Neutrino
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{{Infobox Particle
| bgcolour =
| name = Neutrino
| image =
| caption =
| num_types = 3 – electron, muon and tau
| composition = [[Elementary particle]]
| family = [[Fermion]]
| group = [[Lepton]]
| generation =
| interaction = [[weak interaction]] and [[gravitation]]
| particle =
| antiparticle = [[Antineutrino]] (possibly identical to the neutrino)
| theorized = 1930 by [[Wolfgang Pauli]]
| discovered = 1956 by [[Clyde Cowan]], [[Frederick Reines]], F. B. Harrison, H. W. Kruse, and A. D. McGuire.
| symbol = {{SubatomicParticle|Electron neutrino}}, {{SubatomicParticle|Muon neutrino}}, {{SubatomicParticle|Tau neutrino}}
| mass =
| decay_time =
| decay_particle =
| electric_charge = 0
| color_charge = 0
| spin = ½
| num_spin_states = ?
}}
'''Neutrinos''' are [[elementary particle]]s that travel close to the [[speed of light]], lack an [[electric charge]], are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. [[As of 1999]], it is believed neutrinos have a minuscule, but nonzero [[mass]]. They are usually denoted by the Greek letter <math>\nu_{}^{}</math> ([[Nu (letter)|nu]]).
Neutrinos are created as a result of certain types of [[radioactive decay]] or nuclear reactions such as those that take place in the [[Sun]], in [[nuclear reactor]]s, or when [[cosmic ray]]s hit atoms. There are three types, or "[[Flavour (particle physics)|flavors]]", of neutrinos: ''electron neutrinos'', ''muon neutrinos'' and ''tau neutrinos''; each type also has an [[antimatter]] partner, called an [[antineutrino]]. Electron neutrinos or antineutrinos are generated whenever [[neutron]]s change into [[proton]]s or vice versa, the two forms of [[beta decay]]. Interactions involving neutrinos are generally mediated by the [[weak interaction|weak force]].
Most neutrinos passing through the Earth emanate from the sun, and more than 50 trillion solar electron neutrinos pass through the human body every second<ref>[http://web.mit.edu/newsoffice/2007/neutrino.html MIT News Office, "Experiment confirms famous physics model" April 18, 2007]</ref>.
==History==
[[Image:First neutrino observation.jpg|200px|thumbnail|Observation of a neutrino hitting a proton in a [[bubble chamber]]. The collision occurred at the point where three tracks emanate on the right of the photograph.]]
The neutrino was first postulated in December 1930 by [[Wolfgang Pauli]] to preserve [[conservation of energy]], [[conservation of momentum]], and [[conservation of angular momentum]] in [[beta decay]], the decay of a [[neutron]] into a [[proton]], an [[electron]] and an antineutrino. Pauli theorized that an undetected particle was carrying away the observed difference between the [[energy]], [[momentum]], and [[angular momentum]] of the initial and final particles.
The current name ''neutrino'' was coined by [[Enrico Fermi]], who developed the first theory describing neutrino interactions, as a [[pun]] on ''neutrone'', the [[Italian language|Italian]] name of the [[neutron]]: ''neutrone'' seems to use the ''-one'' suffix (even though it is a complete word, not a compound), which in Italian indicates a large object, whereas ''-ino'' indicates a small one.
In 1942 [[Kan-Chang Wang]] first proposed to use beta-capture to experimentally detect neutrinos.<ref>{{cite journal
| first = Kan Chang
| last = Want
| year = 1942
| month = Jan
| title = A Suggestion on the Detection of the Neutrino
| journal = [[Physical Review]]
| volume = 61
| issue = 1-2
| pages = 97
| id =
| url = http://prola.aps.org/abstract/PR/v61/i1-2/p97_1
| doi = 10.1103/PhysRev.61.97}}</ref> 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 (journal)|Science]]'', a result that was rewarded with the [[Nobel Prize in Physics|1995 Nobel Prize]]. In this experiment, now known as the [[neutrino experiment]], neutrinos created in a nuclear reactor by beta decay were shot into protons producing [[neutron]]s and [[positron]]s both of which could be detected. It is now known that both the proposed and the observed particles were antineutrinos.
