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100-460: Neutrinos are elementary particles first detected in the 1950s, long after their theoretical prediction by theorist Wolfgang Pauli . Neutrinos (or anti-neutrinos) are created during certain nuclear reactions , where protons are transformed into neutrons and vice versa. Neutrinos do not interact with matter via either the electromagnetic , the strong nuclear, or gravitational forces, since they are electrically neutral leptons and their rest mass

200-455: A ν μ (with T 3 = ⁠+ + 1 / 2 ⁠ ) and a μ (as a right-handed antiparticle, ⁠+ + 1 / 2 ⁠ ). For the development of the electroweak theory, another property, weak hypercharge , was invented, defined as where Y W is the weak hypercharge of a particle with electrical charge Q (in elementary charge units) and weak isospin T 3 . Weak hypercharge

300-507: A W  boson or by absorbing a W  boson. More precisely, the down-type quark becomes a quantum superposition of up-type quarks: that is to say, it has a possibility of becoming any one of the three up-type quarks, with the probabilities given in the CKM matrix tables. Conversely, an up-type quark can emit a W  boson, or absorb a W  boson, and thereby be converted into

400-425: A T 3 of ⁠− + 1 / 2 ⁠ and conversely. In any given strong, electromagnetic, or weak interaction, weak isospin is conserved : The sum of the weak isospin numbers of the particles entering the interaction equals the sum of the weak isospin numbers of the particles exiting that interaction. For example, a (left-handed) π , with a weak isospin of +1 normally decays into

500-535: A neutron is heavier than a proton (its partner nucleon ) and can decay into a proton by changing the flavour (type) of one of its two down quarks to an up quark. Neither the strong interaction nor electromagnetism permit flavour changing, so this can only proceed by weak decay ; without weak decay, quark properties such as strangeness and charm (associated with the strange quark and charm quark, respectively) would also be conserved across all interactions. All mesons are unstable because of weak decay. In

600-448: A quark or a lepton (e.g., an electron or a muon ) emits or absorbs a neutral Z boson . For example: Like the W  bosons, the Z  boson also decays rapidly, for example: Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, and / or weak isospin, the neutral-current Z interaction can cause any two fermions in

700-400: 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 packets develop relative phase shifts that change how they combine to produce

800-573: A complex scalar Higgs field doublet. Likewise, there are four massless electroweak vector bosons, each similar to the photon . However, at low energies, this gauge symmetry is spontaneously broken down to the U(1) symmetry of electromagnetism, since one of the Higgs fields acquires a vacuum expectation value . Naïvely, the symmetry-breaking would be expected to produce three massless bosons , but instead those "extra" three Higgs bosons become incorporated into

900-508: A compound symmetry CP to be conserved. CP combines parity P (switching left to right) with charge conjugation C (switching particles with antiparticles). Physicists were again surprised when in 1964, James Cronin and Val Fitch provided clear evidence in kaon decays that CP symmetry could be broken too, winning them the 1980 Nobel Prize in Physics . In 1973, Makoto Kobayashi and Toshihide Maskawa showed that CP violation in

1000-412: A contact force with no range. In the mid-1950s, Chen-Ning Yang and Tsung-Dao Lee first suggested that the handedness of the spins of particles in weak interaction might violate the conservation law or symmetry. In 1957, Chien Shiung Wu and collaborators confirmed the symmetry violation . In the 1960s, Sheldon Glashow , Abdus Salam and Steven Weinberg unified the electromagnetic force and

1100-594: A corresponding antiparticle , called an antineutrino , which also has spin of ⁠ 1  / 2 ⁠ and no electric charge. Antineutrinos are distinguished from neutrinos by having opposite-signed lepton number and weak isospin , and right-handed instead of left-handed chirality. To conserve total lepton number (in nuclear beta decay), electron neutrinos only appear together with positrons (anti-electrons) or electron-antineutrinos, whereas electron antineutrinos only appear with electrons or electron neutrinos. Neutrinos are created by various radioactive decays ;

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1200-433: A corresponding neutrino (with a charge of 0), where the type ("flavour") of neutrino (electron ν e , muon ν μ , or tau ν τ ) is the same as the type of lepton in the interaction, for example: Similarly, a down-type quark ( d , s , or b , with a charge of ⁠− + 1  / 3 ⁠ ) can be converted into an up-type quark ( u , c , or t , with a charge of ⁠+ + 2  / 3 ⁠ ), by emitting

