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{{Infobox particle
| name = Neutrino/Antineutrino
| image = [[File:FirstNeutrinoEventAnnotated.jpg|280px]]
| caption = The first use of a hydrogen [[bubble chamber]] to detect neutrinos, on November 13, 1970. A neutrino hit a proton in a hydrogen atom. The collision occurred at the point where three tracks emanate on the right of the photograph.
| num_types = 3 – electron neutrino, muon neutrino and tau neutrino
| composition = [[Elementary particle]]
| statistics = [[Fermionic]]
| group = [[Lepton]], antilepton
| generation = First, second and third
| interaction = [[Weak interaction]] and [[gravitation]]
| particle =
| antiparticle = Antineutrinos are possibly identical to the neutrino (see ''[[Majorana fermion]]'').
| theorized = {{SubatomicParticle|Electron neutrino}} (Electron neutrino): [[Wolfgang Pauli]] (1930)<br />{{SubatomicParticle|Muon neutrino}} (Muon neutrino): Late 1940s
{{SubatomicParticle|Tau neutrino}} (Tau neutrino): Mid 1970s
| discovered = {{SubatomicParticle|Electron neutrino}}: [[Clyde Cowan]], [[Frederick Reines]] (1956)<br />{{SubatomicParticle|Muon neutrino}}: [[Leon Lederman]], [[Melvin Schwartz]] and [[Jack Steinberger]] (1962)<br />{{SubatomicParticle|Tau neutrino}}: [[DONUT|DONUT collaboration]] (2000)
| symbol = {{SubatomicParticle|Electron neutrino}}, {{SubatomicParticle|Muon neutrino}}, {{SubatomicParticle|Tau neutrino}}, {{SubatomicParticle|Electron antineutrino}}, {{SubatomicParticle|Muon antineutrino}}, {{SubatomicParticle|Tau antineutrino}}
| mass = Small, but non-zero. See the ''[[#Mass|mass]]'' section.
| decay_time =
| decay_particle =
| electric_charge = 0&nbsp;[[elementary charge|e]]
| color_charge =
| spin = {{frac|1|2}}
| X_charge = −3
| weak_hypercharge = −1
| B-L = −1
}}
A '''neutrino''' ({{IPAc-en|n|uː|ˈ|t|r|iː|n|oʊ}} or {{IPAc-en|nj|uː|ˈ|t|r|iː|n|oʊ}}) is an electrically neutral, [[Weak interaction|weakly interacting]] [[elementary particle|elementary subatomic particle]]<ref>
{{cite web
|title=Neutrino
|url=http://www.mpg.de/12928/Glossary
|work=Glossary for the Research Perspectives of the Max Planck Society
|publisher=[[Max Planck Gesellschaft]]
|accessdate=2012-03-27
}}</ref> with [[spin-1/2|half-integer spin]]. The neutrino (meaning "small neutral one" in Italian) is denoted by the Greek letter ν (''[[Nu (letter)|nu]]''). All evidence suggests that neutrinos have [[mass]] but that their mass is tiny even by the standards of subatomic particles. Their mass has never been measured accurately.


Neutrinos do not carry [[electric charge]], which means that they are not affected by the [[electromagnetic force]]s that act on charged particles such as electrons and protons. Neutrinos are affected only by the [[weak interaction|weak sub-atomic force]], of much shorter range than [[electromagnetism]], and [[gravity]], which is relatively weak on the [[subatomic scale]]. Therefore a typical neutrino passes through normal matter unimpeded.
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Neutrinos are created as a result of certain types of [[radioactive decay]], or [[nuclear reaction]]s 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 "[[Flavor (particle physics)|flavors]]", of neutrinos: [[electron neutrino]]s, [[muon neutrino]]s and [[tau neutrino]]s. Each type is associated with an [[antiparticle]], called an "antineutrino", which also has neutral electric charge and half-integer spin. Whether or not the neutrino and its corresponding antineutrino are [[identical particles]] has not yet been resolved, even though the antineutrino has an opposite [[Chirality (physics)|chirality]] to the neutrino.
 
Most neutrinos passing through the Earth emanate from the Sun. About 65&nbsp;billion ({{val|6.5|e=10}}) [[solar neutrino]]s per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.<ref name="J. Bahcall et al. 2005 L85–L88">
{{cite journal
|author1=J. N. Bahcall
|author2=A. M. Serenelli
|author3=S. Basu
|year=2005
|title=New solar opacities, abundances, helioseismology, and neutrino fluxes
|journal=[[The Astrophysical Journal Letters]]
|volume=621 |issue=1 |pages=L85–L88
|arxiv=astro-ph/0412440
|bibcode=2005ApJ...621L..85B
|doi=10.1086/428929
}}</ref>
 
==History==
 
===Pauli's proposal===
The neutrino<ref group=nb>More specifically, the electron neutrino.</ref> was  postulated first by [[Wolfgang Pauli]] in 1930 to explain how [[beta decay]] could conserve [[conservation of energy|energy]], [[conservation of momentum|momentum]], and [[conservation of angular momentum|angular momentum]] ([[Spin (physics)|spin]]). In contrast to [[Niels Bohr]], who proposed a statistical version of the conservation laws to explain the [[Event (particle physics)|event]], Pauli hypothesized an undetected particle that he called a "neutron" in keeping with convention employed for naming both the [[proton]] and the [[electron]], which in 1930 were known to be respective products for alpha and beta decay.<ref>
{{Cite journal
|author=L. M. Brown
|year=1978
|title=The Idea of the Neutrino
|journal=[[Physics Today]]
|volume=31 |issue=9 |pages=23–28
|bibcode=
|doi=10.1063/1.2995181
}}</ref><ref group=nb>Improved understanding between 1930 and 1932 led [[Viktor Ambartsumian]] and [[Dmitri Ivanenko]] to propose the existence of the more massive neutron as it is now known, subsequently demonstrated by [[James Chadwick]] in 1932. These events necessitated renaming Pauli's less massive, momentum-conserving particle. [[Enrico Fermi]] coined "neutrino" in 1933 to distinguish between the neutron and the much lighter neutrino.
{{cite journal
|author=K. Riesselmann
|year=2007
|title=Logbook: Neutrino Invention
|journal=[[Symmetry Magazine]]
|volume=4 |issue=2
|url=http://www.symmetrymagazine.org/article/march-2007/neutrino-invention
}}</ref><ref group=nb>[[Niels Bohr]] was notably opposed to this interpretation of beta decay and was ready to accept that energy, momentum and angular momentum were not conserved quantities.</ref>
:{{SubatomicParticle|Neutron0}} → {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron-}} + {{SubatomicParticle|Electron antineutrino}}
 
[[James Chadwick]] discovered a much more massive nuclear particle in 1932 and also named it a [[neutron]], leaving two kinds of particles with the same name. [[Enrico Fermi]], who developed the theory of beta decay, coined the term ''neutrino'' (the [[Italian language|Italian]] equivalent of "little neutral one") in 1933 as a way to resolve the confusion.<ref name="radioactivity">
{{cite book
|author=M. F. L'Annunziata
|year=2007
|title=Radioactivity
|url=http://books.google.com/books?id=YpEiPPFlNAAC&pg=PA100
|page=100
|publisher=[[Elsevier]]
|isbn=978-0-444-52715-8
}}</ref><ref group=nb>It is a pun on the Italian word for neutron, ''neutrone'', the ''-one'' ending being (though not in this case) an [[Augmentative#Italian|augmentative in Italian]], so ''neutrone'' could be read as the "large neutral one".</ref> Fermi's paper, written in 1934, unified Pauli's neutrino with [[Paul Dirac]]'s [[positron]] and [[Werner Heisenberg]]'s neutron-proton model and gave a solid theoretical basis for future experimental work. However, the journal [[Nature (journal)|Nature]] rejected Fermi's paper, saying that the theory was "too remote from reality". He submitted the paper to an Italian journal, which accepted it, but the general lack of interest in his theory at that early date caused him to switch to experimental physics.<ref>
{{cite book
|author=F. Close
|year=2010
|title=Neutrino
|publisher=[[Oxford University Press]]
|isbn=978-0-19-957459-9
}}</ref><ref>
{{cite journal
|author=E. Fermi
|year=1934
|title=Versuch einer Theorie der β-Strahlen. I
|journal=[[Zeitschrift für Physik A]]
|volume=88 |issue= 3–4|page=161
|bibcode=1934ZPhy...88..161F
|doi=10.1007/BF01351864
}} Translated in {{cite journal
|author=F. L. Wilson
|year=1968
|title=Fermi's Theory of Beta Decay
|url=http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1
|journal=[[American Journal of Physics]]
|volume=36 |issue=12 |page=1150
|bibcode=1968AmJPh..36.1150W
|doi=10.1119/1.1974382
}}</ref>
 