In 1962 [[Leon M. Lederman]], [[Melvin Schwartz]] and [[Jack Steinberger]] showed that more than one type of neutrino exists by first detecting interactions of the [[muon]] neutrino (already hypothesised with the name ''neutretto''<ref>{{cite arXiv |author=Ivan V. Anicin |authorlink= |eprint=0503172 |title=The Neutrino - Its Past, Present and Future |class=physics |year=2005 |version=v1 |accessdate=2008-06-09 }}</ref>), which earned them the [[Nobel Prize in Physics|1988 Nobel Prize]]. When a third type of [[lepton]], the [[tau lepton|tau]], was discovered in 1975 at the [[SLAC|Stanford Linear Accelerator]], it too was expected to have an associated neutrino. First evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay leading to the discovery of the neutrino. The first detection of tau neutrino interactions was announced in summer of 2000 by the [[DONUT]] collaboration at [[Fermilab]], making it the latest particle of the [[Standard Model]] to have been directly observed; its existence had already been inferred by both theoretical consistency and experimental data from [[LEP]].
Starting in the late 1960s, several experiments found that the number of electron neutrinos arriving from the sun was between one third and one half the number predicted by the [[Standard Solar Model]], a discrepancy which became known as the [[solar neutrino problem]] and remained unresolved for some thirty years.
The Standard Model of particle physics assumes massless neutrinos that don't change flavor. However, nonzero neutrino mass and accompanying flavor oscillation remained a possibility.
A practical method for investigating neutrino masses (that is, flavor oscillation) was first suggested by [[Bruno Pontecorvo]] in 1957 using an analogy with the neutral [[kaon]] system; over the subsequent 10 years he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 [[Stanislav Mikheyev]] and [[Alexei Smirnov]] (expanding on 1978 work by [[Lincoln Wolfenstein]]) noted that flavor oscillations can be modified when neutrinos propagate through matter. This so-called [[MSW effect]] is important to understand neutrinos emitted by the Sun, which pass through its dense atmosphere on their way to detectors on Earth.
Starting in 1998, experiments began to show that solar and atmospheric neutrinos change flavors (see [[Super-Kamiokande]], [[Sudbury Neutrino Observatory]]). This resolved the solar neutrino problem: the electron neutrinos produced in the sun had partly changed into other flavors which the experiments could not detect.
Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor
conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations.
Especially relevant in this context are the reactor experiment [[KamLAND]] and the accelerator
experiments such as [[MINOS]]. The KamLAND experiment has indeed identified oscillations as the neutrino flavor conversion mechanism involved in the solar electron neutrinos. Similarly MINOS confirms the oscillation of atmospheric neutrinos and gives a better determination of the mass squared splitting (Maltoni, 2004).
[[Raymond Davis Jr.]] and [[Masatoshi Koshiba]] were jointly awarded the 2002 [[Nobel Prize in Physics]]. Ray Davis for his pioneer work on cosmic neutrinos and Koshiba for the first real time observation of supernova neutrinos. The detection of solar neutrinos, and of neutrinos of [[SN 1987A]] [[supernova]] in 1987 marked the beginning of [[neutrino astronomy]].
==Properties==
The neutrino has half-integer [[spin (physics)|spin]] (<math>\begin{matrix}\frac{1}{2}\hbar\end{matrix}</math>) and is therefore a [[fermion]]. Because it is an electrically neutral [[lepton]], the neutrino interacts neither by way of the [[strong interaction|strong]] nor the [[electromagnetism|electromagnetic]] force, but only through the [[weak nuclear force|weak force]].
An experiment done by [[C. S. Wu]] at [[Columbia University]] showed that neutrinos always have left-handed [[chirality (physics)|chirality]].
Detection of neutrinos is challenging and often requires large detection volumes or high intensity artificial neutrino beams.
To date, not very much is experimentally established concerning the interactions of neutrinos with matter.
