style="background:#efefef"| There was an error working with the wiki: Code[9]

| \bar{\nu}_\tau\,

| &lt 15.5 MeV

|}

|-

|

Notes:Since neutrino flavor There was an error working with the wiki: Code[72] are not the same as neutrino mass eigenstates (see There was an error working with the wiki: Code[73]), the given masses are actually mass There was an error working with the wiki: Code[74]s. If the mass of a neutrino could be measured directly, the value would always be that of one of the three mass eigenstates: ?1, ?2, and ?3. 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.

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There are three known types (There was an error working with the wiki: Code[10]) of neutrinos: There was an error working with the wiki: Code[11] neutrino ??, named after their partner There was an error working with the wiki: Code[12]. 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 The possibility of <pesn type=. The correspondence between the six There was an error working with the wiki: Code[75]s in the There was an error working with the wiki: Code[76] 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 mechanics phenomenon whereby a There was an error working with the wiki: Code[14] (There was an error working with the wiki: Code[15] to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates. Neutrino oscillation is of There was an error working with the wiki: Code[16] and There was an error working with the wiki: Code[17] interest as observation of the phenomenon implies that the neutrino has a non-zero mass, which is not part of the original There was an error working with the wiki: Code[77] of There was an error working with the wiki: Code[78].

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 There was an error working with the wiki: Code[18] (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 "There was an error working with the wiki: Code[79]". 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 There was an error working with the wiki: Code[80] are in fact the same particle, a hypothesis first proposed by the Italian physicist There was an error working with the wiki: Code[81].

##### 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 There was an error working with the wiki: Code[19] There was an error working with the wiki: Code[20] neutrinos using a There was an error working with the wiki: Code[21] detectors confirmed the deficit, but neutrino oscillations weren't conclusively identified as the source of the deficit until the There was an error working with the wiki: Code[82] provided clear evidence of neutrino flavor change.

Solar neutrinos have energies below 20&nbspThere was an error working with the wiki: Code[83] and travel an There was an error working with the wiki: Code[84] between the source and detector. At energies above 5&nbspMeV, solar neutrino oscillation actually takes place in the sun through a resonance known as the There was an error working with the wiki: Code[85], a different process from the vacuum oscillations described later in this article.

##### Atmospheric neutrino oscillation

Large detectors such as There was an error working with the wiki: Code[22], There was an error working with the wiki: Code[23], and There was an error working with the wiki: Code[86] observed a deficit in the ratio of the flux of muon to electron flavor atmospheric neutrinos. The There was an error working with the wiki: Code[87] 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 There was an error working with the wiki: Code[88].

##### Reactor neutrino oscillations

Many experiments have searched for oscillations of electron There was an error working with the wiki: Code[24]-neutrinos produced at There was an error working with the wiki: Code[89]. A high precision observation of reactor neutrino oscillation has been made by the There was an error working with the wiki: Code[90] 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 There was an error working with the wiki: Code[91] 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 There was an error working with the wiki: Code[92] 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 There was an error working with the wiki: Code[93] experiment is currently being tested by There was an error working with the wiki: Code[94]. Results from MiniBooNE are expected in the fall of 2006.

##### Maki-Nakagawa-Sakata matrix

It is generally accepted that neutrino oscillations are due to a mismatch between the flavor and mass There was an error working with the wiki: Code[95]s of neutrinos. The relationship between these eigenstates is given by

: \left| \nu_{\alpha} \right\rangle = \sum_{i} U_{\alpha i}^{} \left| \nu_{i} \right\rangle\,

: \left| \nu_{i} \right\rangle = \sum_{\alpha} U_{\alpha i} \left| \nu_{\alpha} \right\rangle,

where

\left| \nu_{\alpha} \right\rangle is a neutrino with definite flavor. ? = e (electron), \mu (muon) or \tau (tau).

\left| \nu_{i} \right\rangle is a neutrino with definite mass. i = 1, 2, 3.

{}^ represents a There was an error working with the wiki: Code[96] (for There was an error working with the wiki: Code[97]s, the complex conjugate should be dropped from the first equation, and added to the second).

