Editing Neutrino (section)

== History ==

=== Pauli's proposal ===
The neutrino{{efn|
More specifically, Pauli postulated what is now called the ''electron neutrino''. Two other types were discovered later: see ''[[#Neutrino_flavors_anchor|Neutrino flavor]]'' below.
}} 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 observed [[Beta decay#Neutrinos|continuous energy spectra in beta decay]], Pauli hypothesized an undetected particle that he called a "neutron", using the same ''-on'' ending employed for naming both the [[proton]] and the [[electron]]. He considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay and had a mass similar to the electron.<ref name=Brown-1978-idea-ν>
{{cite journal
 |last=Brown 
 |first=Laurie M. 
 |author-link=Laurie Brown (physicist)
 |year=1978
 |title=The idea of the neutrino
 |journal=[[Physics Today]]
 |volume=31 
 |issue=9 
 |pages=23–28
 |bibcode=1978PhT....31i..23B 
 |doi=10.1063/1.2995181
}}
</ref>{{efn|
[[Niels Bohr]] was notably opposed to this interpretation of beta decay—he was ready to accept that energy, momentum, and angular momentum were not conserved quantities at the atomic level.
}}

[[James Chadwick]] discovered a much more massive neutral nuclear particle in 1932 and named it a [[neutron]] also, leaving two kinds of particles with the same name. The word "neutrino" entered the scientific vocabulary through [[Enrico Fermi]], who used it during a conference in Paris in July&nbsp;1932 and at the Solvay Conference in October&nbsp;1933, where Pauli also employed it. The name (the [[Italian language|Italian]] equivalent of "little neutral one") was jokingly coined by [[Edoardo Amaldi]] during a conversation with Fermi at the Institute of Physics of via Panisperna in Rome, in order to distinguish this light neutral particle from Chadwick's heavy neutron.<ref>
{{cite journal
 |last=Amaldi 
 |first=Edoardo
 |author-link=Edoardo Amaldi
 |year=1984
 |title=From the discovery of the neutron to the discovery of nuclear fission
 |journal=[[Physics Reports]]
 |volume=111 |issue=1–4 |page=306
 |bibcode=1984PhR...111....1A
 |doi=10.1016/0370-1573(84)90214-X
}}
</ref>

In [[Fermi's interaction|Fermi's theory of beta decay]], Chadwick's large neutral particle could decay to a proton, electron, and the smaller neutral particle (now called an ''electron antineutrino''):
: {{math| {{SubatomicParticle|Neutron0}} → {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron-}} + {{SubatomicParticle|Electron antineutrino}} }}

Fermi's paper, written in 1934,<ref name=Fermi-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.<ref name=Fermi-1934>
{{cite journal
 |last=Fermi
 |first=Enrico
 |author-link=Enrico Fermi
 |date=March 1934
 |title=Versuch einer Theorie der β-Strahlen. I
 |trans-title=Search for a theory of β-decay. I
 |journal=[[Zeitschrift für Physik A]]
 |language=de
 |volume=88
 |issue=3–4
 |pages=161–177
 |bibcode=1934ZPhy...88..161F
 |doi=10.1007/BF01351864
 |s2cid=125763380
}}</ref><ref>
{{cite journal
 |last=Wilson
 |first=Fred L.
 |date=1 December 1968
 |title=Fermi's theory of beta decay
 |journal=[[American Journal of Physics]]
 |volume=36
 |issue=12
 |pages=1150–1160
 |bibcode=1968AmJPh..36.1150W
 |doi=10.1119/1.1974382
 |url=http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1
 |access-date=5 October 2011
 |archive-date=12 May 2013
 |archive-url=https://web.archive.org/web/20130512011303/http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1
 |url-status=live
}}</ref><ref name=Close-2012-ν>
{{cite book
 |last=Close
 |first=Frank
 |date=12 April 2012
 |orig-date=2010
 |title=Neutrino
 |publisher=[[Oxford University Press]]
 |page=24
 |isbn=978-0-19-969599-7
}}</ref><!--{{rp|page=24}}-->

By 1934, there was experimental evidence against Bohr's idea that energy conservation is invalid for beta decay: At the [[Solvay conference]] of that year, measurements of the energy spectra of beta particles (electrons) were reported, showing that there is a strict limit on the energy of electrons from each type of beta decay. Such a limit is not expected if the conservation of energy is invalid, in which case any amount of energy would be statistically available in at least a few decays. The natural explanation of the beta decay spectrum as first measured in 1934 was that only a limited (and conserved) amount of energy was available, and a new particle was sometimes taking a varying fraction of this limited energy, leaving the rest for the beta particle. Pauli made use of the occasion to publicly emphasize that the still-undetected "neutrino" must be an actual particle.<ref name=Close-2012-ν/>{{rp|page=25}} The first evidence of the reality of neutrinos came in 1938 via simultaneous cloud-chamber measurements of the electron and the recoil of the nucleus.<ref>
{{cite news
 |title=Cloud-chamber test finds neutrino 'real'
 |date=22 May 1938
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/1938/05/22/archives/cloudchamber-test-finds-neutrino-real-drs-crane-and-halpern-decide.html?sq=%2522H.%2520Richard%2520Crane%2522&scp=1&st=cse
 |quote=Drs.&nbsp;Crane and Halpern decide it is no mere hypothesis
}}
</ref>