===Direct detection===
[[File:Clyde Cowan.jpg|thumb|left|Clyde Cowan conducting the neutrino experiment c. 1956]]
In 1942 [[Wang Ganchang]] first proposed the use of beta-capture to experimentally detect neutrinos.<ref>
{{cite journal
|author=K.-C. Wang
|year=1942
|title=A Suggestion on the Detection of the Neutrino
|journal=[[Physical Review]]
|volume=61 |issue=1–2 |page=97
|bibcode=1942PhRv...61...97W
|doi=10.1103/PhysRev.61.97
}}</ref> In the 20 July 1956 issue of [[Science (journal)|''Science'']], [[Clyde Cowan]], [[Frederick Reines]], F. B. Harrison, H. W. Kruse, and A. D. McGuire published confirmation that they had detected the neutrino,<ref>
{{cite journal
|author=C. L Cowan Jr., F. Reines, F. B. Harrison, H. W. Kruse, A. D McGuire
|year=1956
|title=Detection of the Free Neutrino: a Confirmation
|journal=[[Science (journal)|Science]]
|volume=124 |issue=3212 |pages=103–4
|bibcode=1956Sci...124..103C
|doi=10.1126/science.124.3212.103
|pmid=17796274
}}</ref><ref>
{{cite book
|author=K. Winter
|year=2000
|title=Neutrino physics
|url=http://books.google.com/?id=v_tiL2NlfvMC&pg=PA38
|publisher=[[Cambridge University Press]]
|page=38ff
|isbn=978-0-521-65003-8
}}<br>This source reproduces the 1956 paper.</ref> a result that was rewarded almost forty years later with the [[Nobel Prize in Physics|1995 Nobel Prize]].<ref name="noblp">
{{cite web
|accessdate=29 June 2010
|title=The Nobel Prize in Physics 1995
|url=http://nobelprize.org/nobel_prizes/physics/laureates/1995/
|publisher=[[The Nobel Foundation]]
}}</ref>
 
In this experiment, now known as the [[Cowan–Reines neutrino experiment]], antineutrinos created in a nuclear reactor by beta decay reacted with protons producing [[neutron]]s and [[positron]]s:
 
:{{SubatomicParticle|Electron antineutrino}} + {{SubatomicParticle|Proton+}} → {{SubatomicParticle|Neutron0}} + {{SubatomicParticle|Electron+}}
 
The positron quickly finds an electron, and they [[Annihilation|annihilate]] each other. The two resulting [[gamma ray]]s (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events – positron annihilation and neutron capture – gives a unique signature of an antineutrino interaction.
 
===Neutrino flavor===
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=I. V. Anicin
|year=2005
|title=The Neutrino – Its Past, Present and Future
|class=physics
|arxiv=physics/0503172
}}</ref> which earned them the [[Nobel Prize in Physics|1988 Nobel Prize in Physics]]. When the third type of [[lepton]], the [[tau (particle)|tau]], was discovered in 1975 at the [[Stanford Linear Accelerator Center]], it too was expected to have an associated neutrino (the tau 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|DONUT collaboration]] at [[Fermilab]]; its existence had already been inferred by both theoretical consistency and experimental data from the [[Large Electron–Positron Collider]].{{citation needed|date=May 2013}}
 
===Solar neutrino problem===
Starting in the late 1960s, several experiments found that the number of [[electron neutrino]]s arriving from the Sun was between one third and one half the number predicted by the [[Standard Solar Model]]. This discrepancy, which became known as the [[solar neutrino problem]], remained unresolved for some thirty years. It was resolved by discovery of [[neutrino oscillation]] and mass. (The [[Standard Model|Standard Model of particle physics]] had assumed that neutrinos are massless and cannot change flavor. However, if neutrinos had mass, they could change flavor, or ''oscillate'' between flavors).{{citation needed|date=May 2013}}
 
===Oscillation===
A practical method for investigating neutrino oscillations was first suggested by [[Bruno Pontecorvo]] in 1957 using an analogy with [[kaon]] [[kaon oscillation|oscillations]]; over the subsequent 10 years he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 [[Stanislav Mikheyev]] and [[Alexei Smirnov (physicist)|Alexei Smirnov]] (expanding on 1978 work by [[Lincoln Wolfenstein]]) noted that flavor oscillations can be modified when neutrinos propagate through matter. This so-called [[Mikheyev–Smirnov–Wolfenstein effect]] (MSW effect) is important to understand because many neutrinos emitted by fusion in the Sun pass through the dense matter in the [[solar core]] (where essentially all solar fusion takes place) on their way to detectors on Earth.
 
Starting in 1998, experiments began to show that solar and atmospheric neutrinos change flavors (see [[Super-Kamiokande]] and [[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.<ref>
{{cite journal
|author1=M. Maltoni
|author2=T. Schwetz
|author3=M. Tórtola
|author4=J. W. F. Valle
|year=2004
|title=Status of global fits to neutrino oscillations
|journal=[[New Journal of Physics]]
|volume=6 |issue=1 |page=122
|arxiv=hep-ph/0405172
|bibcode=2004NJPh....6..122M
|doi=10.1088/1367-2630/6/1/122
}}</ref>
 
===Supernova neutrinos===
{{See also|Supernova Early Warning System}}
 
[[Raymond Davis Jr.]] and [[Masatoshi Koshiba]] were jointly awarded the 2002 [[Nobel Prize in Physics]]; Davis for his pioneer work on [[cosmic neutrino]]s and Koshiba for the first real time observation of [[supernova neutrino]]s. The detection of [[solar neutrino]]s, and of neutrinos of the [[SN 1987A]] [[supernova]] in 1987 marked the beginning of [[neutrino astronomy]].{{citation needed|date=May 2013}}
 
==Properties and reactions==
The neutrino has half-integer [[spin (physics)|spin]] (½ħ) and is therefore a [[fermion]]. Neutrinos interact primarily through the [[weak nuclear force|weak force]]. The discovery of [[neutrino oscillation|neutrino flavor oscillations]] implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment<ref name="PDG">
{{cite journal
|author=S. Eidelman ''et al.'' ([[Particle Data Group]])
|year=2004
|title=Leptons in the 2005 Review of Particle Physics
|url=http://pdg.lbl.gov/2005/listings/lxxx.html
|journal=[[Physics Letters B]]
|volume=592 |issue=1 |pages=1–5
|arxiv=astro-ph/0406663
|bibcode=2004PhLB..592....1P
|doi=10.1016/j.physletb.2004.06.001
}}</ref> of the order of {{val|e=-19|ul=μ<sub>B</sub>}}, allowing the possibility that neutrinos may interact electromagnetically as well. An experiment done by [[C. S. Wu]] at [[Columbia University]] showed that neutrinos always have left-handed [[chirality (physics)|chirality]].<ref>
{{cite web
|author=S.M. Caroll
|date=25 March 2009
|title=Ada Lovelace Day: Chien-Shiung Wu
|url=http://blogs.discovermagazine.com/cosmicvariance/2009/03/25/ada-lovelace-day-chien-shiung-wu/
|work=[[Discover Magazine]]
|accessdate=2011-09-23
}}</ref> 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 one of the hydrogen nuclei in the water molecules. A hydrogen nucleus is a single proton, so simultaneous nuclear interactions, which would occur within a heavier nucleus, don't need to be considered for the detection experiment. Within a cubic metre of water placed right outside a nuclear reactor, only relatively few such interactions can be recorded, but the setup is now used for measuring the reactor's plutonium production rate.
 