U_{\alpha i} represents the Maki-Nakagawa-Sakata matrix (also called the "MNS matrix", "neutrino mixing matrix", or sometimes "PMNS matrix" to include There was an error working with the wiki: Code[25]). It is the equivalent of the There was an error working with the wiki: Code[98] for There was an error working with the wiki: Code[99]s. If this matrix were the There was an error working with the wiki: Code[100], 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 ?1 and ?2 are non-zero only if neutrinos are There was an error working with the wiki: Code[101]s (whether or not they are is unknown), and do not enter into oscillation phenomena regardless. If There was an error working with the wiki: Code[102] occurs, these factors influence its rate. The phase factor ? is non-zero only if neutrino oscillation violates There was an error working with the wiki: Code[103]. 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 \left| \nu_{i} \right\rangle are mass eigenstates, their propagation can be described by There was an error working with the wiki: Code[104] solutions of the form

: |\nu_{i}(t)\rangle = e^{ -i ( E t - \vec{p} \cdot \vec{x}) }|\nu_{i}(0)\rangle,

where

quantities are expressed in There was an error working with the wiki: Code[105] (c = \hbar = 1)

E is the Energy of the particle,

t is the time from the start of the propagation,

\vec{p} is the 3-dimensional There was an error working with the wiki: Code[106],

\vec{x} 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 There was an error working with the wiki: Code[107]. 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 There was an error working with the wiki: Code[108] ? 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 There was an error working with the wiki: Code[109] 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 ? will later be observed as having flavor ?. It is convenient to plug in the oscillation parameters since:

The mass differences, ?m2, are known to be on the order of 1eV2

Oscillation distances, L, in modern experiments are on the order of There was an error working with the wiki: Code[110]

Neutrino energies, E, in modern experiments are typically on order of GeV.

If there is no There was an error working with the wiki: Code[111] (? 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 There was an error working with the wiki: Code[112], it produces:

:P_{\alpha\rightarrow\beta, \alpha\neq\beta} = \sin^{2}2\theta \sin^{2}\left( 1.267 \frac{\Delta m^2 L}{E} \frac{\rm GeV}{\rm eV^{2}\,\rm km}\right)

This formula is often appropriate for discussing the transition ?? ? ?? in atmospheric mixing, since the electron neutrino plays almost no role in this case. It is also appropriate for the solar case of ?e ? ?x, where ?x is a superposition of ?? and ??. These approximations are possible because the mixing angle ?13 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 There was an error working with the wiki: Code[113] 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 (\sin^{2} 2\theta = 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 There was an error working with the wiki: Code[114], 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 \sin^2 2\theta. The frequency of the oscillation is controlled by \Delta m^2.

##### 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:

\sin^2 \theta_{13} = 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.)

\sin^2 \theta_{23} = 0.95. (It may turn out to be exactly one.)

\sin^2 \theta_{12} = 0.86.

\delta = 0. (If it is actually large, these probabilities will be somewhat distorted and different for neutrinos and antineutrinos.)

\Delta m^2_{12} = 8 \times 10^{-5} {\rm eV}^2.

\Delta m^2_{23} \approx \Delta m^2_{13} = 2.4 \times 10^{-3} {\rm eV}^2.

##### Observed values of oscillation parameters

\sin^2(2\theta_{13}) < 0.19^{}_{} at 90% confidence level (\theta_{13} < 13^\circ)

\tan^2(\theta_{12}) = 0.45^{+0.09}_{-0.07}. This corresponds to \theta_{12}=\theta_{\rm sol}={33.9^\circ}^{+2.4^\circ}_{-2.2^\circ} ("sol" stands for solar)

\sin^2(2\theta_{23}) = 1^{+0}_{-0.1}, corresponding to \theta_{23}=\theta_{\rm atm}=45\pm 7^\circ ("atm" for atmospheric)

\Delta m^2_{21}=\Delta m^2_{\rm sol}= 8.0^{+0.6}_{-0.4}\cdot 10^{-5} {\rm eV}^2

\Delta m^2_{31} \approx \Delta m^2_{32} = \Delta m^2_{\rm atm}= 2.4^{+0.6}_{-0.5}\cdot 10^{-3} {\rm eV}^2