===Popular misconception===
It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. For this reason, in early experiments a special reaction channel was chosen to facilitate the identification: the interaction of an antineutrino with a hydrogen nucleus, which is a single proton. It turns out that an antineutrino would travel about 30 light years through water before it undergoes this specific reaction. This doesn't prove in any way that an antineutrino cannot undergo other reactions with matter. But however it led to popular misconceptions like this one:
''Because the [[cross section (physics)|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 [[electronvolt|MeV]]), it would take approximately one [[light year]] (~10<sup>16</sup> m) of [[lead]] to block half of them.''
For example, in later theories, such as the one describing a so called [[MSW effect]], it is thought that most solar neutrinos are interacting with matter ''inside the sun''.
===Types of neutrinos===
{| border="1" cellpadding="2" cellspacing="0" align="right" style="; margin: 0 0 1em 1em; font-size:"
|+Neutrinos in the Standard Model <br />of elementary particles
|-style="background:#efefef;"
!Fermion
!Symbol
!Mass<ref>Since neutrino flavor [[eigenstates]] are not the same as neutrino mass eigenstates (see [[neutrino oscillation]]), the given masses are actually mass [[expectation value]]s. If the mass of a neutrino could be measured directly, the value would always be that of one of the three mass eigenstates: ν<sub>1</sub>, ν<sub>2</sub>, and ν<sub>3</sub>. 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.</ref>
|-
!colspan="3" style="background:#ffdead;"|Generation 1 (electron)
|-
|style="background:#efefef;"| Electron neutrino
| <math>\nu_e\,</math>
| < 2.2 [[Electronvolt|eV]]
|-
|style="background:#efefef;"| [[Antineutrino|Electron antineutrino]]
| <math>\bar{\nu}_e\,</math>
| < 2.2 eV
|-
!colspan="3" style="background:#ffdead;"|Generation 2 (muon)
|-
|style="background:#efefef;"| Muon neutrino
| <math>\nu_\mu\,</math>
| < 170 [[keV]]
|-
|style="background:#efefef;"| [[Antineutrino|Muon antineutrino]]
| <math>\bar{\nu}_\mu\,</math>
| < 170 keV
|-
!colspan="3" style="background:#ffdead;"|Generation 3 (tau)
|-
|style="background:#efefef;"| Tau neutrino
| <math>\nu_{\tau}\,</math>
| < 15.5 [[MeV]]
|-
|style="background:#efefef;"| [[Antineutrino|Tau antineutrino]]
| <math>\bar{\nu}_\tau\,</math>
| < 15.5 MeV
|}
There are three known types (''[[flavour (particle physics)|flavors]]'') of neutrinos: [[electron]] neutrino ν<sub>e</sub>, [[muon]] neutrino ν<sub>μ</sub> and [[tau lepton|tau]] neutrino ν<sub>τ</sub>, named after their partner [[lepton]]s 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 [[W and Z bosons|Z boson]]. This particle can decay into any light neutrino and its antineutrino, and the more types of light neutrinos available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types (with "light" meaning of less than half the Z mass) is 3.<ref>{{cite journal | url=http://pdg.lbl.gov/2005/listings/lxxx.html | title=Leptons in the 2005 Review of Particle Physics | date=[[2004]] | author=[[Particle Data Group]] (S. Eidelman ''et al.'') | journal=Phys. Lett. B | volume=592 | issue=1 | pages=1–5 | accessdate=2007-11-25}}</ref> The correspondence between the six [[quark]]s in the [[Standard Model]] and the six leptons, among them the three neutrinos, suggests to physicists' intuition that there should be exactly three types of neutrino. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
The possibility of [[Sterile neutrino|''sterile'' neutrinos]] — relatively light 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 hinted by experimental data from the [[LSND]] experiment. However, the currently running [[MiniBooNE]] experiment suggested, until recently, that sterile neutrinos are not required to explain the experimental data,<ref name = "PhysRevD"> {{cite journal
| first = G.
| last = Karagiorgi
| coauthors = A. Aguilar-Arevalo, J. M. Conrad, and M. H. Shaevitz
| year = 2007
| month =
| title = Leptonic CP violation studies at MiniBooNE in the (3+2) sterile neutrino oscillation hypothesis
| journal = [[Physical Review|Phys Rev]] D
| volume = 75
| issue = 013011
| pages = 1–8
| id =
| url = http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVDAQ000075000001013011000001&idtype=cvips&gifs=yes
| doi = 10.1103/PhysRevD.75.013011
}}</ref> although the latest research into this area is on-going and anomalies in the MiniBooNE data may allow for exotic neutrino types, including sterile neutrinos.<ref>{{cite journal
| first = M.