1300-628: 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 or 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 would be allowed, while they would not if neutrinos are Dirac particles. Several experiments have been and are being conducted to search for this process, e.g. GERDA , EXO , SNO+ , and CUORE . The cosmic neutrino background

1400-408: A down-type quark, for example: The W boson is unstable so will rapidly decay, with a very short lifetime. For example: Decay of a W boson to other products can happen, with varying probabilities. In the so-called beta decay of a neutron (see picture, above), a down quark within the neutron emits a virtual W boson and is thereby converted into an up quark, converting

1500-504: A gamma ray. The coincidence of both events—positron annihilation and neutron capture—gives a unique signature of an antineutrino interaction. In February 1965, the first neutrino found in nature was identified by a group including Frederick Reines and Friedel Sellschop . The experiment was performed in a specially prepared chamber at a depth of 3 km in the East Rand ("ERPM") gold mine near Boksburg , South Africa. A plaque in

1600-605: A laboratory, but is predicted to happen within stars and supernovae. The process affects the abundance of isotopes seen in the universe . Neutrino-induced disintegration of deuterium nuclei has been observed in the Sudbury Neutrino Observatory, which uses a heavy water detector. There are three known types ( flavors ) of neutrinos: electron neutrino ν e , muon neutrino ν μ , and tau neutrino ν τ , named after their partner leptons in

1700-455: A life of only about 10  seconds. In contrast, a charged pion can only decay through the weak interaction, and so lives about 10  seconds, or a hundred million times longer than a neutral pion. A particularly extreme example is the weak-force decay of a free neutron, which takes about 15 minutes. All particles have a property called weak isospin (symbol T 3 ), which serves as an additive quantum number that restricts how

1800-516: A lifetime of under 10  seconds. The weak interaction has a coupling constant (an indicator of how frequently interactions occur) between 10 and 10 , compared to the electromagnetic coupling constant of about 10 and the strong interaction coupling constant of about 1; consequently the weak interaction is "weak" in terms of intensity. The weak interaction has a very short effective range (around 10 to 10  m (0.01 to 0.1 fm)). At distances around 10  meters (0.001 fm),

1900-419: A muon or tau neutrino. The three mass values are not yet known as of 2024, but laboratory experiments and cosmological observations have determined the differences of their squares, an upper limit on their sum (<  2.14 × 10  kg ), and an upper limit on the mass of the electron neutrino. Neutrinos are fermions with spin of ⁠ 1  / 2 ⁠ . For each neutrino, there also exists

2000-475: A neutron (an up quark is changed to a down quark), and an electron neutrino is emitted. Due to the large masses of the W ;bosons, particle transformations or decays (e.g., flavour change) that depend on the weak interaction typically occur much more slowly than transformations or decays that depend only on the strong or electromagnetic forces. For example, a neutral pion decays electromagnetically, and so has

2100-473: A new major field of research that still continues. Eventual confirmation of the phenomenon of neutrino oscillation led to two Nobel prizes, one to 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 A.B. McDonald of Sudbury Neutrino Observatory for their joint experiment, which confirmed

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2200-605: A number of predictions, including a prediction of the masses of the Z and W  bosons before their discovery and detection in 1983. On 4 July 2012, the CMS and the ATLAS experimental teams at the Large Hadron Collider independently announced that they had confirmed the formal discovery of a previously unknown boson of mass between 125 and 127 GeV/ c , whose behaviour so far

2300-508: A planned telescope site off the coast of Pylos (which can thus be seen as a continuation of the NESTOR project to build an underwater telescope off the coast of Pylos). Neutrinos A neutrino ( / nj uː ˈ t r iː n oʊ / new- TREE -noh ; denoted by the Greek letter ν ) is an elementary particle that interacts via the weak interaction and gravity . The neutrino

2400-566: A process analogous to 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. Onia 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

2500-418: A proton, electron, and the smaller neutral particle (now called an electron antineutrino ): 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. By 1934, there was experimental evidence against Bohr's idea that energy conservation is invalid for beta decay: At

2600-399: A significant number of neutrinos. After completion, NESTOR will consist of a large number of glass balls (the "eyes") containing photomultiplier tubes. The "eyes" are connected with star-shaped titanium frames. Many frames compose a NESTOR tower. The whole construction is placed at the bottom of the sea to reduce noise from cosmic radiation (depth 4000m). The detectors are connected with

2700-506: A star. This is because it can convert a proton (hydrogen) into a neutron to form deuterium which is important for the continuation of nuclear fusion to form helium. The accumulation of neutrons facilitates the buildup of heavy nuclei in a star. Most fermions decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14 . It can also create radioluminescence , commonly used in tritium luminescence , and in