===Mikheyev–Smirnov–Wolfenstein effect===
{{Main|Mikheyev–Smirnov–Wolfenstein effect}}
Neutrinos traveling through matter, in general, undergo a process analogous to [[Speed of light#In a medium|light traveling through a transparent material]]. This process is not directly observable because it does not produce [[ionizing radiation]], but gives rise to the [[MSW effect]]. Only a small fraction of the neutrino's energy is transferred to the material.
 
===Nuclear reactions===
Neutrinos can interact with a nucleus, changing it to another nucleus. This process is used in radiochemical [[neutrino detector]]s. In this case, the energy levels and spin states within the target nucleus have to be taken into account to estimate the probability for an interaction. In general the interaction probability increases with the number of neutrons and protons within a nucleus.
 
===Induced fission===
Very much like [[neutron]]s do in [[nuclear reactor]]s, neutrinos can induce [[fission reaction]]s within heavy [[atomic nucleus|nuclei]].<ref>
{{cite journal
|author=E. Kolbe, G.M. Fuller
|year=2004
|title=Neutrino-Induced Fission of Neutron-Rich Nuclei
|journal=[[Physical Review Letters]]
|volume=92 |issue=11 |page=1101
|arxiv=astro-ph/0308350
|bibcode=2004PhRvL..92k1101K
|doi=10.1103/PhysRevLett.92.111101
}}</ref> So far, this reaction has not been measured in a laboratory, but is predicted to happen within stars and supernovae. The process affects the [[Abundance of the chemical elements#Abundance of elements in the Universe|abundance of isotopes]] seen in the [[universe]].<ref>
{{cite journal
|author=A. Kelic, K.-H. Schmidt
|year=2005
|title=Cross sections and fragment distributions from neutrino-induced fission on r-process nuclei
|journal=[[Physics Letters B]]
|volume=616 |issue=1–2 |pages=48–48
|arxiv=hep-ex/0312045
|bibcode=2005PhLB..616...48K
|doi=10.1016/j.physletb.2005.04.074
}}</ref> Neutrino fission of [[deuterium]] nuclei has been observed in the [[Sudbury Neutrino Observatory]], which uses a [[heavy water]] detector.
 
===Types===
{| class="wikitable"  style="float:right; margin:0 0 1em 1em;"
|+Neutrinos in the Standard Model<br />of elementary particles
|-
!Fermion
!Symbol
!Mass<ref group=nb>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 decay]]s. 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
|-
|style="background:#efefef;"| Electron neutrino
| {{SubatomicParticle|Electron neutrino}}
| &lt; 2.2 eV
|-
|style="background:#efefef;"| Electron antineutrino
| {{SubatomicParticle|Electron antineutrino}}
| &lt; 2.2 eV
|-
!colspan="3" style="background:#ffdead;"|Generation 2
|-
|style="background:#efefef;"| Muon neutrino
| {{SubatomicParticle|Muon neutrino}}
| &lt; 170 keV
|-
|style="background:#efefef;"| Muon antineutrino
| {{SubatomicParticle|Muon antineutrino}}
| &lt; 170 keV
|-
!colspan="3" style="background:#ffdead;"|Generation 3
|-
|style="background:#efefef;"| Tau neutrino
| {{SubatomicParticle|Tau neutrino}}
| &lt; 15.5 MeV
|-
|style="background:#efefef;"| Tau antineutrino
| {{SubatomicParticle|Tau antineutrino}}
| &lt; 15.5 MeV
|}
 
There are three known types (''[[flavor (particle physics)|flavors]]'') of neutrinos: electron neutrino {{SubatomicParticle|electron neutrino}}, muon neutrino {{SubatomicParticle|muon neutrino}} and tau neutrino {{SubatomicParticle|tau neutrino}}, 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<ref group=nb>In this context, "light neutrino" means neutrinos with less than half the mass of the Z boson.</ref> available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types is 3.<ref name="PDG"/> 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
|author=G. Karagiorgi ''et al.''
|year=2007
|title=Leptonic CP violation studies at MiniBooNE in the (3+2) sterile neutrino oscillation hypothesis
|journal=[[Physical Review D]]
|volume=75 |issue=13011 |pages=1–8
|arxiv=hep-ph/0609177
|bibcode=2007PhRvD..75a3011K
|doi=10.1103/PhysRevD.75.013011
|last2=Aguilar-Arevalo
|first2=A.
|last3=Conrad
|first3=J.
|last4=Shaevitz
|first4=M.
|last5=Whisnant
|first5=K.
|last6=Sorel
|first6=M.
|last7=Barger
|first7=V.
}}</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 web
|author=M. Alpert
|year=2007
|title=Dimensional Shortcuts
|url=http://www.sciam.com/article.cfm?chanID=sa006&colID=5&articleID=B5CB9C67-E7F2-99DF-3BF7368614D46C5D
|work=[[Scientific American]]
|accessdate=2009-10-31
}}{{dead link|date=January 2011}}</ref> A recent re-analysis of reference electron spectra data from the [[Institut Laue-Langevin]]<ref>
{{cite journal
|author=Th. A. Mueller  ''et al.''
|year=2011
|title=Improved Predictions of Reactor Antineutrino Spectra
|journal=[[Physical Review C]]
|volume=83 |issue=5 |page=054615
|arxiv=1101.2663
|doi=10.1103/PhysRevC.83.054615
|bibcode = 2011PhRvC..83e4615M }}</ref> has also hinted at a fourth, sterile neutrino.<ref>
{{cite journal
|author=G. Mention ''et al.''
|date=January 2011
|title=The Reactor Antineutrino Anomaly
|journal=[[Physical Review D]]
|volume=83 |issue=7 |page=073006
|arxiv=1101.2755
|doi=10.1103/PhysRevD.83.073006
|bibcode = 2011PhRvD..83g3006M
|last2=Fechner
|first2=M.
|last3=Lasserre
|first3=Th.
|last4=Mueller
|first4=Th. A.
|last5=Lhuillier
|first5=D.
|last6=Cribier
|first6=M.
|last7=Letourneau
|first7=A. }}</ref>
 
Recently analyzed data from the [[Wilkinson Microwave Anisotropy Probe]] of the [[Cosmic microwave background radiation|cosmic background radiation]] is compatible with either three or four types of neutrinos. It is hoped that the addition of two more years of data from the probe will resolve this uncertainty.<ref>
{{cite web
|author=R. Cowen
|date=2 February 2010
|title=Ancient Dawn's Early Light Refines the Age of the Universe
|url=http://www.sciencenews.org/view/generic/id/55957/title/Ancient_dawns_early_light_refines_age_of_universe
|work=[[Science News]]
|accessdate=2010-02-03
}}</ref>
 
===Antineutrinos===
{{antimatter}}
Antineutrinos are the [[antiparticle]]s of neutrinos, which are [[electric charge|neutral]] particles produced in [[nuclear reaction|nuclear]] [[beta decay]]. These are emitted during [[beta particle]] emissions, when a neutron turns into a proton. They have a [[spin (physics)|spin]] of ½, and are part of the [[lepton]] family of particles. The antineutrinos observed so far all have right-handed [[helicity (particle physics)|helicity]] (i.e. only one of the two possible spin states has ever been seen), while the neutrinos are left-handed. Antineutrinos, like neutrinos, interact with other [[matter]] only through the [[gravitational]] and [[weak force]]s, making them very difficult to detect experimentally. [[Neutrino oscillation]] experiments indicate that antineutrinos have [[mass]], but beta decay experiments constrain that mass to be very small. A neutrino-antineutrino interaction has been suggested in attempts to form a composite photon with the [[neutrino theory of light]].
 