? is unknown

Solar neutrino experiments combined with There was an error working with the wiki: Code[26] have measured the so-called solar parameters \Delta m^2_{\rm sol} and \sin^2\theta_{\rm sol}. Atmospheric neutrino experiments such as There was an error working with the wiki: Code[115] together with the K2K first long baseline accelerator neutrino experiment have determined the so-called atmospheric parameters \Delta m^2_{\rm atm} and \sin^2 2 \theta_{\rm atm}. An additional experiment There was an error working with the wiki: Code[116] is expected to reduce the experimental errors significantly thereby increasing precision. For atmospheric neutrinos (where the relevant difference of masses is about \Delta m^2 =2.5\times 10^{-3}\mbox{ eV}^2 and the typical energies are E\approx 1\,\mbox{ GeV}), oscillations become visible for neutrinos travelling several hundred km, which means neutrinos that reach the detector from below the horizon. From atmospheric and There was an error working with the wiki: Code[117] 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 There was an error working with the wiki: Code[118] 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, There was an error working with the wiki: Code[27]). However, only left-handed neutrinos have been observed so far.

Neutrinos may have another source of mass through the There was an error working with the wiki: Code[119]. 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 There was an error working with the wiki: Code[120]. Note that, in the Standard Model there is just one fundamental mass scale (which can be taken as the scale of SU(2)_L\times U(1)_Y 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 There was an error working with the wiki: Code[28].

##### Other sources

There are other ideas for the origin of neutrino mass, such as There was an error working with the wiki: Code[121] violating There was an error working with the wiki: Code[122], which proposes that the masses for the neutrinos come from interactions with There was an error working with the wiki: Code[123] and There was an error working with the wiki: Code[124], 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 There was an error working with the wiki: Code[29] (see There was an error working with the wiki: Code[30].

The name neutrino was coined by There was an error working with the wiki: Code[31] name of the There was an error working with the wiki: Code[125]. (Neutrone in Italian means big and neutral, and neutrino means small and neutral.)

In 1962 There was an error working with the wiki: Code[32], was discovered in 1975 at the There was an error working with the wiki: Code[33], 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 There was an error working with the wiki: Code[126] collaboration at There was an error working with the wiki: Code[127], making it the latest particle of the There was an error working with the wiki: Code[128] to have been directly observed.

A practical method for investigating neutrino masses (that is, flavour oscillation) was first suggested by There was an error working with the wiki: Code[129] in 1957 using an analogy with the neutral There was an error working with the wiki: Code[130] 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 There was an error working with the wiki: Code[131]) noted that flavour oscillations can be modified when neutrinos propagate through matter. This so-called There was an error working with the wiki: Code[132] is important to understand neutrinos emitted by the Sun, which pass through its dense atmosphere on their way to detectors on Earth.

#### Mass

The There was an error working with the wiki: Code[133] 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 There was an error working with the wiki: Code[134] requires neutrinos to have non-zero masses.

The strongest upper limit on the masses of neutrinos comes from There was an error working with the wiki: Code[34]: the There was an error working with the wiki: Code[35]. If the total energy of all three types of neutrinos exceeded an average of 50 There was an error working with the wiki: Code[135]s 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, There was an error working with the wiki: Code[136]s and the There was an error working with the wiki: Code[137]. These indicate that the sum of the neutrino masses must be less than 0.3 There was an error working with the wiki: Code[135]s (Goobar, 2006).

In 1998, research results at the There was an error working with the wiki: Code[139] 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 There was an error working with the wiki: Code[140] in 2005: ?m212&nbsp=&nbsp0.000079&nbspeV2

In 2006, the There was an error working with the wiki: Code[36] . 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 There was an error working with the wiki: Code[37] ( 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 There was an error working with the wiki: Code[38] is observed for either particle. These are the only chiralities included in the There was an error working with the wiki: Code[141] 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 There was an error working with the wiki: Code[142] - see There was an error working with the wiki: Code[143] ), 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_{\nu}/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} ). [http://www.nu.to.infn.it/pap/0102320/

#### Neutrino sources

##### Artificially produced neutrinos

There was an error working with the wiki: Code[39] 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: There was an error working with the wiki: Code[144]-235, There was an error working with the wiki: Code[144]-238, There was an error working with the wiki: Code[146]-239 and There was an error working with the wiki: Code[146]-241. An average plant may generate over 1020 anti-neutrinos per second.

