Editing Neutrino (section)

== Sources ==
=== Artificial ===

==== Reactor neutrinos ====
Nuclear reactors are the major source of human-generated neutrinos. The majority of energy in a nuclear reactor is generated by fission (the four main fissile isotopes in nuclear reactors are {{SimpleNuclide|uranium|235|link=yes}}, {{SimpleNuclide|uranium|238|link=yes}}, {{SimpleNuclide|plutonium|239|link=yes}} and {{SimpleNuclide|plutonium|241|link=yes}}), the resultant neutron-rich daughter nuclides rapidly undergo additional beta decays, each converting one neutron to a proton and an electron and releasing an electron antineutrino. Including these subsequent decays, the average nuclear fission releases about {{val|200|u=MeV}} of energy, of which roughly 95.5% remains in the core as heat, and roughly 4.5% (or about {{val|9|u=MeV}})<ref>
{{cite book
 |title=Kay & Laby Tables of Physical and Chemical Constants
 |year=2008
 |chapter=Nuclear Fission and Fusion, and Nuclear Interactions
 |publisher=[[National Physical Laboratory (United Kingdom)|National Physical Laboratory]]
 |url=http://www.kayelaby.npl.co.uk/
 |chapter-url=http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html
 |access-date=2009-06-25 
 |archive-url=https://web.archive.org/web/20060425181600/http://www.kayelaby.npl.co.uk/
 |archive-date=2006-04-25
}}
</ref> is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of {{val|4000|ul=MW}},{{efn|
Like all [[thermal power plant]]s, only about one third of the heat generated can be converted to electricity, so a {{val|4000|u=MW}} reactor would produce only {{val|1300|u=MW}} of electric power, with {{val|2700|u=MW}} being [[waste heat]].
}} 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 antineutrinos penetrate all building materials practically without interaction.

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
 |first1=Adam |last1=Bernstein
 |first2=Yifang |last2=Wang |author-link2=Wang Yifang
 |first3=Giorgio |last3=Gratta   
 |first4=Todd |last4=West    
 |date=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 |s2cid=6569332
}}
</ref> There is no established experimental method to measure the flux of low-energy antineutrinos, though experiments to demonstrate the capacity of low-energy neutrino detection via the threshold-less [[Coherent Elastic Neutrino-Nucleus Scattering|CEνNS]] interaction are ongoing. Only antineutrinos with an energy above threshold of {{val|1.8|u=MeV}} can trigger [[inverse beta decay]] and thus be unambiguously identified (see ''{{section link||Detection}}'' below).

An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above that threshold. Thus, an average nuclear power plant may generate over {{val|e=20}} antineutrinos per second above the threshold, but also a much larger number ({{nowrap|97% / 3% ≈ 30}} times this number) below the energy threshold; these lower-energy antineutrinos are invisible to present detector technology.

==== Accelerator neutrinos ====
{{main|Accelerator neutrinos}}
Some [[particle accelerator]]s have been used to make neutrino beams. The technique is to collide [[proton]]s with a fixed target, producing charged [[pion]]s or [[kaon]]s. These unstable particles are then magnetically focused into a long tunnel where they decay while in flight. Because of the [[Lorentz transformation|relativistic boost]] of the decaying particle, the neutrinos are produced as a beam rather than isotropically. Efforts to design an accelerator facility where neutrinos are produced through muon decays are ongoing.<ref name=Bandypdhy-Choubey-Gandhi-Goswami-etal-2009/> Such a setup is generally known as a [[Neutrino Factory|"neutrino factory"]].

==== Collider neutrinos ====
Unlike other artificial sources, colliders produce both neutrinos and anti-neutrinos of all flavors at very high energies. The first direct observation of collider neutrinos was reported in 2023 by the [[FASER experiment]] at the [[Large Hadron Collider]].<ref name="colliderneutrino">{{cite journal |last1=Worcester |first1=Elizabeth |title=The Dawn of Collider Neutrino Physics |journal=Physics |date=July 19, 2023 |volume=16 |page=113 |doi=10.1103/Physics.16.113 |bibcode=2023PhyOJ..16..113W |s2cid=260749625 |access-date=July 23, 2023 |url=https://physics.aps.org/articles/v16/113 |doi-access=free |archive-date=30 July 2023 |archive-url=https://web.archive.org/web/20230730054959/https://physics.aps.org/articles/v16/113 |url-status=live }}</ref>