=== Direct detection ===
[[File:Clyde Cowan.jpg|thumb|upright=1.1|Fred Reines and Clyde Cowan conducting the neutrino experiment c. 1956]]
In 1942, [[Wang Ganchang]] first proposed the use of [[Electron capture|beta capture]] to experimentally detect neutrinos.<ref>
{{cite journal
 |last=Wang |first=Kan Chang |author-link=Wang Ganchang
 |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&nbsp;July 1956 issue of [[Science (journal)|''Science'']], [[Clyde Cowan]], [[Frederick Reines]], Francis B. "Kiko" Harrison, Herald W. Kruse, and Austin D. McGuire published confirmation that they had detected the neutrino,<ref>
{{cite journal
 |last1=Cowan |first1=Clyde L. Jr. |author1-link=Clyde Cowan
 |last2=Reines |first2=Frederick |author2-link=Frederick Reines
 |last3=Harrison |first3=Francis B. "Kiko"
 |last4=Kruse |first4=Herald W.
 |last5=McGuire |first5=Austin D.
 |year=1956
 |title=Detection of the free neutrino: A confirmation
 |journal=[[Science (journal)|Science]]
 |volume=124 |issue=3212 |pages=103–104
 |bibcode=1956Sci...124..103C
 |doi=10.1126/science.124.3212.103 |pmid=17796274
}}
</ref><ref>
This source reproduces the 1956 paper:<br />{{cite book |last=Winter |first=Klaus |url=https://books.google.com/books?id=v_tiL2NlfvMC&pg=PA38 |title=Neutrino Physics |publisher=[[Cambridge University Press]] |year=2000 |isbn=978-0-521-65003-8 |pages=38&nbsp;ff}}
</ref> a result that was rewarded almost forty years later with the [[Nobel Prize in Physics|1995 Nobel Prize]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1995
 |publisher=[[The Nobel Foundation]]
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1995/
 |access-date=29 June 2010
 |archive-date=30 June 2018
 |archive-url=https://web.archive.org/web/20180630025246/https://www.nobelprize.org/nobel_prizes/physics/laureates/1995/
 |url-status=live
}}</ref>

In this experiment, now known as the [[Cowan–Reines neutrino experiment]], antineutrinos created in a nuclear reactor by beta decay reacted with protons to produce [[neutron]]s and positrons:
: {{math| {{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.

In February&nbsp;1965, the first neutrino found in nature was identified by a group including Frederick Reines and [[Friedel Sellschop]].<ref>{{cite web |url=https://www.space.com/what-are-neutrinos |title=What are neutrinos? |date=2022-09-21 |access-date=2023-12-22 |website=Space.com |last=Cooper |first=Keith |archive-date=22 December 2023 |archive-url=https://web.archive.org/web/20231222154103/https://www.space.com/what-are-neutrinos |url-status=live }}</ref><ref>{{cite journal |last1=Reines |first1=F. |last2=Crouch |first2=M. F. |last3=Jenkins |first3=T. L. |last4=Kropp |first4=W. R. |last5=Gurr |first5=H. S. |last6=Smith |first6=G. R. |last7=Sellschop |first7=J. P. F. |last8=Meyer |first8=B. |title=Evidence for High-Energy Cosmic-Ray Neutrino Interactions |journal=Physical Review Letters |date=30 August 1965 |volume=15 |issue=9 |pages=429–433 |doi=10.1103/PhysRevLett.15.429 |bibcode=1965ICRC....2.1051R |url=https://adsabs.harvard.edu/full/1965ICRC....2.1051R |access-date=22 December 2023}}</ref> The experiment was performed in a specially prepared chamber at a depth of 3&nbsp;km in the [[East Rand Mine|East Rand ("ERPM") gold mine]] near [[Boksburg]], South Africa. A plaque in the main building commemorates the discovery. The experiments also implemented a primitive neutrino astronomy and looked at issues of neutrino physics and weak interactions.<ref>
{{cite journal
 |last1=Johnson |first1=C.D.
 |last2=Tegen |first2=Rudolph
 |title=The ''little neutral one'': An overview of the neutrino
 |date=January 1999
 |journal=[[South African Journal of Science]]
 |volume=95
 |number=95 |pages=13–20
 |hdl=10520/AJA00382353_7822
 |url=https://journals.co.za/doi/pdf/10.10520/AJA00382353_7822
}}
</ref>

=== Neutrino flavor <span class="anchor" id="Neutrino_flavors_anchor"></span> ===
The antineutrino discovered by [[Clyde Cowan]] and [[Frederick Reines]] was the antiparticle of the electron neutrino.