Some There was an error working with the wiki: Code[40] of the decaying particle the neutrinos are produced as a beam rather than isotropically.

There was an error working with the wiki: Code[148]s also produce very large numbers of neutrinos. There was an error working with the wiki: Code[149] and There was an error working with the wiki: Code[150] 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 There was an error working with the wiki: Code[151]. In particular, the decay chains of There was an error working with the wiki: Code[152]-238 and There was an error working with the wiki: Code[153]-232 isotopes, as well as There was an error working with the wiki: Code[154]-40, include There was an error working with the wiki: Code[155]s 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 There was an error working with the wiki: Code[156] 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 There was an error working with the wiki: Code[157]s with atomic nuclei in the There was an error working with the wiki: Code[158], 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 There was an error working with the wiki: Code[159] powering the There was an error working with the wiki: Code[160] and other stars.

There was an error working with the wiki: Code[161] and There was an error working with the wiki: Code[162] were jointly awarded the 2002 There was an error working with the wiki: Code[163] for their work in the detection of cosmic neutrinos.

##### Other astrophysical phenomena

Neutrinos are an important product of There was an error working with the wiki: Code[41] 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 There was an error working with the wiki: Code[164] were detected. It is thought that neutrinos would also be produced from other events such as the collision of There was an error working with the wiki: Code[165]s.

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 There was an error working with the wiki: Code[42] 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 There was an error working with the wiki: Code[166] 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 There was an error working with the wiki: Code[167]. 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 There was an error working with the wiki: Code[168] 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 There was an error working with the wiki: Code[169] left over from the There was an error working with the wiki: Code[170] includes a background of low energy neutrinos. In the 1980s it was proposed that these may be the explanation for the There was an error working with the wiki: Code[171] 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 There was an error working with the wiki: Code[43] structures that we see.

Further, these same galaxies and There was an error working with the wiki: Code[44] 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 There was an error working with the wiki: Code[45]. 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 1.9 K = 1.7 \times 10^{-4} {\rm eV}. 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) There was an error working with the wiki: Code[172]s.

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. There was an error working with the wiki: Code[46] and There was an error working with the wiki: Code[47] 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 There was an error working with the wiki: Code[173] 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 There was an error working with the wiki: Code[48] near There was an error working with the wiki: Code[174], containing 520 There was an error working with the wiki: Code[175]s (470 There was an error working with the wiki: Code[176]s) of fluid, made the first measurement of the deficit of electron neutrinos from the sun (see There was an error working with the wiki: Code[177]). A similar detector design uses a There was an error working with the wiki: Code[178] ? There was an error working with the wiki: Code[179] transformation which is sensitive to lower energy neutrinos. This latter method is nicknamed the "There was an error working with the wiki: Code[180]" 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 There was an error working with the wiki: Code[49] produced by charged particles moving through a medium faster than the There was an error working with the wiki: Code[50]. In these detectors, a large volume of clear material (e.g., water) is surrounded by light-sensitive There was an error working with the wiki: Code[51]) recorded the neutrino burst from There was an error working with the wiki: Code[181]. The largest such detector is the water-filled Super-Kamiokande.

The There was an error working with the wiki: Code[182] (SNO) uses There was an error working with the wiki: Code[183]. 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 There was an error working with the wiki: Code[184] detector employs pure mineral oil as its detection medium. Mineral oil is a natural There was an error working with the wiki: Code[185], 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 There was an error working with the wiki: Code[186] 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 There was an error working with the wiki: Code[187] 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 (\mu^+ versus \mu^-) 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 There was an error working with the wiki: Code[188]s 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

{| border="1" cellpadding="2" cellspacing="0" align="center" style=" margin: 0 0 1em 1em font-size:"|

|-style="background:#9bb0ff"

! colspan="6" | General data

|-

|-style="background:#cad7ff"