Because antineutrinos and neutrinos are neutral particles it is possible that they are actually the same particle. Particles which have this property are known as [[Majorana particle]]s. If neutrinos are indeed Majorana particles then the [[neutrinoless double beta decay]], as well as a range of other lepton number violating phenomena, are allowed. Several experiments have been proposed to search for this process.
 
Researchers around the world have begun to investigate the possibility of using antineutrinos for reactor monitoring in the context of preventing the [[nuclear proliferation|proliferation of nuclear weapons]].<ref>
{{cite web
|year=2006
|title=LLNL/SNL Applied Antineutrino Physics Project. LLNL-WEB-204112
|url=http://neutrinos.llnl.gov/ neutrinos.llnl.gov
}}</ref><ref>
{{cite web
|year=2007
|title=Applied Antineutrino Physics 2007 workshop
|url=http://www.apc.univ-paris7.fr/AAP2007/ apc.univ-paris7.fr
}}</ref><ref>
{{cite web
|date=13 March 2008
|title=New Tool To Monitor Nuclear Reactors Developed
|publisher=[[ScienceDaily]]
|accessdate=2008-03-16
|url=http://www.sciencedaily.com/releases/2008/03/080313091522.htm
}}</ref>
 
Antineutrinos were first detected as a result of their interaction with protons in a large tank of water. This was installed next to a nuclear reactor as a controllable source of the antineutrinos. (See: [[Cowan–Reines neutrino experiment]])
 
===Flavor oscillations===
{{Main|Neutrino oscillation}}
Neutrinos are most often created or detected with a well defined [[Flavor (particle physics)|flavor]] (electron, muon, tau). However, in a phenomenon known as neutrino flavor oscillation, neutrinos are able to oscillate among the three available flavors while they propagate through space. Specifically, this occurs because the neutrino flavor [[Eigenvalues and eigenvectors|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 nonzero differences between the neutrino masses, because the amount of mixing between neutrino flavors at a given time depends on the differences between their squared masses. There are other possibilities in which neutrino can oscillate even if they are massless. If Lorentz invariance is not an exact symmetry, neutrinos can experience [[Lorentz-violating neutrino oscillations|Lorentz-violating oscillations]].<ref>
{{cite journal
|author=V.A. Kostelecky, M. Mewes
|year=2004
|title=Lorentz and CPT violation in neutrinos
|journal=[[Physical Review D]]
|volume=69 |issue=1 |page=016005
|arxiv=hep-ph/0309025
|bibcode= 2004PhRvD..69a6005A
|doi=10.1103/PhysRevD.69.016005
}}</ref>
 
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.<ref name="neutrino-fundamentals">
{{cite book
|author=C. Giunti, C.W. Kim
|year=2007
|title=Fundamentals of neutrino physics and astrophysics
|url=http://books.google.com/?id=SdAcSwTR0CgC&lpg=PA255&dq=majorana%20neutrino%20helicity&pg=PA255
|page=255
|publisher=[[Oxford University Press]]
|isbn=0-19-850871-9
}}</ref>
 
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.
 
On July 19, 2013 the results from the [[T2K experiment]] presented at the [[European Physical Society]] Conference on High Energy Physics in Stockholm, Sweden, confirmed the Neutrino oscillation theory.<ref name="PhysicsNews20130719">[http://www.physnews.com/physics-news/cluster637855326/ "Neutrino shape-shift points to new physics"] ''Physics News'', 19 July 2013.</ref><ref name="BBC20130719">[http://www.bbc.co.uk/news/science-environment-23366318 "Neutrino 'flavour' flip confirmed"] ''BBC News'', 19 July 2013.</ref>
 
===Speed===
{{Main|Measurements of neutrino speed}}
 
Before neutrinos were found to oscillate, they were generally assumed to be massless, propagating at the [[speed of light]]. According to the theory of [[special relativity]], the question of neutrino [[velocity]] is closely related to their [[mass]]. If neutrinos are massless, they must travel at the speed of light. However, if they have mass, they cannot reach the speed of light.
 
Also some [[Lorentz violation|Lorentz violating]] variants of [[quantum gravity]] might allow faster-than-light neutrinos. A comprehensive framework for Lorentz violations is the [[Standard-Model Extension]] (SME).
 
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 was repeated in 2007 using the [[MINOS]] detectors, which found the speed of {{val|3|ul=GeV}} neutrinos to be {{val|1.000051|(29)|ul=c}} at 68% confidence level, and at 99% confidence level a range between {{val|0.999976|ul=c}} and {{val|1.000126|ul=c}}. The central value is higher than the speed of light and is consistent with superluminal velocity; however, the uncertainty is great enough that the result also does not rule out speeds less than or equal to light at this high confidence level. This measurement set an upper bound on the mass of the muon neutrino of {{val|50|u=MeV}} at 99% [[confidence interval|confidence]].<ref>
{{cite journal
|author=P. Adamson ''et al.'' ([[MINOS|MINOS Collaboration]])
|year=2007
|title=Measurement of neutrino velocity with the MINOS detectors and NuMI neutrino beam
|volume=76 |issue=7 |page=072005
|journal=[[Physical Review D]]
|arxiv=0706.0437
|bibcode=2007PhRvD..76g2005A
|doi=10.1103/PhysRevD.76.072005
}}</ref><ref>
{{cite news
|author=D. Overbye
|date=22 September 2011
|title=Tiny neutrinos may have broken cosmic speed limit
|url=http://www.nytimes.com/2011/09/23/science/23speed.html
|newspaper=[[New York Times]]
|quote=That group found, although with less precision, that the neutrino speeds were consistent with the speed of light.
}}</ref> After the detectors for the project were upgraded in 2012, MINOS corrected their initial result and found agreement with the speed of light, with limits for the difference in the arrival time of light and neutrinos of <math>\delta t=-15\pm31</math> nanoseconds. Further measurements are going to be conducted.<ref>{{cite web|title=MINOS reports new measurement of neutrino velocity |publisher=Fermilab today|url=http://www.fnal.gov/pub/today/archive_2012/today12-06-08.html|date=June 8, 2012|accessdate= June 8, 2012}}</ref>
 
The same observation was made, on a somewhat larger scale, with [[supernova 1987A]] (SN 1987A). 10-MeV antineutrinos 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.
 
In September 2011, the [[OPERA experiment|OPERA collaboration]] released calculations showing velocities of 17-GeV and 28-GeV neutrinos exceeding the speed of light in their experiments (see [[Faster-than-light neutrino anomaly]]).  In November 2011, OPERA repeated its experiment with changes so that the speed could be determined individually for each detected neutrino. The results showed the same faster-than-light speed. However, in February 2012 reports came out that the results may have been caused by a loose fiber optic cable attached to one of the atomic clocks which measured the departure and arrival times of the neutrinos. An independent recreation of the experiment in the same laboratory by [[ICARUS (experiment)|ICARUS]] found no discernible difference between the speed of a neutrino and the speed of light.<ref name="Arxiv-20120316">{{Cite journal|author=ICARUS Collaboration|title=Measurement of the neutrino velocity with the ICARUS detector at the CNGS beam|journal=Physics Letters B|volume=713|issue=1|pages=17–22|doi=10.1016/j.physletb.2012.05.033|arxiv=1203.3433|date=June 18, 2012|bibcode = 2012PhLB..713...17I|last2=Aprili|first2=P.|last3=Baiboussinov|first3=B.|last4=Baldo Ceolin|first4=M.|last5=Benetti|first5=P.|last6=Calligarich|first6=E.|last7=Canci|first7=N.|last8=Centro|first8=S.|last9=Cesana|first9=A. }}</ref>
In June 2012, CERN announced that new measurements conducted by four Gran Sasso experiments (OPERA, ICARUS, [[Borexino]] and [[Large Volume Detector|LVD]]) found agreement between the speed of light and the speed of neutrinos, finally refuting the initial OPERA result.<ref>{{cite web|title=Neutrinos sent from CERN to Gran Sasso respect the cosmic speed limit|publisher=CERN press release|url=http://press.web.cern.ch/press-releases/2011/09/opera-experiment-reports-anomaly-flight-time-neutrinos-cern-gran-sasso
/PR19.11E.html|date=2012-06-08|accessdate=2012-06-08}}</ref>
 