2800-605: 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. There are other possibilities in which neutrinos could oscillate even if they were massless: If Lorentz symmetry were not an exact symmetry, neutrinos could experience Lorentz-violating oscillations . Neutrinos traveling through matter, in general, undergo

2900-470: A weak isospin value of either ⁠+ + 1 / 2 ⁠ or ⁠− + 1 / 2 ⁠ ; all right-handed fermions have 0 isospin. For example, the up quark has T 3 = ⁠+ + 1 / 2 ⁠ and the down quark has T 3 = ⁠− + 1 / 2 ⁠ . A quark never decays through the weak interaction into a quark of the same T 3 : Quarks with a T 3 of ⁠+ + 1 / 2 ⁠ only decay into quarks with

3000-472: 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. Neutrinos can interact with a nucleus, changing it to another nucleus. This process is used in radiochemical neutrino detectors . In this case, the energy levels and spin states within the target nucleus have to be taken into account to estimate

3100-493: 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. 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

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3200-463: 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:

3300-488: Is less than the diameter of a proton. The Standard Model of particle physics provides a uniform framework for understanding electromagnetic, weak, and strong interactions. An interaction occurs when two particles (typically, but not necessarily, half-integer spin fermions ) exchange integer-spin, force-carrying bosons . The fermions involved in such exchanges can be either elementary (e.g. electrons or quarks ) or composite (e.g. protons or neutrons ), although at

3400-533: Is no experimental evidence for a non-zero magnetic moment in neutrinos. Weak interactions create neutrinos in one of three leptonic flavors : electron neutrinos ( ν e ), muon neutrinos ( ν μ ), or tau neutrinos ( ν τ ), associated with the corresponding charged leptons, the electron ( e ), muon ( μ ), and tau ( τ ), respectively. Although neutrinos were long believed to be massless, it

3500-408: Is nonzero. The second type is called the " neutral-current interaction " because the weakly interacting fermions form a current with total electric charge of zero. It is responsible for the (rare) deflection of neutrinos . The two types of interaction follow different selection rules . This naming convention is often misunderstood to label the electric charge of the W and Z bosons , however

3600-456: Is now known that there are three discrete neutrino masses; each neutrino flavor 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, experiments have shown that these masses are tiny compared to any other particle. From cosmological measurements, it has been calculated that the sum of the three neutrino masses must be less than one-millionth that of

3700-423: Is responsible for the radioactive decay of atoms: The weak interaction participates in nuclear fission and nuclear fusion . The theory describing its behaviour and effects is sometimes called quantum flavordynamics ( QFD ); however, the term QFD is rarely used, because the weak force is better understood by electroweak theory (EWT). The effective range of the weak force is limited to subatomic distances and

3800-438: Is so named because it is electrically neutral and because its rest mass is so small ( -ino ) that it was long thought to be zero . The rest mass of the neutrino is much smaller than that of the other known elementary particles (excluding massless particles ). The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in

3900-485: Is the generator of the U(1) component of the electroweak gauge group ; whereas some particles have a weak isospin of zero, all known spin- ⁠ 1 / 2 ⁠ particles have a non-zero weak hypercharge. There are two types of weak interaction (called vertices ). The first type is called the " charged-current interaction " because the weakly interacting fermions form a current with total electric charge that

4000-494: Is then close to zero, so these mostly interact with the Z  boson through the axial coupling. The Standard Model of particle physics describes the electromagnetic interaction and the weak interaction as two different aspects of a single electroweak interaction. This theory was developed around 1968 by Sheldon Glashow , Abdus Salam , and Steven Weinberg , and they were awarded the 1979 Nobel Prize in Physics for their work. The Higgs mechanism provides an explanation for

4100-596: Is typically several orders of magnitude less than that of the electromagnetic force, which itself is further orders of magnitude less than the strong nuclear force. The weak interaction is the only fundamental interaction that breaks parity symmetry , and similarly, but far more rarely, the only interaction to break charge–parity symmetry . Quarks , which make up composite particles like neutrons and protons, come in six "flavours" – up, down, charm, strange, top and bottom – which give those composite particles their properties. The weak interaction

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4200-433: Is unique in that it allows quarks to swap their flavour for another. The swapping of those properties is mediated by the force carrier bosons. For example, during beta-minus decay , a down quark within a neutron is changed into an up quark, thus converting the neutron to a proton and resulting in the emission of an electron and an electron antineutrino. Weak interaction is important in the fusion of hydrogen into helium in