| last = Alpert
| year = 2007
| month = August
| title = Dimensional Shortcuts
| journal = Scientific American
| url = http://www.sciam.com/article.cfm?chanID=sa006&colID=5&articleID=B5CB9C67-E7F2-99DF-3BF7368614D46C5D}}</ref>
===Flavor oscillations===
{{Main|Neutrino oscillation}}
Neutrinos are most often created or detected with a well defined [[Flavour (particle physics)|flavor]] (electron, muon, tau). However, in a phenomenon known as [[neutrino oscillation|neutrino flavor oscillation]], neutrinos are able to oscillate between the three available flavors while they propagate through space. Specifically, this occurs because the neutrino flavor [[Eigenvalue, eigenvector and eigenspace|eigenstates]] are not the same as the neutrino mass eigenstates (simply called 1, 2, 3). This allows for a neutrino that was produced as an electron neutrino at a given location to have a calculable probability to be detected as either a muon or tau neutrino after it has traveled to another location. This [[quantum mechanical]] effect was first hinted by the discrepancy between the number of electron neutrinos detected from the [[Sun]]'s core failing to match the expected numbers, dubbed as the "[[solar neutrino problem]]." In the [[Standard Model]] the existence of flavor oscillations implies a nonzero neutrino mass, because the amount of mixing between neutrino flavors at a given time depends on the differences in their squared-masses.
It is possible that the neutrino and [[antineutrino]] are in fact the same particle, a hypothesis first proposed by the Italian physicist [[Ettore Majorana]]. The neutrino could transform into an antineutrino (and vice versa) by flipping the orientation of its [[Spin (physics)|spin]] state.
This change in spin would require the neutrino and antineutrino to have nonzero mass, and therefore travel slower than light, because such a spin flip caused only by a change in point of view, can take place only if [[inertial frame of reference|inertial frames of reference]] exist that move faster than the particle: such a particle has a spin of one orientation when seen from a frame which moves slower than the particle, but the opposite spin when observed from a frame that moves faster than the particle.
===Speed===
Before the idea of neutrino oscillations came up, it was generally assumed that neutrinos travel at the [[speed of light]]. The question of neutrino [[velocity]] is closely related to their [[mass]]. According to [[relativity]], if neutrinos are massless, they must travel at the speed of light. However, if they carry a mass, they cannot reach the speed of light.
In the early 1980s, first measurements of neutrino speed were done using pulsed pion beams (produced by pulsed proton beams hitting a target). The pions decayed producing neutrinos, and the neutrino interactions observed within a time window in a detector at a distance were consistent with the speed of light. This measurement has been repeated using the [[MINOS]] detectors, which found the speed of 3 GeV neutrinos to be (1 − (5.1 ± 2.9)×10<sup>−5</sup>) times the speed of light. While the central value is lower than the speed of light, the uncertainty is great enough that it is very likely that the true velocity is too close to the speed of light to see the difference. This measurement set an upper bound on the mass of the muon neutrino of 50 MeV at 99% confidence.<ref>Measurement of neutrino velocity with the MINOS detectors and NuMI neutrino beam, Adamson ''et al.'', {{arxiv|archive=hep-ex|id=0706.0437}}</ref>
The same observation was made, on a somewhat larger scale, with [[supernova 1987a]]. The neutrinos from the supernova were detected within a time window that was consistent with a speed of light for the neutrinos. So far, the question of neutrino masses cannot be decided based on measurements of the neutrino speed.