===Mass===
{{unsolved|physics|Can we measure the neutrino masses? Do neutrinos follow [[Fermi-Dirac statistics|Dirac]] or [[Majorana fermion|Majorana]] statistics?}}
The [[Standard Model]] of particle physics assumed that neutrinos are massless. However the experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavour states with neutrino mass states (analagously to [[Cabibbo–Kobayashi–Maskawa matrix|CKM mixing]]), requires neutrinos to have nonzero masses.<ref name="PhysRevD">
{{cite journal
|author=J. Schechter, J.W.F. Valle
|year=1980
|title=Neutrino Masses in SU(2) x U(1) Theories
|journal=[[Physical Review D]]
|volume=22 |issue=9 |page=2227
|bibcode=1980PhRvD..22.2227S
|doi=10.1103/PhysRevD.22.2227
}}</ref> Massive neutrinos were originally conceived by [[Bruno Pontecorvo]] in the 1950s. Enhancing the basic framework to accommodate their mass is straightforward by adding a [[right-handed Lagrangian]]. This can be done in two ways. If, like other fundamental Standard Model particles, mass is generated by the [[Dirac fermion|Dirac mechanism]], then the framework would require a [[SU(2) singlet]]. This particle would have no other Standard Model interactions (apart from the [[Yukawa interaction]]s with the neutral component of the [[Higgs doublet]]), so is called a sterile neutrino. Or, mass can be  generated by the [[Majorana mass|Majorana mechanism]], which would require the neutrino and antineutrino to be the same particle.
 
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 {{val|50|ul=eV}} per neutrino, there would be so much mass in the universe that it would collapse.{{Citation needed|date=September 2010}} 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 summed masses of the three neutrino varieties must be less than {{val|0.3|u=eV}}.<ref>
{{cite journal
|author=A. Goobar, S. Hannestad, E. Mörtsell, H. Tu
|year=2006
|title=The neutrino mass bound from WMAP 3 year data, the baryon acoustic peak, the SNLS supernovae and the Lyman-α forest
|journal=[[Journal of Cosmology and Astroparticle Physics]]
|volume=606 |issue=6 |page=19
|arxiv=astro-ph/0602155
|bibcode=2006JCAP...06..019G
|doi=10.1088/1475-7516/2006/06/019
}}</ref>
 
In 1998, research results at the [[Super-Kamiokande]] neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass.<ref>
{{cite journal
|author=Fukuda, Y., ''et al''
|year=1998
|title=Measurements of the Solar Neutrino Flux from Super-Kamiokande's First 300 Days
|journal=[[Physical Review Letters]]
|volume=81 |issue=6 |pages=1158–1162
|arxiv=hep-ex/9805021
|bibcode=1998PhRvL..81.1158F
|doi=10.1103/PhysRevLett.81.1158
}}</ref> While this shows that neutrinos have mass, the absolute neutrino mass scale is still not known. This is because neutrino oscillations are sensitive only to the difference in the squares of the masses.<ref>
{{cite journal
|author=R.N. Mohapatra ''et al.'' ([[American Physical Society|APS]] neutrino theory working group)
|year=2007
|title=Theory of Neutrinos: A White Paper
|journal=[[Reports on Progress in Physics]]
|volume=70 |issue=11 |page=1757
|arxiv=hep-ph/0510213
|bibcode=2007RPPh...70.1757M
|doi=10.1088/0034-4885/70/11/R02
}}</ref> The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by [[KamLAND]] in 2005: Δ''m''{{Su|b=21|p=2}}&nbsp;=&nbsp;{{val|0.000079|u=eV<sup>2</sup>}}.<ref>
{{cite journal
|author=T. Araki ''et al.'' ([[KamLAND|KamLAND Collaboration]])
|year=2005
|title=Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion
|journal=[[Physical Review Letters]]
|volume=94 |issue=8 |page=081801
|arxiv=hep-ex/0406035
|bibcode=2005PhRvL..94h1801A
|doi=10.1103/PhysRevLett.94.081801
|pmid=15783875
}}</ref> 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 |Δ''m''{{Su|b=32|p=2}}|&nbsp;=&nbsp;{{val|0.0027|u=eV<sup>2</sup>}}, consistent with previous results from Super-Kamiokande.<ref>
{{cite press
|title=MINOS experiment sheds light on mystery of neutrino disappearance
|url=http://www.fnal.gov/pub/presspass/press_releases/minos_3-30-06.html
|publisher=[[Fermilab]]
|date=30 March 2006
|accessdate=2007-11-25
}}</ref> Since |Δ''m''{{Su|b=32|p=2}}| is the difference of two squared masses, at least one of them has to have a value which is at least the square root of this value. Thus, there exists at least one neutrino mass eigenstate with a mass of at least {{val|0.04|u=eV}}.<ref>
{{cite journal
|author=C. Amsler ''et al.'' ([[Particle Data Group]])
|year=2008
|title=The Review of Particle Physics: Neutrino Mass, Mixing, and Flavor Change
|url=http://pdg.lbl.gov/2008/reviews/rpp2008-rev-neutrino-mixing.pdf
|journal=[[Physics Letters B]]
|volume=667 |issue= |page=1
|bibcode=2008PhLB..667....1P
|doi=10.1016/j.physletb.2008.07.018
}}</ref>
 
In 2009 lensing data of a galaxy cluster were analyzed to predict a neutrino mass of about {{val|1.5|u=eV}}.<ref>
{{cite journal
|author=Th. M. Nieuwenhuizen
|year=2009
|title=Do non-relativistic neutrinos constitute the dark matter?
|journal=[[Europhysics Letters]]
|volume=86 |issue=5 |page=59001
|bibcode=2009EL.....8659001N
|doi=10.1209/0295-5075/86/59001
|arxiv = 0812.4552 }}</ref> All neutrino masses are then nearly equal, with neutrino oscillations of order meV. They lie below the Mainz-Troitsk upper bound of {{val|2.2|u=eV}} for the electron antineutrino.<ref>"The most sensitive analysis on the neutrino mass [...]  is compatible with a neutrino mass of zero. Considering its uncertainties this value corresponds to an upper limit on the electron neutrino mass of ''m'' < 2.2 eV/''c''<sup>2</sup> (95% Confidence Level)" [http://www.physik.uni-mainz.de/exakt/neutrino/en_experiment.html The Mainz Neutrino Mass Experiment]</ref> The latter will be tested in 2015 in the [[KATRIN]] experiment, that searches for a mass between {{val|0.2|u=eV}} and {{val|2|u=eV}}.
 
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 [[Double beta decay#Neutrinoless double-beta decay|neutrinoless double beta decay]] (e.g. [[GERDA]], [[CUORE]]/[[Cuoricino]], [[Neutrino Ettore Majorana Observatory|NEMO-3]] and others).
 