! Abbreviation

! Experiment

! Place

! homepage

! Cooperation

! scheduled to start

|-

|BOREXINO || BORon EXperiment|| Gran Sasso, There was an error working with the wiki: Code[189] || [http://borex.lngs.infn.it/ ||LNGS, INFN||

|-

|CLEAN ||Cryogenic Low-Energy Astrophysics with Neon|| || ([[PDF])||LANL||futureexperiment

|-

| GALLEX ||GALLium EXperiment ||Gran Sasso, Italy|| ||LNGS, INFN ||<pesn type=

|-

| GNO || Gallium Neutrino Observatory ||Gran Sasso, Italy|| ||LNGS, INFN ||[[1998] -

|-

| HERON || Helium Roton Observation of Neutrinos|| || Berkeley National Laboratory|LBNL]||

|-

|HOMESTAKE–CHLORINE ||Homestake chlorine experiment||Homestake mine, There was an error working with the wiki: Code[52] - There was an error working with the wiki: Code[191]

|-

|HOMESTAKE–IODINE ||Homestake iodine experiment||Homestake mine, South Dakota, USA|| ||<pesn type= -

|-

|ICARUS ||Imaging Cosmic And Rare Underground Signal ||Gran Sasso, There was an error working with the wiki: Code[192]||||[[CERN] to CNGS ||

|-

|Kamiokande|| Kamioka Nucleon Decay Experiment||Kamioka, There was an error working with the wiki: Code[54]

|-

|LENS||Low Energy Neutrino Spectroscopy|| ||<pesn type= [http://laser.physics.sunysb.edu/~thomas/report1/lens_report.html"></pesn>||LANL ||

|-

|MOON ||Molybdenum Observatory Of Neutrinos|| There was an error working with the wiki: Code[193], There was an error working with the wiki: Code[194]||http://ewi.npl.washington.edu/moon/ || ||

|-

|SAGE ||Soviet–American Gallium Experiment||There was an error working with the wiki: Code[55]

|-

|SNO ||There was an error working with the wiki: Code[56], There was an error working with the wiki: Code[57]||There was an error working with the wiki: Code[195] (- 2006)

|-

|SK ||There was an error working with the wiki: Code[58]|| <pesn type= || ||There was an error working with the wiki: Code[196]

|-

|UNO ||There was an error working with the wiki: Code[197]||Henderson mine, Colorado|| || [[National Underground Science Laboratory|NUSL]||future experiment

|-

|IceCube || There was an error working with the wiki: Code[59], There was an error working with the wiki: Code[198] || http://icecube.wisc.edu/ || ||futureexperiment

|}

##### Technical data

{| border="1" cellpadding="2" cellspacing="0" align="center" style=" margin: 0 0 1em 1em font-size:"|

|-style="background:#9bb0ff"

! colspan="8" | Technical data

|-

|-style="background:#cad7ff"

! Abbreviation

! Sensitivity (1)

! Sensitivity (2)

! Induced reaction

! Type ofreaction

! Detector

! Type of detector

! thresholdenergy

|-

|BOREXINO||lS ||E||vx + e? ? vx + e? ||ES|| H2O + PC+PPOPC=C6H3(CH3)3PPO=C15H11NO]||liquid scintillation||250–665 keV

|-

|CLEAN || lS, SN, WIMP ||E|| vx + e? ? vx + e?ve + 20Ne ? ve + 20Ne ||ESES|| 10 t liquid There was an error working with the wiki: Code[60]||scintillation|| ???

|-

| GALLEX || S ||E ||ve+71Ga ? 71Ge+e? ||CC ||GaCl3 (30 t There was an error working with the wiki: Code[61])||radiochemical ||233.2 keV

|-

| GNO || lS ||E|| ve+71Ga ? 71Ge+e? ||CC || GaCl3 (30 t There was an error working with the wiki: Code[62])||radiochemical ||233.2 keV

|-

| HERON || lS ||mainly E||ve + e? ? ve + e?|| NC||There was an error working with the wiki: Code[63]|| 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?|| ESCC||NaI|| radiochemical||789 keV

|-

|ICARUS || S, ATM, GSN ||E, M, T|| ve + e? ? ve + e?||ES||liquid There was an error working with the wiki: Code[64] ||Cherenkov||5900 keV

