Editing Neutrino (section)

== Properties and reactions ==
Neutrinos have half-integer [[Spin (physics)|spin]] ({{sfrac| 1 |2}}{{math|''ħ''}}); therefore they are [[fermion]]s. Neutrinos are [[leptons]]; therefore they are [[color charge|colorless]] fermions that cannot interact with the [[gluon]]s of the [[strong interaction|strong force]]. They have only been observed to interact through the [[weak nuclear force|weak force]], although it is assumed that they also interact gravitationally. Since they have non-zero mass, some theories permit neutrinos to interact magnetically, but do not require them to; as yet there is no experimental evidence for a non-zero [[magnetic moment]] in neutrinos.<ref>{{Cite journal |last=Giunti |first=Carlo |last2=Studenikin |first2=Alexander |date=2015-06-16 |title=Neutrino electromagnetic interactions: A window to new physics |url=https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.87.531 |journal=Reviews of Modern Physics |volume=87 |issue=2 |pages=531–591 |doi=10.1103/RevModPhys.87.531|arxiv=1403.6344 }}</ref>

=== Flavor, mass, and their mixing<span id="neutrino_flavor_anchor" class="anchor"></span> ===
<!-- "Neutrino flavor" redirects here -->
Weak interactions create neutrinos in one of three leptonic [[Flavor (particle physics)|flavors]]: [[electron neutrino]]s ({{math|{{SubatomicParticle|Electron neutrino}}}}), [[muon neutrino]]s ({{math|{{SubatomicParticle|Muon neutrino}}}}), or [[tau neutrino]]s ({{math|{{SubatomicParticle|Tau neutrino}}}}), associated with the corresponding charged leptons, the [[electron]] ({{math|{{SubatomicParticle|Electron}}}}), [[muon]] ({{math|{{SubatomicParticle|Muon}}}}), and [[tau (particle)|tau]] ({{math|{{SubatomicParticle|Tau}}}}), respectively.<ref name=Nakamura-Petcov-PDG-2016-νmix>
{{cite journal
 |last1=Nakamura |first1=Kengo
 |last2=Petcov   |first2=Serguey Todorov
 |year=2016
 |title=Neutrino mass, mixing, and oscillations
 |journal=Chinese Physics C
 |volume=40  |page=100001
 |url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-neutrino-mixing.pdf
 |access-date=13 December 2016  |url-status=live  |via=pdg.lbl.gov
 |archive-url=https://web.archive.org/web/20180417202541/http://pdg.lbl.gov/2016/reviews/rpp2016-rev-neutrino-mixing.pdf
 |archive-date=17 April 2018
}}</ref>

Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses; each neutrino flavor state is a linear combination of the three distinct mass [[Quantum state#Eigenstates_and_pure_states|eigenstates]]. Although only differences of squares of the three mass values are known as of 2016,<ref name=Capozzi-Lisi-Marrone-etal-2016/> experiments have shown that these masses are tiny compared to any other particle. From [[cosmology|cosmological]] measurements, it has been calculated that the sum of the three neutrino masses must be less than one-millionth that of the electron (below 0.5&nbsp;[[electron volt|eV]]).<ref name=Mertens-2016-mν/><ref name=Olive-PDG-2016-Σmν/> Since the largest difference between squared masses is {{val|24.4|e=−4|u=eV<sup>2</sup>}}, the mass of at least one neutrino must be no less than {{val|0.049|ul=eV}}.<ref name=pdg2010b>{{cite journal |last1=Nakamura |first1=K. |display-authors=etal |collaboration=Particle Data Group |year=2010 |title=Review of particle physics |journal=[[Journal of Physics G]] |volume=37 |issue=7A |pages=1–708 |pmid=10020536 |bibcode=2010JPhG...37g5021N |doi=10.1088/0954-3899/37/7a/075021 |doi-access=free |hdl=10481/34593 |hdl-access=free }}</ref>

More formally, neutrino flavor eigenstates (creation and annihilation combinations) are not the same as the neutrino mass eigenstates (simply labeled "1", "2", and "3"). As of 2024, it is not known which of these three is the heaviest. The [[neutrino mass hierarchy]] consists of two possible configurations. In analogy with the mass hierarchy of the charged leptons, the configuration with mass&nbsp;2 being lighter than mass&nbsp;3 is conventionally called the "normal hierarchy", while in the "inverted hierarchy", the opposite would hold. Several major experimental efforts are underway to help establish which is correct.<ref name=hierarchy>
{{cite press release
 |title=Neutrino mass hierarchy
 |publisher=[[Kamioka Observatory#Hyper-Kamiokande|Hyper-Kamiokande]]
 |url=http://www.hyper-k.org/en/physics/phys-hierarchy.html
 |access-date=14 December 2016 |url-status=live
 |archive-url=https://web.archive.org/web/20170102020153/http://www.hyper-k.org/en/physics/phys-hierarchy.html
 |archive-date=2 January 2017
}}</ref>