===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 nonzero masses.<ref name = "PhysRevD"> {{cite journal
| first = J.
| last = Schechter
| coauthors = J. W. F. Valle
| year = 1980
| month =
| title = Neutrino Masses in SU(2) x U(1) Theories
| journal = [[Physical Review|Phys Rev]] D
| volume = 22
| issue =
| pages = 2227
| id =
| url = http://www.slac.stanford.edu/spires/find/hep/wwwrefs?key=681288
| doi = 10.1103/PhysRevD.22.2227
}}</ref>
The strongest upper limit on the masses of neutrinos comes from [[physical cosmology|cosmology]]: the [[Big Bang]] model predicts that there is a fixed ratio between the number of neutrinos and the number of [[photon]]s in the [[cosmic microwave background radiation|cosmic microwave background]]. If the total energy of all three types of neutrinos exceeded an average of 50 [[electronvolt]]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, [[galaxy survey]]s and the [[Lyman-alpha forest]]. These indicate that the sum of the neutrino masses must be less than 0.3 electronvolt (Goobar, 2006).
In 1998, research results at the [[Super-Kamiokande]] neutrino detector determined that neutrinos do indeed flavor oscillate, and therefore have mass. The experiment is only sensitive to the difference in the squares of the masses (Mohapatra, 2005).
The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by [[KamLAND]] in 2005: Δm<sub>21</sub><sup>2</sup> = 0.000079 eV<sup>2</sup>
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 Δm<sub>23</sub><sup>2</sup> = 0.003 eV<sup>2</sup>, consistent with previous results from [[Super-Kamiokande|Super-K]].<ref>{{cite web | url=http://www.fnal.gov/pub/presspass/press_releases/minos_3-30-06.html | title=MINOS experiment sheds light on mystery of neutrino disappearance | publisher=[[Fermilab]] | date=[[30 March]] [[2006]] | accessdate=2007-11-25}}</ref>
Currently a number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments. The methods applied involve nuclear beta decay ([[KATRIN]] and MARE) or neutrinoless double beta decay (e.g. GERDA, CUORE/Cuoricino, NEMO 3 and others).
===Handedness===
Experimental results show that (nearly) all produced and observed neutrinos have left-handed [[Helicity (particle physics)|helicities]] (spins antiparallel to [[Momentum|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 [[chirality (physics)|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 <math>m_{\nu}/E</math>. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small (for example, most solar neutrinos have energies on the order of 100 keV–1 MeV, so the fraction of neutrinos with "wrong" helicity among them cannot exceed 10<sup>-10</sup>).<ref>{{cite web | url=http://pdg.lbl.gov/2006/reviews/numixrpp.pdf | title=Neutrino mass, mixing, and flavor change | author=B. Kayser | date=[[2005]] | publisher=[[Particle Data Group]] | accessdate=2007-11-25}}</ref><ref>{{cite journal | url=http://www.nu.to.infn.it/pap/0102320/ | title=Lepton Numbers in the framework of Neutrino Mixing | last=Bilenky | first=S.M. | coauthors=Giunti, C. | journal=Int. J. Mod. Phys. A | volume=16 | issue=3931 | date=[[2001]] | accessdate=2007-11-25}}</ref>
==Neutrino sources==
===Artificially produced neutrinos===
[[Nuclear reactor]]s are the major source of human-generated neutrinos. Anti-neutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the anti-neutrino flux are: [[uranium]]-235, [[uranium]]-238, [[plutonium]]-239, [[plutonium]]-241 (e.g. the anti-neutrinos emitted during [[beta decay|beta-minus decay]] of their respective fission fragments). The average nuclear fission releases about 200 MeV of energy, of which roughly 6% (or 9 MeV, depending on quoted reference) are radiated away as anti-neutrinos. For a typical nuclear reactor with a thermal power of 4,000 MW and an electrical power generation of 1,300 MW, this corresponds to a total power production of 4,250 [[MW]] (Mega-Watts), of which 250 MW is radiated away, and ''disappears'', as anti-neutrino radiation. This is to say, 250 MW of fission energy is ''lost'' from this reactor and does not appear as heat, since the anti-neutrinos penetrate all normal building materials essentially tracelessly. The exact energy spectrum is mostly uncertain and depends, for example, on the degree to which the fuel is burned.
There is no established experimental method to measure the flux of low energy anti-neutrinos. Only anti-neutrinos with an energy above threshold of 1.8 MeV can be uniquely identified (see "Neutrino Detection" below). An estimated 3% of all anti-neutrinos from a nuclear reactor carry an energy above this threshold. An average nuclear power plant may generate over 10<sup>20</sup> anti-neutrinos per second above this threshold, and a much larger number which cannot be seen with present detector technology.