|-

|Kamiokande|| S, ATM||E ||ve + e? ? ve + e? ||ES|| Water||There was an error working with the wiki: Code[65]||7500 keV

|-

|LENS|| lS||E ||ve + 176Yb ? 176Lu+e? ||CC||In(acc)3http://www.mpi-hd.mpg.de/nubis/lens/images/inacac.html||scintillation||120 keV

|-

|MOON || lS, lSN ||E||ve+100Mo ? 100Tc+e?||CC ||There was an error working with the wiki: Code[66] (1 t) + MoF6 (gas)|| scintillation|| 168 keV

|-

|SAGE || lS ||E||ve+71Ga ? 71Ge+e? ||CC|| GaCl3|| radiochemical||233.2 keV

|-

|There was an error working with the wiki: Code[67] || S, ATM, GSN ||E, M, T||ve + 21D ?p++p++e? vx + 21D ?vx+no+p+ ve + e? ? ve + e?||CCNCES|| 1000 t D2O ||heavy water Cherenkov||6.75 MeV

|-

|There was an error working with the wiki: Code[199] || S, ATM, GSN ||E, M, T||ve + e? ? ve + e? ve + no ? e? + p+ve + p+ ? e+ + no ||ESCC ||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

There was an error working with the wiki: Code[200] (BNL)

Conseil Européen pour la Recherche Nucleaire (There was an error working with the wiki: Code[201])

CERN Neutrino to Gran Sasso (There was an error working with the wiki: Code[202])

There was an error working with the wiki: Code[203] (INFN)

There was an error working with the wiki: Code[204] (LANL)

There was an error working with the wiki: Code[205] (LNGS)

There was an error working with the wiki: Code[206] (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 There was an error working with the wiki: Code[207], in the form of fast-moving protons and atomic nuclei, are not able to travel more than about 100 There was an error working with the wiki: Code[208]s due to the There was an error working with the wiki: Code[209]. Neutrinos can travel this distance, and greater distances, with very little attenuation.

The galactic core of the There was an error working with the wiki: Code[210] 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 There was an error working with the wiki: Code[211]e, 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 There was an error working with the wiki: Code[212]). 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 There was an error working with the wiki: Code[213] 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 There was an error working with the wiki: Code[214] of particle physics. For example, one would expect that if there is a fourth class of There was an error working with the wiki: Code[215]s 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 There was an error working with the wiki: Code[216] effects. Because they are not affected by either the There was an error working with the wiki: Code[217] 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

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neutrino physicists

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#### References and external articles

Super-Kamiokande Super-Kamiokande at UC Irvine

G. A. Tammann, F. K. Thielemann, D. Trautmann Opening new windows in observing the Universe 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. (There was an error working with the wiki: Code[69] 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"

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

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

Limit from the Solar and KamLAND Experiments, Phys. Rev. C 72, 055502 (2005)

Double Chooz Letter of Intent

M.C. Gonzalez-Garcia, Y. Nir, "A review of evidence of neutrino masses and the implications", Reviews of Modern Physics 75 (2003) p.345-402.

The Neutrino Oscillation Industry page: An index of experiments and subjects related to neutrino mass and oscilations

NEUTRINO UNBOUND: On-line review and e-archive on Neutrino Physics and Astrophysics

Maury Goodman, "The Neutrino Oscillation Industry" (2005). (Provides links to many other neutrino oscillation websites.)

Nova: The Ghost Particle: Documentary on US public television from WGBH

SNEWS: Using neutrino detectors to receive early warning of supernovae

Ultimate neutrino page

Super-Kamiokande neutrino detector finds neutrino mass

Measuring the density of the earth's core with neutrinos

The IceCube Neutrino Observatory Web site

GENIE Neutrino MC Generator

Fermilab's MINOS experiment

Ray Davis Press release

Bulletin Board -

Laboratori Nazionali del Gran Sasso

GALLEX

John Bahcall Website

SNO Home page

Super-Kamiokande neutrino detector finds neutrino mass

Measuring the density of the earth's core with neutrinos

The IceCube Neutrino Observatory Web site

MiniBooNE

Images of Super-Kamiokande events from tscan

Evidence for oscillation of atmospheric neutrinos

Super-Kamiokande

All neutrino data in one plot

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