A neutrino created in a specific flavor eigenstate is in an associated specific [[quantum superposition]] of all three mass eigenstates. The three masses differ so little that they cannot possibly be distinguished experimentally within any practical flight path. The proportion of each mass state in the pure flavor states produced has been found to depend profoundly on the flavor. The relationship between flavor and mass eigenstates is encoded in the [[PMNS matrix]]. Experiments have established moderate- to low-precision values for the elements of this matrix, with the single complex phase in the matrix being only poorly known, as of 2016.<ref name=Capozzi-Lisi-Marrone-etal-2016/>

A non-zero mass means neutrinos could have a tiny [[magnetic moment]]. If so, neutrinos would interact electromagnetically, albeit probably undetectably considering their enormous velocities. No such interaction has ever been observed.<ref>
{{cite journal
 |last1=Giunti     |first1=Carlo
 |last2=Studenikin |first2=Alexander I.
 |year=2015
 |title=Neutrino electromagnetic interactions: A window to new physics
 |journal=[[Reviews of Modern Physics]]
 |volume=87  |issue=2  |pages=531–591
 |doi=10.1103/RevModPhys.87.531  |arxiv=1403.6344
 |bibcode=2015RvMP...87..531G  |s2cid=119261485
}}
</ref>

=== Flavor oscillations ===
{{Main|Neutrino oscillation}}
Neutrinos [[Neutrino oscillation|oscillate]] between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino, as defined by the flavor of the charged lepton produced in the detector. This oscillation occurs because the three mass state components of the produced flavor travel at slightly different speeds, so that their quantum mechanical [[wave packet]]s develop relative [[Phase (waves)#phase shift|phase shift]]s that change how they combine to produce a varying superposition of three flavors. Each flavor component thereby oscillates as the neutrino travels, with the flavors varying in relative strengths. The relative flavor proportions when the neutrino interacts represent the relative probabilities for that flavor of interaction to produce the corresponding flavor of charged lepton.<ref name=Grossman-Lipkin-1997/><ref name=Bilenky-2016-ν-osc/>

There are other possibilities in which neutrinos could oscillate even if they were massless: If [[Lorentz covariance|Lorentz symmetry]] were not an exact symmetry, neutrinos could experience [[Lorentz-violating neutrino oscillations|Lorentz-violating oscillations]].<ref>
{{cite journal
 |last1=Kostelecký |first1=V. Alan |author-link1=Alan Kostelecky
 |last2=Mewes |first2=Matthew
 |year=2004
 |title=Lorentz and CPT violation in neutrinos
 |journal=Physical Review D
 |volume=69 |issue=1 |page=016005
 |hdl=2022/18691 |s2cid=119024343
 |arxiv=hep-ph/0309025 |bibcode=2004PhRvD..69a6005A
 |doi=10.1103/PhysRevD.69.016005
}}
</ref>

=== 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 [[Mikheyev–Smirnov–Wolfenstein effect]]. Only a small fraction of the neutrino's energy is transferred to the material.<ref>
{{cite web
 |title=Neutrino Oscillations
 |year=2015
 |series=Scientific background on the Nobel Prize in Physics
 |department=Class for Physics of the RSAC
 |publisher=[[Royal Swedish Academy of Sciences]]
 |website=Nobelprize.org
 |url=https://www.nobelprize.org/uploads/2017/09/advanced-physicsprize2015.pdf
 |access-date=2015-11-01 |pages=15–16
}}
</ref>

=== Antineutrinos ===
{{antimatter}}
For each neutrino, there also exists a corresponding [[antiparticle]], called an ''antineutrino'', which also has no electric charge and half-integer spin. They are distinguished from the neutrinos by having opposite signs of [[lepton number]] and opposite [[Chirality (physics)|chirality]] (and consequently opposite-sign weak isospin). As of 2016, no evidence has been found for any other difference.

