Neutrino: Difference between revisions

[[Weak interactions]] create neutrinos in one of three leptonic [[FlavourFlavor (particle physics)|flavoursflavors]]:

Each flavourflavor is associated with the correspondingly named charged [[lepton]].<ref>{{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}}</ref> Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values (the smallest of which could even be zero<ref>{{cite journal |last1 = Boyle |first1 = Latham |last2 = Finn |first2 = Kiernan |last3 = Turok |first3 = Neil |year = 2022 |title = The Big Bang, CPT, and neutrino dark matter |journal = Annals of Physics |volume = 438 |page = 168767 |doi = 10.1016/j.aop.2022.168767 |arxiv = 1803.08930 |bibcode = 2022AnPhy.43868767B |s2cid = 119252778}}</ref>), but the three masses do not uniquely correspond to the three flavoursflavors: A neutrino created with a specific flavourflavor is a specific mixture of all three mass states (a ''[[quantum superposition]]''). Similar to some [[neutral particle oscillation|other neutral particles]], [[Neutrino oscillation|neutrinos oscillate]] between different flavoursflavors in flight as a consequence. For example, an electron neutrino produced in a [[beta decay]] reaction may interact in a distant detector as a muon or tau neutrino.<ref name=Grossman-Lipkin-1997>{{cite journal |last1=Grossman |first1=Yuval |last2=Lipkin |first2=Harry J. |author2-link=Harry J. Lipkin |year=1997 |title=Flavor oscillations from a spatially localized source — A simple general treatment |journal=[[Physical Review|Physical Review D]] |volume=55 |issue=5 |page=2760 |arxiv=hep-ph/9607201 |bibcode=1997PhRvD..55.2760G |doi=10.1103/PhysRevD.55.2760 |s2cid=9032778}}</ref><ref name=Bilenky-2016-ν-osc>{{cite journal |first=Samoil M. |last=Bilenky |year=2016 |arxiv=1602.00170 |bibcode=2016NuPhB.908....2B |doi=10.1016/j.nuclphysb.2016.01.025 |title=Neutrino oscillations: From a historical perspective to the present status |journal=[[Nuclear Physics (journal)|Nuclear Physics B]] |volume=908 |pages=2–13 |s2cid=119220135}}</ref> The three mass values are not yet known as of 2024, but laboratory experiments and [[Cosmology|cosmological]] observations have determined the differences of their squares,<ref name=Capozzi-Lisi-Marrone-etal-2016>{{cite journal |last1=Capozzi |first1=Francesco |last2=Lisi |first2=Eligio |last3=Marrone |first3=Antonio |last4=Montanino |first4=Daniele |last5=Palazzo |first5=Antonio |year=2016 |title=Neutrino masses and mixings: Status of known and unknown 3ν parameters |journal=Nuclear Physics B |volume=908 |pages=218–234 |arxiv=1601.07777 |bibcode=2016NuPhB.908..218C |doi=10.1016/j.nuclphysb.2016.02.016 |s2cid=119292028}}</ref> an upper limit on their sum (<&nbsp;{{val|2.14|e=-37|u=kg}}),<ref name=Mertens-2016-mν/><ref name=Olive-PDG-2016-Σmν>{{cite journal |last1=Olive |first1=Keith A. |author-link=Keith Olive |collaboration=[[Particle Data Group]] |year=2016 |title=Sum of neutrino masses |journal=Chinese Physics C |volume=40 |issue=10 |page=100001 |bibcode=2016ChPhC..40j0001P |doi=10.1088/1674-1137/40/10/100001 |s2cid=125766528 |url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-sum-neutrino-masses.pdf}}</ref> and an upper limit on the mass of the electron neutrino.<ref name=KATRIN-2022-NatPhys/>

The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun. At the surface of the Earth, the flux is about 65&nbsp;billion ({{val|6.5|e=10}}) [[solar neutrino]]s, per second per square centimetrecentimeter.<ref name=Armitage-JILA-2003-sol-ν>

===Neutrino flavourflavor{{anchor|Neutrino_flavors_anchor}}===

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 flavoursflavors 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|SNO]] for their joint experiment, which confirmed the existence of all three neutrino flavoursflavors and found no deficit.<ref name=CERN-2001-12-04-SNO/>

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 flavourflavor 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 flavoursflavors (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 flavoursflavors 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 flavourflavor 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 flavourflavor 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>

</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 flavoursflavors.