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 journal
 |first=Ivan V. |last=Aničin |author-link=Ivan Aničin
 |year=2005
 |title=The neutrino – its past, present, and future
 |journal=SFIN (Institute of Physics, Belgrade) Year&nbsp;XV
 |series=A: Conferences
 |id=No. A (00)
 |volume=2 |issue=2002 |pages=3–59
 |arxiv=physics/0503172 |bibcode=2005physics...3172A
}}
</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 was also expected to have an associated neutrino (the tau neutrino). The 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 electron neutrino. The first detection of tau neutrino interactions was announced in 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]].<ref>
{{cite news
 |title=Physicists find first direct evidence for Tau neutrino at Fermilab
 |publisher=[[Fermilab]]
 |date=20 July 2000
 |quote=In 1989, experimenters at CERN found proof that the tau neutrino is the third and last light neutrino of the Standard Model, but a direct observation was not yet feasible.
 |url=http://www.fnal.gov/pub/presspass/press_releases/donut.html
 |access-date=9 October 2015
 |archive-date=20 October 2016
 |archive-url=https://web.archive.org/web/20161020025730/http://www.fnal.gov/pub/presspass/press_releases/donut.html
 |url-status=live
}}</ref>

=== Solar neutrino problem ===
{{Main|Solar neutrino problem}}
In the 1960s, the now-famous [[Homestake experiment]] made the first measurement of the flux of electron neutrinos arriving from the core of the Sun and found a value that 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, while possible problems with both the experiment and the solar model were investigated, but none could be found. Eventually, it was realized that both were actually correct and that the discrepancy between them was due to neutrinos being more complex than was previously assumed. It was postulated that the three neutrinos had nonzero and slightly different masses, and could therefore oscillate into undetectable flavors on their flight to the Earth. This hypothesis was investigated by a new series of experiments, thereby opening a new major field of research that still continues. Eventual confirmation of the phenomenon of neutrino oscillation led to two Nobel prizes, one to [[Raymond Davis, Jr.|R. Davis]], who conceived and led the Homestake experiment and [[Masatoshi Koshiba]] of Kamiokande, whose work confirmed it, and one to [[Takaaki Kajita]] of [[Super-Kamiokande]] and [[Arthur B. McDonald|A.B. McDonald]] of [[Sudbury Neutrino Observatory]] for their joint experiment, which confirmed the existence of all three neutrino flavors and found no deficit.<ref name=CERN-2001-12-04-SNO/>

=== Oscillation ===
{{main|Neutrino oscillation}}
A practical method for investigating neutrino oscillations was first suggested by [[Bruno Pontecorvo]] in 1957 using an analogy with [[kaon]] oscillations; over the subsequent 10&nbsp;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
 |last1=Maltoni |first1=Michele
 |last2=Schwetz |first2=Thomas
 |last3=Tórtola |first3=Mariam A.
 |last4=Valle |first4=José W.F. |author-link4=José 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 |s2cid=119459743
}}
</ref> Takaaki Kajita of Japan, and Arthur B. McDonald of Canada, received the 2015 Nobel Prize for Physics for their landmark finding, theoretical and experimental, that neutrinos can change flavors.

=== Cosmic neutrinos ===
{{main|cosmic neutrino background|diffuse supernova neutrino background}}
As well as specific sources, a general background level of neutrinos is expected to pervade the universe, theorized to occur due to two main sources.

; Cosmic neutrino background (Big Bang-originated) :

Around 1&nbsp;second after the [[Big Bang]], neutrinos decoupled, giving rise to a background level of neutrinos known as the [[cosmic neutrino background]] (CNB).

; Diffuse supernova neutrino background (supernova-originated):

[[Raymond Davis, Jr.]] and [[Masatoshi Koshiba]] were jointly awarded the 2002 Nobel Prize in Physics. Both conducted pioneering work on [[solar neutrino]] detection, and Koshiba's work also resulted in the first real-time observation of neutrinos from the [[SN 1987A]] supernova in the nearby [[Large Magellanic Cloud]]. These efforts marked the beginning of [[neutrino astronomy]].<ref name=Pagliarl-Vissani-etal-2009>
{{cite journal
 |last1=Pagliaroli |first1=Giulia
 |last2=Vissani |first2=Francesco
 |last3=Costantini |first3=Maria Laura
 |last4=Ianni |first4=Aldo
 |year=2009
 |title=Improved analysis of SN1987A antineutrino events
 |journal=[[Astroparticle Physics (journal)|Astroparticle Physics]]
 |volume=31 |issue=3 |pages=163–176
 |arxiv=0810.0466 |doi=10.1016/j.astropartphys.2008.12.010
 |bibcode=2009APh....31..163P |s2cid=119089069 
}}
</ref>

[[SN 1987A]] represents the only verified detection of neutrinos from a supernova. However, many stars have exploded as supernovae in the universe, leaving a theorized [[diffuse supernova neutrino background]].