Some [[particle accelerator]]s have been used to make neutrino beams. The technique is to smash [[protons]] into a fixed target, producing charged [[pions]] or [[kaon]]s. These unstable particles are then magnetically focused into a long tunnel where they decay while in flight. Because of the [[Lorentz Boost|relativistic boost]] of the decaying particle the neutrinos are produced as a beam rather than isotropically.
[[Nuclear bomb]]s also produce very large quantities of neutrinos. [[Fred Reines]] and [[Clyde Cowan]] considered the detection neutrinos from a bomb prior to their search for reactor neutrinos.
===Geologically produced neutrinos===
Neutrinos are produced as a result of natural [[background radiation]]. In particular, the decay chains of [[uranium]]-238 and [[thorium]]-232 isotopes, as well as [[potassium]]-40, include [[beta decay]]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 [[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.
[[Image:Proton proton cycle.png|350px|thumb|Solar neutrinos ([[Proton-proton chain reaction|proton-proton chain]]) in the Standard Solar Model]]
===Atmospheric neutrinos===
Atmospheric neutrinos result from the interaction of [[cosmic ray]]s with atomic nuclei in the [[Earth's atmosphere]], creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from Tata Institute of Fundamental Research (TIFR), India, Osaka City University, Japan and Durham University, UK recorded the first cosmic ray neutrino interaction in an underground laboratory in [[Kolar Gold Fields|KGF]] gold mines in India in 1965.
===Solar neutrinos===
Solar neutrinos originate from the [[nuclear fusion]] powering the [[sun]] and other stars.
The details of the operation of the sun are explained by the [[Standard Solar Model]]. In short: when four protons fuse to become one [[helium]] nucleus, two of them have to convert into neutrons, and each such conversion releases one electron neutrino.
The sun sends enormous numbers of neutrinos in all directions. Every second, about 70 [[1000000000 (number)|billion]] (7×10<sup>10</sup>) solar neutrinos pass through every square centimeter on Earth that faces the sun.<ref name=encarta>"Neutrino." ''Microsoft Encarta Online Encyclopedia'', 2006</ref> Since neutrinos are insignificantly absorbed by the mass of the Earth, the surface area on the side of the Earth opposite the Sun receives about the same number of neutrinos as the side facing the Sun.
===Supernovae===
[[Image:Supernova-1987a.jpg|350px|thumb|[[supernova 1987a|SN 1987A]]]]
Neutrinos are an important product of Types Ib, Ic and II (core-collapse) [[supernova]]e. In such events, the [[pressure]] at the core becomes so high (10<sup>14</sup> g/cm³) that the [[degeneracy pressure|degeneracy]] of electrons is not enough to prevent protons and electrons from combining to form a neutron and an electron neutrino. A second and more important neutrino source is the thermal energy (100 billion kelvins) of the newly formed neutron core, which is dissipated via the formation of neutrino-antineutrino pairs of all flavors.<ref name=Mannbook>{{cite book
| author = [http://www.physics.upenn.edu/facultyinfo/mann.html Mann, Alfred K.]
| title = Shadow of a star: The neutrino story of Supernova 1987A
| publisher = [[W. H. Freeman]]
| date = 1997
| location = New York
| pages = page 122
| url = http://www.whfreeman.com/GeneralReaders/book.asp?disc=TRAD&id_product=1058001008&@id_course=1058000240
| isbn = 0716730979 }}</ref> Most of the energy produced in supernovas is thus radiated away in the form of an immense burst of neutrinos. The first experimental evidence of this phenomenon came in the year 1987, when neutrinos from [[SN 1987A|supernova 1987A]] were detected. The water-based detectors [[Kamiokande II]] and [[Irvine-Michigan-Brookhaven (detector)|IMB]] detected 11 and 8 antineutrinos of thermal origin,<ref name=Mannbook /> respectively, while the [[gallium]]-71-based [[Baksan Neutrino Observatory|Baksan]] detector found 5 neutrinos ([[lepton number]] = 1) of either thermal or electron-capture origin, in a burst lasting less than 13 seconds. It is thought that neutrinos would also be produced from other events such as the collision of [[neutron star]]s. What was particularly interesting about this event was that the neutrino signature of the supernova arrived at earth approximately 18 hours ''before'' the arrival of the first photon signature. The exceptionally weak interaction with normal matter allowed the neutrinos to pass through the churning mass of the exploding star, while the electromagnetic photons were retarded, with the photon signature of the supernova not being released until the outermost layers of the star were superheated and released a much brighter visible light signature, observed telescopically on earth some 18 hours after the neutrinos had already arrived. This point shows how weakly interacting neutrinos truly are.