On 31 May 2010, [[OPERA experiment|OPERA]] researchers observed the first [[tau neutrino]] candidate event in a [[muon neutrino]] beam, the first time a transformation in neutrinos had been observed, giving evidence that they have mass.<ref>
{{cite journal
|author=N. Agafonova ''et al.'' (OPERA Collaboration)
|year=2010
|title=Observation of a first ν<sub>τ</sub> candidate event in the OPERA experiment in the CNGS beam
|journal=[[Physics Letters B]]
|volume=691  |issue=3 |pages=138–145
|arxiv=1006.1623
|bibcode=2010PhLB..691..138A
|doi=10.1016/j.physletb.2010.06.022
}}</ref>
 
In July 2010 the 3-D MegaZ DR7 galaxy survey reported that they had measured a limit of the combined mass of the three neutrino varieties to be less than {{val|0.28|u=eV}}.<ref>
{{cite journal
|author=S. Thomas, F. Abdalla, O. Lahav
|year=2010
|title=Upper Bound of 0.28 eV on Neutrino Masses from the Largest Photometric Redshift Survey
|journal=[[Physical Review Letters]]
|volume=105 |issue=3 |page=031301
|arxiv= 0911.5291
|bibcode=2010PhRvL.105c1301T
|doi=10.1103/PhysRevLett.105.031301
}}</ref> A tighter upper bound yet for this sum of masses,  {{val|0.23|u=eV}}, was reported in March 2013 by the [[Planck (spacecraft)|Planck collaboration]].<ref>Planck Collaboration, [http://arxiv.org/abs/1303.5076  arXiv:1303.5076].</ref>
 
If the neutrino is a [[Majorana fermion|Majorana particle]], the mass can be calculated by finding the [[half life]] of [[Double beta decay|neutrinoless double-beta decay]] of certain nuclei. The lowest upper limit, on the Majorana mass of the neutrino, has been set by [[EXO-200]] 140–380 meV<ref>{{cite journal|last=Auger|first=M|coauthors=et. al|title=Search for Neutrinoless Double-Beta Decay in 136Xe with EXO-200|journal=Phys Rev Lett|date=19|year=2012|month=07|volume=109|issue=3|page=6|doi=10.1103/PhysRevLett.109.032505|arxiv = 1205.5608 |bibcode = 2012PhRvL.109c2505A }}</ref>
 
===Size===
The physical size of neutrinos can be defined using their electroweak radius (apparent size in [[electroweak interaction]]). The average electroweak characteristic size is ⟨r²⟩ = n × 10<sup>−33</sup> cm² (n × 1 [[Barn (unit)|nanobarn]]), where n = 3.2 for electron neutrino, n = 1.7 for muon neutrino and 1.0 for tau neutrino; it depends on no other properties than mass.<ref>
{{cite journal
|author=J. Lucio, A. Rosado, A. Zepeda
|year=1985
|title=Characteristic size for the neutrino
|journal=[[Physical Review D]]
|volume=31 |issue=5 |page=1091
|arxiv=
|bibcode= 1985PhRvD..31.1091L
|doi=10.1103/PhysRevD.31.1091
}}</ref> However, this is best understood as being relevant only to probability of scattering. Since the neutrino does not interact electromagnetically, and is defined quantum mechanically by a wavefunction instead of a single point in space, it does not have a size in the same sense as everyday objects.<ref>{{citation |last=Choi |first=Charles Q. |date=2 June 2009|journal=National Geographic News |url=http://news.nationalgeographic.com/news/2009/06/090602-particles-larger-than-galaxies.html |title=Particles Larger Than Galaxies Fill the Universe? }}</ref> Furthermore, processes that produce neutrinos impart such high energies to them that they travel at almost the speed of light. Nevertheless, neutrinos are fermions, and thus obey the [[Pauli exclusion principle]], i.e. in principle they are solid objects that cannot be packed into arbitrary densities.
 
===Chirality===
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 neutrino]]s), 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''<sub>ν</sub>/''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 {{val|100|u=keV}}–{{val|1|u=MeV}}, so the fraction of neutrinos with "wrong" helicity among them cannot exceed {{val|e=-10}}.<ref>
{{cite web
|author=B. Kayser
|year=2005
|title=Neutrino mass, mixing, and flavor change
|url=http://pdg.lbl.gov/2006/reviews/numixrpp.pdf
|publisher=[[Particle Data Group]]
|accessdate=2007-11-25
}}</ref><ref>
{{cite journal
|author=S.M. Bilenky, C. Giunti
|year=2001
|title=Lepton Numbers in the framework of Neutrino Mixing
|url=http://www.nu.to.infn.it/pap/0102320/
|journal=[[International Journal of Modern Physics A]]
|volume=16 |issue=24 |pages=3931–3949
|arxiv=hep-ph/0102320
|bibcode=2001IJMPA..16.3931B
|doi=10.1142/S0217751X01004967
}}</ref>
 
==Sources==
 
===Artificial===
====Reactor neutrinos====
[[Nuclear reactor]]s are the major source of human-generated neutrinos. Antineutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the antineutrino flux are {{SimpleNuclide2|uranium|235|link=yes}}, {{SimpleNuclide2|uranium|238|link=yes}}, {{SimpleNuclide2|plutonium|239|link=yes}} and {{SimpleNuclide2|plutonium|241|link=yes}} (i.e. via the antineutrinos emitted during [[beta decay|beta-minus decay]] of their respective fission fragments). The average nuclear fission releases about {{val|200|u=MeV}} of energy, of which roughly 4.5% (or about {{val|9|u=MeV}})<ref>
{{cite web
|year=2008
|url=http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html
|title=Nuclear Fission and Fusion, and Nuclear Interactions
|publisher=[[NLP National Physical Laboratory]]
|accessdate=2009-06-25
}}</ref> is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of {{val|4000|ul=MW}}, meaning that the core produces this much heat, and an electrical power generation of {{val|1300|u=MW}}, the total power production from fissioning atoms is actually {{val|4185|u=MW}}, of which {{val|185|u=MW}} is radiated away as antineutrino radiation and never appears in the engineering. This is to say, {{val|185|u=MW}} of fission energy is ''lost'' from this reactor and does not appear as heat available to run turbines, since the antineutrinos penetrate all building materials essentially without any trace, and disappear.<ref group="nb">Typically about one third of the heat which is deposited in a reactor core is available to be converted to electricity, and a {{val|4000|u=MW}} reactor would produce only {{val|2700|u=MW}} of actual heat, with the rest being converted to its {{val|1300|u=MW}} of electric power production.</ref>
 
The antineutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission antineutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the ''detectable'' antineutrinos from fission have a peak energy between about 3.5 and {{val|4|u=MeV}}, with a maximum energy of about {{val|10|u=MeV}}.<ref>
{{cite journal
|author=A. Bernstein ''et al.''
|year=2002
|title=Nuclear reactor safeguards and monitoring with antineutrino detectors
|journal=[[Journal of Applied Physics]]
|volume=91 |issue=7 |page=4672
|arxiv=nucl-ex/0108001
|bibcode=2002JAP....91.4672B
|doi=10.1063/1.1452775
|last2=Wang
|first2=Y.
|last3=Gratta
|first3=G.
|last4=West
|first4=T.
}}</ref> There is no established experimental method to measure the flux of low energy antineutrinos. Only antineutrinos with an energy above threshold of {{val|1.8|u=MeV}} can be uniquely identified (see ''neutrino detection'' below). An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above this threshold. Thus, an average nuclear power plant may generate over {{val|e=20}} antineutrinos per second above this threshold, but also a much larger number (97%/3% = ~30 times this number) below the energy threshold, which cannot be seen with present detector technology.
 
====Accelerator neutrinos====
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. Efforts to construct an accelerator facility where neutrinos are produced through [[muon]] decays are ongoing.<ref>
{{cite journal
|author=A. Bandyopadhyay ''et al.'' ([http://www.hep.ph.ic.ac.uk/iss/wg1-phys-phen/index.html ISS Physics Working Group])
|year=2007
|title=Physics at a future Neutrino Factory and super-beam facility
|journal=[[Reports on Progress in Physics]]
|volume=72 |issue=10 |page=6201
|arxiv=0710.4947
|bibcode=2009RPPh...72j6201B
|doi=10.1088/0034-4885/72/10/106201
}}</ref> Such a setup is generally known as a [[Neutrino Factory]].
 