4300-466: Is very small. They interact with the nucleons (neutrons and protons) only via weak nuclear interactions . Since they do not interact with matter via the electromagnetic or gravitational forces, it is extremely difficult to detect them. Since their mass is very small (less than 14 eV ) they travel with speeds that are very close to the speed of light in vacuum . Because neutrinos are very weakly interacting, neutrino detectors must be very large to detect

4400-475: The 1995 Nobel Prize . 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 neutrons and positrons: The positron quickly finds an electron, and they annihilate each other. The two resulting gamma rays (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing

4500-590: The DUMAND neutrino telescope in the Pacific ocean and therefore, it is possible to compare observations and study correlations between the observed neutrino. Original surveys of the seafloor were conducted in 1989, 1991, 1992 and scientific conferences of the NESTOR Collaboration were held in the 1990s. In March 2003, the NESTOR prototype was lowered to the depth of 3800 meters some 30 kilometers off

4600-455: 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

4700-520: The Standard Model (see table at right). The current best measurement of the number of neutrino types comes from observing the decay of the Z boson . This particle can decay into any light neutrino and its antineutrino, and the more available types of light neutrinos, the shorter the lifetime of the Z ;boson. Measurements of the Z lifetime have shown that three light neutrino flavors couple to

4800-427: The Standard Model at lower energies, but dramatically different above symmetry breaking. The laws of nature were long thought to remain the same under mirror reflection . The results of an experiment viewed via a mirror were expected to be identical to the results of a separately constructed, mirror-reflected copy of the experimental apparatus watched through the mirror. This so-called law of parity conservation

4900-468: The cosmic neutrino background (CNB). R. Davis and M. 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 . SN 1987A represents

5000-489: The electromagnetic interaction or the strong interaction . Thus, neutrinos typically pass through normal matter unimpeded and undetected. Weak interactions create neutrinos in one of three leptonic flavors : Each flavor is associated with the correspondingly named charged lepton . 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 ), but

5100-529: The muon neutrino (already hypothesised with the name neutretto ), which earned them the 1988 Nobel Prize in Physics . When the third type of lepton, the 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

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5200-405: 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. 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

5300-454: The 2015 Nobel Prize for Physics for their landmark finding, theoretical and experimental, that neutrinos can change flavors. 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. Around 1 second after the Big Bang , neutrinos decoupled, giving rise to a background level of neutrinos known as

5400-537: The Z. The correspondence between the six quarks 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. There are several active research areas involving the neutrino with aspirations of finding: International scientific collaborations install large neutrino detectors near nuclear reactors or in neutrino beams from particle accelerators to better constrain

5500-529: 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 collaboration at Fermilab ; its existence had already been inferred by both theoretical consistency and experimental data from the Large Electron–Positron Collider . In the 1960s, the now-famous Homestake experiment made the first measurement of

5600-471: 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. The first evidence of the reality of neutrinos came in 1938 via simultaneous cloud-chamber measurements of

5700-464: The coast of Greece . The prototype's results were published in 2005. The spokesperson for the project is Leonidas Resvanis from University of Athens . In 2014 the project was still applying funding to build the actual telescope. The NESTOR collaboration is now (2018) part of the KM3NeT -collaboration. As such, they are not developing the NESTOR telescope anymore as its own instance, but Km3Net has

5800-409: 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

5900-466: The context of preventing the proliferation of nuclear weapons . Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Rather than conventional Dirac fermions , neutral particles can be another type of spin  ⁠ 1  / 2 ⁠ particle called Majorana particles , named after the Italian physicist Ettore Majorana who first proposed

6000-441: The deepest levels, all weak interactions ultimately are between elementary particles . In the weak interaction, fermions can exchange three types of force carriers, namely W , W , and Z  bosons . The masses of these bosons are far greater than the mass of a proton or neutron, which is consistent with the short range of the weak force. In fact, the force is termed weak because its field strength over any set distance

6100-417: The electron and the recoil of the nucleus. In 1942, Wang Ganchang first proposed the use of beta capture to experimentally detect neutrinos. In the 20 July 1956 issue of Science , Clyde Cowan , Frederick Reines , Francis B. "Kiko" Harrison, Herald W. Kruse, and Austin D. McGuire published confirmation that they had detected the neutrino, a result that was rewarded almost forty years later with