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 [[Supernova Early Warning System|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 a few to several tens 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 remnant]]s. The connection between cosmic rays and supernova remnants was suggested by [[Walter Baade]] and [[Fritz 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 (which is still under 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, [[Antarctic Muon And Neutrino Detector Array|AMANDA]], [ICECUBE][http://icecube.wisc.edu/index.php], Antares, NEMO and [[Nestor Project|Nestor]]. Related information is provided by very high energy [[gamma ray]] observatories, such as [[High Energy Stereoscopic System|HESS]] and [[MAGIC (telescope)|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===
{{main|Cosmic neutrino background}}
It is thought that, just like the [[cosmic microwave background radiation]] left over from the [[Big Bang]], there is a background of low energy neutrinos in our Universe. 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 [[galaxy|galactic]] structures that we see.
Further, these same galaxies and [[galaxy groups and clusters|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 [[galaxy formation and evolution|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 of each type per cubic centimeter and temperature 1.9 K (1.7×10<sup>-4</sup> eV) if they are massless, much colder if their mass exceeds 0.001 eV. Although their density is quite high, due to extremely low neutrino cross-sections at sub-eV energies, the relic neutrino background has not yet been observed in the laboratory (e.g. boron-8 solar neutrinos -- which are emitted with a higher energy -- have been detected definitively despite having a space density that is lower than that of relic neutrinos by some 6 orders of magnitude).
== Neutrino detection ==
{{Main|Neutrino detector}}
Because neutrinos are very weakly interacting, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground in order to isolate the detector from [[cosmic ray]]s and other background radiation.
Antineutrinos were first detected in the 1950s near a nuclear reactor. [[Frederick Reines|Reines]] and [[Clyde Cowan|Cowan]] used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrino with an energy above the [[threshold]] of 1.8 [[MeV]] caused charged current interactions with the protons in the water, producing 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.
Since then, various detection methods have been used. [[Super Kamiokande]] is a large volume of water surrounded by [[photomultiplier tube]]s that watch for the [[Cherenkov radiation]] emitted when an incoming neutrino creates an [[electron]] or [[muon]] in the water. The [[Sudbury Neutrino Observatory]] is similar, but uses [[heavy water]] as the detecting medium, which uses the same effects, but also allows the additional reaction any-flavor neutrino photo-dissociation of deuterium, resulting in a free neutron which is then detected from gamma radiation after chlorine-capture. Other detectors have consisted of large volumes of [[chlorine]] or [[gallium]] which are periodically checked for excesses of [[argon]] or [[germanium]], respectively, which are created by electron-neutrinos interacting with the original substance. [[MINOS]] uses a solid plastic [[scintillator]] coupled to [[photomultiplier tube]]s, while [[Borexino]] uses a liquid [[pseudocumene]] [[scintillator]] also watched by [[photomultiplier tube]]s while the proposed [[NOνA]] detector will use liquid scintillator watched by [[avalanche photodiode]]s.
==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. <!-- Although a vast number of photons are produced in the solar core, these photons also interact with the atomic nuclei present and therefore are unable to rapidly diffuse to the surface for emission. It is typically estimated that photons of light produced in the solar core take 1000 years to reach the surface. Moreover, such photons would be indistinguishable from photons produced at or near the surface, so observation of the solar core would be difficult or impossible using optical techniques. --> On the other hand, neutrinos generated in stellar fusion reactions are thought to very weakly interact with matter and therefore pass right through the sun with few or no interactions. (However, the claim of an almost interaction-free passage is not backed by experimental evidence and purely theoretical so far.) While photons emitted by the solar core may require some 40,000<ref>J.N. Bahcall, Neutrino Astrophysics, Cambridge, 1989.</ref> 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 [[megaparsec]]s 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 telescope]]s in the next decade.