====Nuclear bombs====
[[Nuclear bomb]]s also produce very large quantities of neutrinos. [[Fred Reines]] and [[Clyde Cowan]] considered the detection of neutrinos from a bomb prior to their search for reactor neutrinos; a fission reactor was recommended as a better alternative by Los Alamos physics division leader J.M.B. Kellogg.<ref>
{{cite journal
|author=F. Reines, C. Cowan Jr.
|year=1997
|title=The Reines-Cowan Experiments: Detecting the Poltergeist
|url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
|journal=[[Los Alamos Science]]
|volume=25 |page=3
|arxiv=
|bibcode=
|doi=
}}</ref>
 
===Geologic===
{{main|Geoneutrino}}
Neutrinos are part of the natural [[background radiation]]. In particular, the decay chains of {{SimpleNuclide2|uranium|238|link=yes}} and {{SimpleNuclide2|thorium|232|link=yes}} isotopes, as well as{{SimpleNuclide2|potassium|40|link=yes}}, include [[beta decay]]s which emit antineutrinos. 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 antineutrinos coming from reactors. Several future experiments aim at improving the geoneutrino measurement and these will necessarily have to be far away from reactors.
[[File:Proton proton cycle.svg|350px|thumb|Solar neutrinos ([[Proton-proton chain reaction|proton-proton chain]]) in the Standard Solar Model]]
 
===Atmospheric===
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]] (India), [[Osaka City University]] (Japan) and [[Durham University]] (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in [[Kolar Gold Fields]] in India in 1965.
 
===Solar===
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. Each second, about 65 [[1000000000 (number)|billion]] ({{val|6.5|e=10}}) solar neutrinos pass through every square centimeter on the part of the Earth that faces the Sun.<ref name="J. Bahcall et al. 2005 L85–L88"/> 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|thumb|[[supernova 1987a|SN 1987A]]]]
In 1966 Colgate and White<ref name="S. A. Colgate and R. H. White"</ref>
{{cite journal
|author=S. A. Colgate and R. H. White
|year=1966
|title=The Hydrodynamic Behavior of Supernova Explosions
|journal=[[The Astrophysical Journal]]
|volume=143 |issue= |page=626
|arxiv=
|bibcode=1966ApJ...143..626C
|doi=10.1086/148549
}}</ref>
calculated that neutrinos carry away most of the gravitational energy released by the collapse of massive stars, events now categorized as [[Type Ib and Ic supernovae|Type Ib and Ic]] and [[Type II supernova|Type II]] [[supernova]]e. When such stars collapse, matter [[densities]] at the core becomes so high ({{val|e=17|u=kg/m3}}) 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 [[kelvin]]s) 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=A.K. Mann
|year=1997
|title=Shadow of a star: The neutrino story of Supernova 1987A
|url=http://www.whfreeman.com/GeneralReaders/book.asp?disc=TRAD&id_product=1058001008&@id_course=1058000240
|page=122
|publisher=[[W. H. Freeman]]
|isbn=0-7167-3097-9
}}</ref>
 
Colgate and White’s theory of supernova neutrino production was confirmed in 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 scintillator-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. The neutrino signal from the supernova arrived at earth several hours before the arrival of the first electromagnetic radiation, as expected from the evident fact that the latter emerges along with the shock wave. The exceptionally feeble interaction with normal matter allowed the neutrinos to pass through the churning mass of the exploding star, while the electromagnetic photons were slowed.
 
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 depends on the velocity of the shock wave and on the thickness of the outer layer of the star. 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 emerge hours later. The [[Supernova Early Warning System|SNEWS]] project uses a network of neutrino detectors to monitor the sky for candidate supernova events; the neutrino signal will provide a useful advance warning of a star exploding in the Milky Way.
 
===Supernova remnants===
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 at least one million times more energetic, produced from turbulent gaseous environments left over by supernova explosions: the [[supernova remnant]]s. The origin of the cosmic rays was attributed to supernovas by [[Walter Baade]] and [[Fritz Zwicky]]; this hypothesis was refined by [[Vitaly L. Ginzburg]] and [[Sergei I. Syrovatsky]] who attributed the origin to supernova remnants, and supported their claim by the crucial remark, that the cosmic ray losses of the Milky Way is compensated, if the efficiency of acceleration in supernova remnants is about 10 percent. Ginzburg and Syrovatskii's hypothesis is supported by the specific mechanism of "shock wave acceleration" happening in supernova remnants, which is consistent with the original theoretical picture drawn by [[Enrico Fermi]], and is receiving support from observational data. 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 Deep Underwater Neutrino Telescope|Baikal]], [[Antarctic Muon And Neutrino Detector Array|AMANDA]], [[IceCube]], [[ANTARES (telescope)|ANTARES]], [[Neutrino Mediterranean Observatory|NEMO]] and [[Nestor Project|Nestor]]. Related information is provided by [[very high energy gamma ray]] observatories, such as [[VERITAS]], [[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, and 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 [[Pierre Auger Observatory]] or with the dedicated experiment named [[ANtarctic Impulse Transient Antenna|ANITA]].
 
===Big Bang===
{{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 easily move at speeds close to the [[speed of light]]. 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 that 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 {{val|1.9|u=K}} ({{val|1.7|e=-4|u=eV}}) if they are massless, much colder if their mass exceeds {{val|0.001|u=eV}}. Although their density is quite high, they have not yet been observed in the laboratory, as their energy is below thresholds of most detection methods, and due to extremely low neutrino interaction cross-sections at sub-eV energies. In contrast, [[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.
 
==Detection==
{{Main|Neutrino detector}}
 
Neutrinos cannot be detected directly, because they do not ionize the materials they are passing through (they do not carry electric charge and other proposed effects, like the MSW effect, do not produce traceable radiation). A unique reaction to identify antineutrinos, sometimes referred to as [[inverse beta decay]], as applied by Reines and Cowan (see below), requires a very large detector in order to detect a significant number of neutrinos. All detection methods require the neutrinos to carry a minimum threshold energy. So far, there is no detection method for low energy neutrinos, in the sense that potential neutrino interactions (for example by the MSW effect) cannot be uniquely distinguished from other causes. 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. Antineutrinos with an energy above the threshold of {{val|1.8|u=MeV}} caused charged current interactions with the protons in the water, producing positrons and neutrons. This is very much like {{SubatomicParticle|Beta+}} decay, where energy is used to convert a proton into a neutron, a [[positron]] ({{SubatomicParticle|Positron}}) and an [[electron neutrino]] ({{SubatomicParticle|Electron Neutrino}}) is emitted:
 
From known {{SubatomicParticle|Beta+}} decay:
 
: Energy + {{SubatomicParticle|Proton}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}}
 
In the Cowan and Reines experiment, instead of an outgoing neutrino, you have an incoming antineutrino ({{SubatomicParticle|Electron Antineutrino}}) from a nuclear reactor:
 
: Energy (>{{val|1.8|u=MeV}}) + {{SubatomicParticle|Proton}} + {{SubatomicParticle|Electron antineutrino}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Positron}}
 
The resulting positron annihilation with electrons in the detector material created photons with an energy of about {{val|0.5|u=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 {{val|8|u=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 tubes, while [[Borexino]] uses a liquid [[pseudocumene]] scintillator also watched by photomultiplier tubes and the proposed [[NOνA]] detector will use liquid scintillator watched by [[avalanche photodiode]]s. The [[IceCube Neutrino Observatory]] uses {{val|1|u=km<sup>3</sup>}} of the [[Antarctic ice sheet]] near the [[south pole]] with photomultiplier tubes distributed throughout the volume.
 
==Motivation for scientific interest==
Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.
 