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6200-430: The electron neutrino, with other approaches to this problem in the planning stages. Weak interactions In nuclear physics and particle physics , the weak interaction , also called the weak force , is one of the four known fundamental interactions , with the others being electromagnetism , the strong interaction , and gravitation . It is the mechanism of interaction between subatomic particles that

6300-405: 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

6400-451: The electron. 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 2 being lighter than mass 3

6500-600: The existence of all three neutrino flavors and found no deficit. 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 years, he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 Stanislav Mikheyev and 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)

6600-486: 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

6700-451: The following list is not exhaustive, but includes some of those processes: 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 billion ( 6.5 × 10 ) solar neutrinos , per second per square centimeter. Neutrinos can be used for tomography of the interior of the Earth. The neutrino

6800-593: 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. Very much like neutrons do in nuclear reactors , neutrinos can induce fission reactions within heavy nuclei . So far, this reaction has not been measured in

6900-513: 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. 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 (i.e., only one of

7000-505: The main building commemorates the discovery. The experiments also implemented a primitive neutrino astronomy and looked at issues of neutrino physics and weak interactions. 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

7100-521: The momentum difference (called " running ") between the particles involved. Hence since by convention ⁠ sgn ⁡ T 3 ≡ sgn ⁡ Q {\displaystyle \operatorname {sgn} T_{3}\equiv \operatorname {sgn} Q} ⁠ , and for all fermions involved in the weak interaction ⁠ T 3 = ± 1 2 {\displaystyle T_{3}=\pm {\tfrac {1}{2}}} ⁠ . The weak charge of charged leptons

7200-420: The naming convention predates the concept of the mediator bosons, and clearly (at least in name) labels the charge of the current (formed from the fermions), not necessarily the bosons. In one type of charged current interaction, a charged lepton (such as an electron or a muon , having a charge of −1) can absorb a W  boson (a particle with a charge of +1) and be thereby converted into

7300-513: The neutrino masses and the values for the magnitude and rates of oscillations between neutrino flavors. 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. The KATRIN experiment in Germany began to acquire data in June 2018 to determine the value of the mass of

7400-404: The neutrinos by having opposite signs of lepton number and opposite 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

7500-436: The neutron into a proton. Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual W boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products. At the quark level, the process can be represented as: In neutral current interactions,

7600-578: The only verified detection of neutrinos from a supernova. However, many stars have gone supernova in the universe, leaving a theorized diffuse supernova neutrino background . Neutrinos have half-integer spin ( ⁠ 1  / 2 ⁠ ħ ); therefore they are fermions . Neutrinos are leptons. They have only been observed to interact through the weak force , although it is assumed that they also interact gravitationally. Since they have non-zero mass, theoretical considerations permit neutrinos to interact magnetically, but do not require them to. As yet there

7700-415: The particle can interact with the W of the weak force. Weak isospin plays the same role in the weak interaction with W as electric charge does in electromagnetism , and color charge in the strong interaction ; a different number with a similar name, weak charge , discussed below , is used for interactions with the Z . All left-handed fermions have

7800-488: The presence of three massive gauge bosons ( W , W , Z , the three carriers of the weak interaction), and the photon ( γ , the massless gauge boson that carries the electromagnetic interaction). According to the electroweak theory, at very high energies, the universe has four components of the Higgs field whose interactions are carried by four massless scalar bosons forming

7900-409: The probability for an interaction. In general the interaction probability increases with the number of neutrons and protons within a nucleus. 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

8000-413: The process known as beta decay , a down quark in the neutron can change into an up quark by emitting a virtual W  boson, which then decays into an electron and an electron antineutrino . Another example is electron capture  – a common variant of radioactive decay  – wherein a proton and an electron within an atom interact and are changed to

8100-688: 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. A non-zero mass allows neutrinos to possibly have a tiny magnetic moment ; if so, neutrinos would interact electromagnetically, although no such interaction has ever been observed. Neutrinos oscillate between different flavors in flight. For example, an electron neutrino produced in

8200-428: 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. Takaaki Kajita of Japan, and Arthur B. McDonald of Canada, received

8300-428: The related field of betavoltaics (but not similar radium luminescence ). The electroweak force is believed to have separated into the electromagnetic and weak forces during the quark epoch of the early universe . In 1933, Enrico Fermi proposed the first theory of the weak interaction, known as Fermi's interaction . He suggested that beta decay could be explained by a four- fermion interaction, involving

8400-947: The scientific vocabulary through Enrico Fermi , who used it during a conference in Paris in July ;1932 and at the Solvay Conference in October ;1933, where Pauli also employed it. The name (the 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. In Fermi's theory of beta decay , Chadwick's large neutral particle could decay to