==== Nuclear weapons ====
[[Nuclear weapon]]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
 |first1=Reines
 |last1=Frederick
 |last2=Cowan
 |first2=Clyde L. Jr.
 |date=1997
 |title=The Reines-Cowan experiments: Detecting the poltergeist
 |journal=[[Los Alamos Science]]
 |volume=25
 |page=3
 |url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
 |access-date=20 October 2009
 |archive-date=21 February 2013
 |archive-url=https://web.archive.org/web/20130221123519/http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
 |url-status=live
}}</ref> Fission weapons produce antineutrinos (from the fission process), and fusion weapons produce both neutrinos (from the fusion process) and antineutrinos (from the initiating fission explosion).

=== Geologic ===
{{main|Geoneutrino}}
[[File:41598 2015 Article BFsrep13945 Fig1 HTML.webp|thumb|upright=1.5|AGM2015: A worldwide v̄<sub>e</sub> flux map combining [[geoneutrino]]s from natural [[Uranium-238]] and [[Thorium-232]] decay in the Earth’s crust and mantle as well as manmade reactor-v̄<sub>e</sub> emitted by power reactors worldwide.]]
Neutrinos are produced together with the natural [[background radiation]]. In particular, the decay chains of {{SimpleNuclide|uranium|238|link=yes}} and {{SimpleNuclide|thorium|232|link=yes}} isotopes, as well as {{SimpleNuclide|potassium|40|link=yes}}, include beta decays 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, updated results have been presented by KamLAND,<ref name=Gando-Gando-Hanakago-Ikeda-etal-2013-KamLAND/> and [[Borexino]].<ref name=Agostini-Appel-Bellini-Benziger-etal-2015/> The main background in the geoneutrino measurements are the antineutrinos coming from reactors.
[[File:Proton proton cycle.svg|upright=1.5|thumb|Solar neutrinos ([[proton–proton chain]]) in the Standard Solar Model]]

=== Atmospheric ===
Atmospheric neutrinos result from the interaction of cosmic rays 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.<ref name=Krishnswmy-Menon-Narasmhn-etal-1971/>

=== Solar ===
{{main|Solar neutrino}}
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 orthogonal to the direction of the Sun.<ref name=Bahcall-Serenelli-Basu-2005/> 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 ===
{{See also|Supernova neutrinos|SuperNova Early Warning System|Diffuse supernova neutrino background}}

[[Image:Supernova-1987a.jpg|thumb|[[SN 1987A]]]]
[[Stirling Colgate|Colgate]] & White (1966)<ref name=Colgate-White-1966/> calculated that neutrinos carry away most of the gravitational energy released during the collapse of massive stars,<ref name=Colgate-White-1966/> events now categorized as [[Type Ib and Ic supernovae|Type&nbsp;Ib and Ic]] and [[Type II supernova|Type&nbsp;II]] supernovae. When such stars collapse, matter [[densities]] at the core become 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. [[Alfred K. Mann|Mann]] (1997)<ref name=Mann-1997-SN1987A/> found a second and more profuse neutrino source is the thermal energy (100&nbsp;billion&nbsp;[[kelvin]]s) of the newly formed neutron core, which is dissipated via the formation of neutrino–antineutrino pairs of all flavors.<ref name=Mann-1997-SN1987A/>

Colgate and White's theory of supernova neutrino production was confirmed in 1987, when neutrinos from Supernova&nbsp;1987A were detected. The water-based detectors [[Kamiokande II]] and [[Irvine–Michigan–Brookhaven (detector)|IMB]] detected 11 and 8&nbsp;antineutrinos (lepton number&nbsp;=&nbsp;&minus;1) of thermal origin,<ref name=Mann-1997-SN1987A/> respectively, while the scintillator-based [[Baksan Neutrino Observatory|Baksan]] detector found 5&nbsp;neutrinos (lepton number&nbsp;=&nbsp;+1) of either thermal or electron-capture origin, in a burst less than 13&nbsp;seconds long. 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, and thus delayed. The neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays, or radio waves. The exact time delay of the electromagnetic waves' arrivals depends on the velocity of the shock wave and on the thickness of the outer layer of the star. For a Type&nbsp;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, after the explosion shock wave has had time to reach the surface of the star. The [[SuperNova Early Warning System]] 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]].<ref>{{Cite journal |last1=Antonioli |first1=Pietro |last2=Fienberg |first2=Richard Tresch |last3=Fleurot |first3=Fabrice |last4=Fukuda |first4=Yoshiyuki |last5=Fulgione |first5=Walter |last6=Habig |first6=Alec |last7=Heise |first7=Jaret |last8=McDonald |first8=Arthur B. |last9=Mills |first9=Corrinne |last10=Namba |first10=Toshio |last11=Robinson |first11=Leif J. |date=2 September 2004 |title=SNEWS: the SuperNova Early Warning System |url=https://iopscience.iop.org/article/10.1088/1367-2630/6/1/114 |journal=[[New Journal of Physics]] |volume=6 |pages=114 |doi=10.1088/1367-2630/6/1/114 |arxiv=astro-ph/0406214 |bibcode=2004NJPh....6..114A |s2cid=119431247 |issn=1367-2630}}</ref>