===FlavourFlavor, mass, and their mixing===

Weak interactions create neutrinos in one of three leptonic [[FlavourFlavor (particle physics)|flavoursflavors]]: electron neutrinos ({{math|{{SubatomicParticle|Electron neutrino}}}}), muon neutrinos ({{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>

Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses; each neutrino flavourflavor state is a linear combination of the three discrete mass 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.<ref name=Mertens-2016-mν/><ref name=Olive-PDG-2016-Σmν/>

More formally, neutrino flavourflavor [[Eigenvalues and eigenvectors|eigenstates]] (creation and annihilation combinations) are not the same as the neutrino mass eigenstates (simply labelledlabeled "1", "2", and "3"). As of 2016, 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>

A neutrino created in a specific flavourflavor 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 flavourflavor states produced has been found to depend profoundly on the flavourflavor. The relationship between flavourflavor 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/>

===FlavourFlavor oscillations===

Neutrinos [[Neutrino oscillation|oscillate]] between different flavoursflavors 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 flavourflavor of the charged lepton produced in the detector. This oscillation occurs because the three mass state components of the produced flavourflavor 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 flavoursflavors. Each flavourflavor component thereby oscillates as the neutrino travels, with the flavoursflavors varying in relative strengths. The relative flavourflavor proportions when the neutrino interacts represent the relative probabilities for that flavourflavor of interaction to produce the corresponding flavourflavor of charged lepton.<ref name=Grossman-Lipkin-1997/><ref name=Bilenky-2016-ν-osc/>

There are three known types (''flavoursflavors'') 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|

the shorter the lifetime of the Z&nbsp;boson. Measurements of the Z lifetime have shown that three light neutrino flavoursflavors 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.

International scientific collaborations install large neutrino detectors near nuclear reactors or in neutrino beams from particle accelerators to better constrain the neutrino masses and the values for the magnitude and rates of oscillations between neutrino flavoursflavors. These experiments are thereby searching for the existence of [[CP violation]] in the neutrino sector; that is, whether or not the laws of physics treat neutrinos and antineutrinos differently.<ref name=Capozzi-Lisi-Marrone-etal-2016/>

The three known neutrino flavoursflavors are the only candidates for [[dark matter]] that are experimentally established elementary particles – specifically, they would be [[hot dark matter]]. However, the currently known neutrino types seem to be essentially ruled out as a substantial proportion of dark matter, based on observations of the [[cosmic microwave background]]. It still seems plausible that heavier, sterile neutrinos might compose [[warm dark matter]], if they exist.<ref>

Other efforts search for evidence of a [[sterile neutrino]] – a fourth neutrino flavourflavor that would not interact with matter like the three known neutrino flavoursflavors.<ref>

In 1998, research results at the [[Super-Kamiokande]] neutrino detector determined that neutrinos can oscillate from one flavourflavor to another, which requires that they must have a nonzero mass.<ref name=Fukuda-Hayakawa-Ichihara-Inoue-etal-1998/> 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 name=Mohapatra-Antusch-Babu-Barenboim-etal-2007/>

Unlike other artificial sources, colliders produce both neutrinos and anti-neutrinos of all flavoursflavors 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 }}</ref>

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 centimetrecentimeter 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.

[[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 flavoursflavors.<ref name=Mann-1997-SN1987A/>

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 used [[heavy water]] as the detecting medium, which uses the same effects, but also allows the additional reaction any-flavourflavor 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 used a solid plastic [[scintillator]] coupled to photomultiplier tubes, while Borexino uses a liquid [[pseudocumene]] scintillator also watched by photomultiplier tubes and the [[NOνA]] detector uses liquid scintillator watched by [[avalanche photodiode]]s. The [[IceCube Neutrino Observatory]] uses {{val|1|u=km3}} of the [[Antarctic ice sheet]] near the [[south pole]] with photomultiplier tubes distributed throughout the volume.