The most important use of the neutrino is in the observation of [[supernova]]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 [[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 [[fermion]]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 could also be used for studying [[quantum gravity]] effects. Because they are not affected by either the [[strong interaction]] or [[electromagnetism]], and because they are not normally found in composite particles (unlike quarks) or prone to near instantaneous decay (like many other standard model particles) it might be possible to isolate and measure gravitational effects on neutrinos at a quantum level.
== See also ==
* [[List of neutrino experiments]]
* [[Neutrino Factory]]
== Notes ==
{{reflist|2}}
== References ==
<div class="references-small">
*{{cite web | title=Super-Kamiokande | work=Super-Kamiokande at UC Irvine | url=http://www.ps.uci.edu/~superk/ | accessmonthday=July 14 | accessyear=2003}}
* {{cite web | author = G. A. Tammann, F. K. Thielemann, D. Trautmann |date= 2003 | url = http://www.europhysicsnews.com/full/20/article8/article8.html | title = Opening new windows in observing the Universe | publisher = Europhysics News | language = English | accessdate = 2006-06-08 }}
*{{cite book | author=Bahcall, John N. | title=Neutrino Astrophysics | publisher=Cambridge University Press | year=1989 | id=ISBN 0-521-35113-8}}
*{{cite book | author=Griffiths, David J. | title=Introduction to Elementary Particles | publisher=Wiley, John & Sons, Inc | year=1987 | id=ISBN 0-471-60386-4}}
*{{cite book | author=Perkins, Donald H. | title=Introduction to High Energy Physics | publisher=Cambridge University Press | year=1999 | id=ISBN 0-521-62196-8}}
*{{cite book | author=Povh, Bogdan | title=Particles and Nuclei: An Introduction to the Physical Concepts | publisher=Springer-Verlag | year=1995 | id=ISBN 0-387-59439-6}}
*{{cite book | author=Tipler, Paul; Llewellyn, Ralph | title=Modern Physics (4th ed.) | publisher=W. H. Freeman | year=2002 | id=ISBN 0-7167-4345-0}}
*{{cite journal | author=M. Maltoni ''et al.'' |title = Status of global fits to neutrino oscillations| year=2004 |journal = NJP|volume=06|pages=122 |doi = 10.1088/1367-2630/6/1/122}} {{arxiv|archive=hep-ph|id=0405172}}
*{{cite journal | author=R. N. Mohapatra ''et al.'' ([[American Physical Society|APS]] neutrino theory working group) | title = Theory of neutrinos: a white paper | year=2005 |journal = preprint}} {{arxiv|archive=hep-ph|id=0510213}}
*{{cite journal | author=A. Goobar, S. Hannestad, E. Mörtsell and H. Tu|title = A new bound on the neutrino mass from the SDSS baryon acoustic peak| year=2006 |journal = JCAP|volume=06|pages=019}} {{arxiv|archive=astro-ph|id=0602155}}
*Neutrino Oscillations, Masses And Mixing: W.M.Alberico, Torino University&S.M. Bilenky, Dubna NRI; 2003;http://arxiv.org/abs/hep-ph/0306239v1
</div>
== External links ==
* [http://www.nu.to.infn.it NEUTRINO UNBOUND]: On-line review and e-archive on Neutrino Physics and Astrophysics
* [http://www.pbs.org/wgbh/nova/neutrino/ Nova: The Ghost Particle]: Documentary on US public television from WGBH
* [http://snews.bnl.gov SNEWS]: Using neutrino detectors to receive early warning of supernovae
* [http://www.newscientist.com/channel/fundamentals/mg18524885.900 Measuring the density of the earth's core with neutrinos]
* [http://www.sns.ias.edu/~jnb/ John Bahcall Website]
* [http://space.newscientist.com/article/dn13414-universe-submerged-in-a-sea-of-chilled-neutrinos.html?feedId=online-news_rss20 Universe submerged in a sea of chilled neutrinos], ''New Scientist'', [[5 March]] [[2008]]
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