Although neutrinos pass through the outer gases of a supernova without scattering, they provide information about the deeper supernova core with evidence that here, even neutrinos scatter to a significant extent. In a supernova core the densities are those of a neutron star (which is expected to be formed in this type of supernova),<ref>{{cite web |last=Bartusiak |first=Marcia |author-link=Marcia Bartusiak |title=The short life and violent death of Sanduleak-69 |url=http://www.marciabartusiak.com/uploads/8/5/8/9/8589314/sanduleak.pdf |website=marciabartusiak.com |access-date=2 October 2014 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304041206/http://www.marciabartusiak.com/uploads/8/5/8/9/8589314/sanduleak.pdf |url-status=live }}</ref> becoming large enough to influence the duration of the neutrino signal by delaying some neutrinos. The 13-second-long neutrino signal from SN&nbsp;1987A lasted far longer than it would take for unimpeded neutrinos to cross through the neutrino-generating core of a supernova, expected to be only 3,200&nbsp;kilometers in diameter for SN&nbsp;1987A.

The number of neutrinos counted was also consistent with a total neutrino energy of {{val|2.2|e=46|u=joules}}, which was estimated to be nearly all of the total energy of the supernova.<ref name=Pagliarl-Vissani-etal-2009/>

For an average supernova, approximately {{val|e=57}} (an [[octodecillion]]) neutrinos are released, but the actual number detected at a terrestrial detector <math>N</math> will be far smaller, at the level of
<math display="block">N \sim 10^4 \left(\frac{M}{25 \,\mathsf{kton}}\right) \left(\frac{10 \,\mathsf{kpc}}{d}\right)^2,</math>
where <math>M</math> is the mass of the detector (with e.g. [[Super Kamiokande]] having a mass of 50&nbsp;kton) and <math>d</math> is the distance to the supernova.<ref name=Beacom-Vogel-1999-ν-loc-SN/>
Hence in practice it will only be possible to detect neutrino bursts from supernovae within or nearby the [[Milky Way]] (our own galaxy). In addition to the detection of neutrinos from individual supernovae, it should also be possible to detect the [[diffuse supernova neutrino background]], which originates from all supernovae in the Universe.<ref name=Beacom-2010-diffu-SN-ν/>

=== Supernova remnants ===
The energy of supernova neutrinos ranges from a few to several tens of MeV. 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: [[Supernova remnant]]s. The origin of the cosmic rays was attributed to supernovas by [[Walter Baade|Baade]] and [[Fritz Zwicky|Zwicky]]; this hypothesis was refined by [[Vitaly L. Ginzburg|Ginzburg]] and [[Sergei I. Syrovatsky|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&nbsp;percent. [[Vitaly L. Ginzburg|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]], [[KM3NeT|NEMO]] and [[Nestor Project|Nestor]]. Related information is provided by [[ultra-high-energy gamma ray|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, neutral pions, and gamma rays the environment of a supernova remnant, which 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 Impulsive Transient Antenna|ANITA]].

=== Big Bang ===
{{Main|Cosmic neutrino background}}
It is thought that, just like the cosmic microwave background radiation leftover 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: They are known to exist. This idea also has 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]]. For this reason, 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.

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 cannot make up a significant part of the total amount of dark matter.

From cosmological arguments, relic background neutrinos are estimated to have density of 56&nbsp;of&nbsp;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/c2}}. 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 six [[orders of magnitude]].