8500-419: The standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs. The quantum number weak charge ( Q W ) serves the same role in the neutral current interaction with the Z that electric charge ( Q , with no subscript) does in the electromagnetic interaction : It quantifies

8600-409: The terminal station through a 31-km-long deep-sea, optic fiber cable for data collection. Pylos was selected for the installation of the telescope for several reasons. It combines deep water with close proximity to the shore, a convenient combination for the installation of the NESTOR towers and the communication and power supply cables. It is also located in an anti-diametric point with respect to

8700-412: The three masses do not uniquely correspond to the three flavors: A neutrino created with a specific flavor is a specific mixture of all three mass states (a quantum superposition ). Similar to some other neutral particles , neutrinos oscillate between different flavors in flight as a consequence. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as

8800-409: The three weak bosons, which then acquire mass through the Higgs mechanism . These three composite bosons are the W , W , and Z  bosons actually observed in the weak interaction. The fourth electroweak gauge boson is the photon ( γ ) of electromagnetism, which does not couple to any of the Higgs fields and so remains massless. This theory has made

8900-449: The two possible spin states has ever been seen), while neutrinos were all left-handed. 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

9000-550: The vector part of the interaction. Its value is given by: Since the weak mixing angle ⁠ θ w ≈ 29 ∘ {\displaystyle \theta _{\mathsf {w}}\approx 29^{\circ }} ⁠ , the parenthetic expression ⁠ ( 1 − 4 sin 2 ⁡ θ w ) ≈ 0.060 {\displaystyle (1-4\,\sin ^{2}\theta _{\mathsf {w}})\approx 0.060} ⁠ , with its value varying slightly with

9100-416: The weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity. The V − A theory was developed before the discovery of the Z boson, so it did not include the right-handed fields that enter in the neutral current interaction. However, this theory allowed

9200-420: The weak interaction by showing them to be two aspects of a single force, now termed the electroweak force. The existence of the W and Z  bosons was not directly confirmed until 1983. The electrically charged weak interaction is unique in a number of respects: Due to their large mass (approximately 90 GeV/ c ) these carrier particles, called the W and Z  bosons, are short-lived with

9300-460: The weak interaction has an intensity of a similar magnitude to the electromagnetic force, but this starts to decrease exponentially with increasing distance. Scaled up by just one and a half orders of magnitude, at distances of around 3 × 10  m, the weak interaction becomes 10,000 times weaker. The weak interaction affects all the fermions of the Standard Model , as well as the Higgs boson ; neutrinos interact only through gravity and

9400-401: The weak interaction required more than two generations of particles, effectively predicting the existence of a then unknown third generation. This discovery earned them half of the 2008 Nobel Prize in Physics. Unlike parity violation, CP  violation occurs only in rare circumstances. Despite its limited occurrence under present conditions, it is widely believed to be the reason that there

9500-404: The weak interaction was once described by Fermi's theory , the discovery of parity violation and renormalization theory suggested that a new approach was needed. In 1957, Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a V − A ( vector minus axial vector or left-handed) Lagrangian for weak interactions. In this theory,

9600-469: The weak interaction. The weak interaction does not produce bound states , nor does it involve binding energy  – something that gravity does on an astronomical scale , the electromagnetic force does at the molecular and atomic levels, and the strong nuclear force does only at the subatomic level, inside of nuclei . Its most noticeable effect is due to its first unique feature: The charged weak interaction causes flavour change . For example,

9700-522: Was "consistent with" a Higgs boson, while adding a cautious note that further data and analysis were needed before positively identifying the new boson as being a Higgs boson of some type. By 14 March 2013, a Higgs boson was tentatively confirmed to exist. In a speculative case where the electroweak symmetry breaking scale were lowered, the unbroken SU(2) interaction would eventually become confining . Alternative models where SU(2) becomes confining above that scale appear quantitatively similar to

9800-430: Was known to be respected by classical gravitation , electromagnetism and the strong interaction ; it was assumed to be a universal law. However, in the mid-1950s Chen-Ning Yang and Tsung-Dao Lee suggested that the weak interaction might violate this law. Chien Shiung Wu and collaborators in 1957 discovered that the weak interaction violates parity, earning Yang and Lee the 1957 Nobel Prize in Physics . Although

9900-423: Was postulated first by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy , momentum , and angular momentum ( spin ). In contrast to Niels Bohr , who proposed a statistical version of the conservation laws to explain the observed 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

10000-404: 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

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