So far, despite extensive and continuing searches for exceptions, in all observed leptonic processes there has never been any change in total lepton number; for example, if the total lepton number is zero in the initial state, then the final state has only matched lepton and anti-lepton pairs: electron neutrinos appear in the final state together with only positrons (anti-electrons) or electron antineutrinos, and electron antineutrinos with electrons or electron neutrinos.<ref name=FourPeaksAZ-ghostν/><ref name=HypPhys-GSU-consℓ/>

Antineutrinos are produced in nuclear beta decay together with a [[beta particle]] (in beta decay a neutron decays into a proton, electron, and antineutrino). All antineutrinos observed thus far had right-handed [[helicity (particle physics)|helicity]] (i.e., only one of the two possible spin states has ever been seen), while neutrinos were all left-handed.{{efn|Nevertheless, because neutrinos have mass, their helicity is [[Frame of reference|frame]]-dependent, so particle physicists have fallen back on the frame-independent property of [[chirality]] that is closely related to helicity, and for practical purposes the same as the helicity of the ultra-relativistic neutrinos that can be observed in detectors.}}

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]]''). 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
 |publisher=[[Lawrence Livermore National Laboratory]] / [[Sandia National Laboratory]]
 |year=2006
 |title=Applied Antineutrino Physics Project
 |id=LLNL-WEB-204112
 |url=http://neutrinos.llnl.gov/
 |access-date=9 April 2010
 |archive-date=21 April 2021
 |archive-url=https://web.archive.org/web/20210421150739/https://neutrinos.llnl.gov/
 |url-status=live
}}</ref><ref>
{{cite conference
 |title=Workshop
 |conference=Applied Antineutrino Physics
 |year=2007
 |place=Paris, FR
 |url=http://www.apc.univ-paris7.fr/AAP2007/
 |archive-url=https://web.archive.org/web/20071112083328/http://www.apc.univ-paris7.fr/AAP2007/
 |archive-date=2007-11-12
}}
</ref>

=== Majorana mass ===
{{See also|Seesaw mechanism}}
Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Rather than conventional [[Dirac fermion]]s, neutral particles can be another type of spin&nbsp;{{sfrac| 1 |2}} particle called ''[[Majorana particle]]s'', named after the Italian physicist [[Ettore Majorana]] who first proposed the concept. For the case of neutrinos this theory has gained popularity as it can be used, in combination with the [[seesaw mechanism]], to explain why neutrino masses are so small compared to those of the other elementary particles, such as electrons or quarks. Majorana neutrinos would have the property that the neutrino and antineutrino could be distinguished only by chirality; what experiments observe as a difference between the neutrino and antineutrino could simply be due to one particle with two possible chiralities.

{{As of|2019}}, it is not known whether neutrinos are [[Majorana fermion|Majorana]] or [[Dirac fermion|Dirac]] particles. It is possible to test this property experimentally. For example, if neutrinos are indeed Majorana particles, then lepton-number violating processes such as [[neutrinoless double beta decay|neutrinoless double-beta decay]] would be allowed, while they would not if neutrinos are [[Dirac fermion|Dirac]] particles. Several experiments have been and are being conducted to search for this process, e.g. [[GERDA]],<ref name=Giunti-Kim-2007-Fundm-νν/> [[Enriched Xenon Observatory|EXO]],<ref>
{{cite journal |last=Albert |first=J B |date=June 2014 |title=Search for Majorana neutrinos with the first two years of EXO-200 data |journal=[[Nature (journal)|Nature]] |volume=510 |issue=7504 |pages=229–234 |arxiv=1402.6956 |bibcode=2014Natur.510..229T |doi=10.1038/nature13432 |issn=0028-0836 |pmid=24896189 |s2cid=2740003 |collaboration=EXO-200 Collaboration}}
</ref> [[SNO+]],<ref>
{{cite journal
 |last1=Andringa |first1=Sofia |last2=Arushanova |first2=Evelina
 |last3=Asahi |first3=Shigeo |last4=Askins |first4=Morgan
 |last5=Auty |first5=David John |last6=Back |first6=Asheley R.
 |last7=Barnard |first7=Zachariah |last8=Barros |first8=Nuno
 |last9=Beier |first9=Eugene W. |author-link9=Eugene W. Beier
 |year=2016
 |title=Current Status and Future Prospects of the SNO+ Experiment
 |journal=[[Advances in High Energy Physics]]
 |volume=2016 |pages=1–21
 |doi=10.1155/2016/6194250 |issn=1687-7357
 |arxiv=1508.05759 |s2cid=10721441
 |doi-access=free
}}
</ref> and [[CUORE]].<ref>
{{cite journal
 |first1=K. |last1=Alfonso
 |collaboration=CUORE Collaboration
 |year=2015
 |title=Search for Neutrinoless Double-Beta Decay of Te 130 with CUORE-0
 |journal=[[Physical Review Letters]]
 |volume=115 |issue=10 |page=102502
 |doi=10.1103/PhysRevLett.115.102502 |pmid=26382673
 |bibcode=2015PhRvL.115j2502A |arxiv=1504.02454 |s2cid=30807808
}}</ref> The [[cosmic neutrino background]] is also a probe of whether neutrinos are [[Majorana particles]], since there should be a different number of cosmic neutrinos detected in either the Dirac or Majorana case.<ref>
{{cite journal
 |last1=Long |first1=Andrew J.
 |last2=Lunardini |first2=Cecilia |author2-link=Cecilia Lunardini
 |last3=Sabancilar |first3=Eray
 |year=2014
 |title=Detecting non-relativistic cosmic neutrinos by capture on tritium: Phenomenology and physics potential
 |journal=[[Journal of Cosmology and Astroparticle Physics]]
 |volume=1408 |issue=8 |page=038
 |doi=10.1088/1475-7516/2014/08/038 |arxiv=1405.7654
 |bibcode=2014JCAP...08..038L |s2cid=119102568
}}
</ref>