Using neutrinos as a probe was first proposed in the mid 20<sup>th</sup> century as a way to detect conditions at the core of the Sun. The solar core cannot be imaged directly because electromagnetic radiation (such as light) is diffused by the great amount and density of matter surrounding the core. On the other hand, neutrinos pass through the Sun with few interactions. Whereas photons emitted from the solar core may require 40,000 years to diffuse to the outer layers of the Sun, neutrinos generated in stellar fusion reactions at the core cross this distance practically unimpeded at nearly the speed of light.<ref>
{{cite book
|author=J.N. Bahcall
|year=1989
|title=Neutrino Astrophysics
|publisher=[[Cambridge University Press]]
|isbn=0-521-37975-X
}}</ref><ref>
{{cite journal
|author=D.R. David Jr.
|year=2003
|title=Nobel Lecture: A half-century with solar neutrinos
|journal=[[Reviews of Modern Physics]]
|volume=75 |issue=3 |page=10
|arxiv=
|bibcode=2003RvMP...75..985D
|doi=10.1103/RevModPhys.75.985
}}</ref>
 
Neutrinos are also useful for probing astrophysical sources beyond our solar system because they are the only known particles that are not significantly [[attenuation|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 swift protons and atomic nuclei, are unable to travel more than about 100 [[megaparsec]]s due to the [[Greisen–Zatsepin–Kuzmin limit]] (GZK cutoff). Neutrinos, in contrast, can travel even greater distances barely attenuated.{{citation needed|date=March 2012}}
 
The galactic core of the [[Milky Way]] is fully obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core might be measurable by Earth-based [[neutrino telescope]]s.{{citation needed|date=September 2011}}
 
Another 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 extremely 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 radiant energy in a short (10-second) burst of neutrinos.<ref>{{cite web|url=http://focus.aps.org/story/v24/st4 |title=Physics – Supernova Starting Gun: Neutrinos |publisher=Focus.aps.org |date=2009-07-17 |accessdate=2012-04-05}}</ref> These neutrinos are a very useful probe for core collapse studies.
 
The rest mass of the neutrino (see above) is an important test of cosmological and astrophysical theories (see ''[[Dark matter]]''). The neutrino's significance in probing cosmological phenomena is as great as any other method, and is thus a major focus of study in astrophysical communities.<ref name="SciAm-Gelmini">
{{cite journal
|author=G.B. Gelmini, A. Kusenko, T.J. Weiler
|date=May 2010
|title=Through Neutrino Eyes
|url=http://www.scientificamerican.com/article.cfm?id=through-neutrino-eyes
|journal=[[Scientific American]]
|volume=302 |issue=5 |pages=38–45
|arxiv=
|bibcode=2010SciAm.302e..38G
|doi=10.1038/scientificamerican0510-38
}}</ref>
 
The study of neutrinos is important in [[particle physics]] because neutrinos typically have the lowest mass, and hence are examples of the lowest energy particles theorized in extensions of the [[Standard Model]] of particle physics.
 
In November 2012 American scientists used a particle accelerator to send a coherent neutrino message through 780 feet of rock. This marks the first use of neutrinos for communication, and future research may permit binary neutrino messages to be sent immense distances through even the densest materials, such as the Earth's core. [http://www.popsci.com/science/article/2012-03/first-time-neutrinos-send-message-through-bedrock?google_editors_picks=true (PopSci)] {{doi-inline|10.1142/S0217732312500770|(''Mod. Phys. Lett. A.'')}}
 
==See also==
{{Book bar|1=Leptons|3=Particles of the Standard Model}}
* [[List of neutrino experiments]]
 
==Notes==
{{Reflist|group=nb|30em}}
 
==References==
{{Reflist|30em}}
 
==Bibliography==
{{refbegin}}
* {{cite web
|author=Tammann, G.A.; Thielemann, F.K.; Trautmann, D.
|year=2003
|title=Opening new windows in observing the Universe
|url=http://www.europhysicsnews.com/full/20/article8/article8.html
|publisher=Europhysics News
|accessdate=2006-06-08
}}
* {{Cite book
|author=Bahcall, John N.
|year=1989
|title=Neutrino Astrophysics
|publisher=[[Cambridge University Press]]
|isbn=0-521-35113-8
}}
* {{Cite book
|author=Close, Frank
|year=2010
|title=Neutrino
|publisher=[[Oxford University Press]]
|isbn=978-0-19-957459-9
}}
* {{Cite book
|author=Griffiths, David J.
|year=1987
|title=Introduction to Elementary Particles
|publisher=[[John Wiley & Sons]]
|isbn=0-471-60386-4
}}
* {{Cite book
|author=Perkins, Donald H.
|year=1999
|title=Introduction to High Energy Physics
|publisher=[[Cambridge University Press]]
|isbn=0-521-62196-8
}}
* {{cite web
|author=Riazuddin
|year=2005
|title=Neutrinos
|url=http://www.ncp.edu.pk/docs/12th_rgdocs/Riazuddin.pdf
|publisher=[[National Center for Physics]]
|accessdate=2010
|authorlink=Riazuddin (physicist)
}}
* {{Cite book
|author=Povh, Bogdan
|year=1995
|title=Particles and Nuclei: An Introduction to the Physical Concepts
|publisher=[[Springer-Verlag]]
|isbn=0-387-59439-6
}}
* {{Cite book
|author=Tipler, Paul; Llewellyn, Ralph
|year=2002
|title=Modern Physics
|edition=4th
|publisher=[[W.H. Freeman]]
|isbn=0-7167-4345-0
}}
* {{cite journal
|title=Neutrino Oscillations, Masses And Mixing
|author=Alberico, W.M. Bilenky, S.M.
|year=2003
|journal=Phys.Part.Nucl. 35 (2004) 297-323; Fiz.Elem.Chast.Atom.Yadra 35 (2004) 545-596
|arxiv=hep-ph/0306239
|bibcode = 2003hep.ph....6239A
|last2=Bilenky
|page=6239 }}
* {{cite web
|author=Bumfiel, Geoff
|date=1 October 2001
|title=The Milky Way's Hidden Black Hole
|url=http://www.sciam.com/article.cfm?id=the-milky-ways-hidden-bla
|work=[[Scientific American]]
|accessdate=2010-04-23
}}
* {{Cite book
|author=Zuber, Kai
|year=2003
|title=Neutrino Physics
|publisher=Institute of Physics Publishing
|isbn=978-0-7503-0750-5
}}
* {{Cite arxiv
|last1=Adam |first1=T.
|coauthors=''et al.'' ([[OPERA|OPERA collaboration]])
|year=2011
|title=Measurement of the neutrino velocity with the OPERA detector in the CNGS beam
|class=hep-ex
|eprint=1109.4897
}}
{{refend}}
 
==External links==
{{Wiktionary|neutrino}}
* [http://www.ps.uci.edu/~superk/neutrino.html "What's a Neutrino?"], Dave Casper ([[University of California, Irvine]])
* [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://www.newscientist.com/channel/fundamentals/mg18524885.900 Measuring the density of the earth's core with neutrinos]
* [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
* [http://www.ps.uci.edu/~superk/neutrino.html What's a neutrino?]
* [http://web.archive.org/web/20070927012833/http://www.mpi-hd.mpg.de/non_acc/POSITIVE-EVID/NEW-2004/PL586-2004.pdf Search for neutrinoless double beta decay with enriched 76Ge in Gran Sasso 1990–2003]
* [http://press.web.cern.ch/press/PressReleases/Releases2010/PR08.10E.html Neutrino caught in the act of changing from muon-type to tau-type], [[CERN]] press release
* [http://www.nytimes.com/2002/04/28/weekinreview/ideas-trends-cosmic-weight-gain-a-wispy-particle-bulks-up.html?pagewanted=all&src=pm Cosmic Weight Gain: A Wispy Particle Bulks Up by George Johnson]
* [http://news.bbc.co.uk/1/hi/science_and_environment/10364160.stm Neutrino 'ghost particle' sized up by astronomers] BBC News 22 June 2010
* [http://www.stuff.co.nz/world/5671848/Pillar-of-physics-challenged Pillar of physics challenged]
* {{cite web|last=Merrifield|first=Michael|title=Neutrinos|url=http://www.sixtysymbols.com/videos/neutrinos.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|coauthors=Copeland, Ed; Bowley, Roger|year=2010}}
* [http://www.youtube.com/watch?v=AYqEtm0X2Sc&feature=youtu.be The Neutrino with Dr. Clyde L. Cowan (Lecture on Project Poltergeist by Clyde Cowan)]
 
{{particles}}
 
[[Category:Dark matter]]
[[Category:Exotic matter]]
[[Category:Leptons]]
[[Category:Neutrinos]]
 
{{Link FA|hu}}

Latest revision as of 06:55, 22 December 2014

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