=== Nuclear reactions ===
Neutrinos can interact with a nucleus, changing it to different 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.<ref name=CERN-2001-12-04-SNO>
{{cite periodical
 |title=The Sudbury Neutrino Observatory – Canada's eye on the universe
 |date=4 December 2001
 |periodical=[[CERN Courier]]
 |publisher=[[European Center for Nuclear Research]]
 |url=http://cerncourier.com/cws/article/cern/28553
 |access-date=2008-06-04
 |quote=The detector consists of a 12&nbsp;meter diameter acrylic sphere containing 1000 tonnes of heavy water ... [Solar neutrinos] are detected at SNO via the charged current process of electron neutrinos interacting with deuterons to produce two protons and an electron.
 |archive-date=25 June 2016
 |archive-url=https://web.archive.org/web/20160625002601/http://cerncourier.com/cws/article/cern/28553
 |url-status=live
}}</ref><ref name=Kelić-Zinner-Kolbe-etal-2005/>

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, do not need to be considered for the detection experiment. Within a cubic meter 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.

=== Induced fission and other disintegration events ===
Very much like neutrons do in [[nuclear reactor]]s, neutrinos can induce [[fission reaction]]s within heavy [[atomic nucleus|nuclei]].<ref name=Kolbe-Langanke-Fuller-2004/> 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 isotopes]] seen in the [[universe]].<ref name=Kelić-Zinner-Kolbe-etal-2005/> Neutrino-induced disintegration of [[deuterium]] nuclei has been observed in the Sudbury Neutrino Observatory, which uses a [[heavy water]] detector.<ref>{{cite journal |last1=Bellerive |first1=A |last2=Klein |first2=J.R. |last3=McDonald |first3=A.B. |last4=Noble |first4=A.J. |last5=Poon |first5=A.W.P. |title=The Sudbury Neutrino Observatory |journal=Nuclear Physics B |date=July 2016 |volume=908 |issue= |pages=30–51 |doi=10.1016/j.nuclphysb.2016.04.035 |arxiv=1602.02469 |bibcode=2016NuPhB.908...30B |s2cid=117005142 |url=https://www.sciencedirect.com/science/article/pii/S0550321316300736 |access-date=Nov 20, 2022 |archive-date=21 November 2022 |archive-url=https://web.archive.org/web/20221121044105/https://www.sciencedirect.com/science/article/pii/S0550321316300736 |url-status=live }}</ref>

=== Types ===
{| class="wikitable floatright"
|+Neutrinos in the Standard Model of elementary particles
|-
! Fermion
! Symbol
|-
!colspan="2" style="background:#ffdead;"| Generation&nbsp;1
|-
|style="background:#efefef;"| Electron neutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Electron neutrino}}}}
|-
|style="background:#efefef;"| Electron antineutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Electron antineutrino}}}}
|-
!colspan="3" style="background:#ffdead;"| Generation&nbsp;2
|-
|style="background:#efefef;"| Muon neutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Muon neutrino}}}}
|-
|style="background:#efefef;"| Muon antineutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Muon antineutrino}}}}
|-
!colspan="3" style="background:#ffdead;"| Generation&nbsp;3
|-
|style="background:#efefef;"| Tau neutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Tau neutrino}}}}
|-
|style="background:#efefef;"| Tau antineutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Tau antineutrino}}}}
|}

There are three known types (''flavors'') of neutrinos: electron neutrino {{math|{{SubatomicParticle|electron neutrino}}}}, muon neutrino {{math|{{SubatomicParticle|muon neutrino}}}}, and tau neutrino {{math|{{SubatomicParticle|tau neutrino}}}}, named after their partner leptons in the [[Standard Model]] (see table at right). The current best measurement of the number of neutrino types comes from observing the decay of the [[W and Z bosons|Z boson]]. This particle can decay into any light neutrino and its antineutrino, and the more available types of light neutrinos,{{efn|
In this context, "light neutrino" means neutrinos with less than half the mass of the Z&nbsp;boson.
}} the shorter the lifetime of the Z&nbsp;boson. Measurements of the Z lifetime have shown that three light neutrino flavors couple to the Z.<ref name=Nakamura-Petcov-PDG-2016-νmix/> 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.