Muon neutrino

The muon neutrino is a lepton, an elementary subatomic particle which has the symbol
and no net electric charge. Together with the muon it forms the second generation of leptons, hence the name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by Leon Lederman, Melvin Schwartz and Jack Steinberger. The discovery was rewarded with the 1988 Nobel Prize in Physics.

Muon neutrino
CompositionElementary particle
InteractionsWeak, Gravity
AntiparticleMuon antineutrino (
DiscoveredLeon Lederman, Melvin Schwartz and Jack Steinberger (1962)
MassSmall but non-zero. See neutrino mass.
Electric charge0 e
Color chargeNo
Weak isospin1/2
Weak hypercharge−1
Chiralityleft-handed (for right-handed neutrinos, see sterile neutrino)


In 1962 Leon M. Lederman, Melvin Schwartz and Jack Steinberger established by performing an experiment at the Brookhaven National Laboratory[1] that more than one type of neutrino exists by first detecting interactions of the muon neutrino (already hypothesised with the name neutretto[2]), which earned them the 1988 Nobel Prize.[3]


In September 2011 OPERA researchers reported that muon neutrinos were apparently traveling at faster than light speed. This result was confirmed again in a second experiment in November 2011. These results have been viewed skeptically by the scientific community at large, and more experiments have/are investigating the phenomenon. In March 2012 the ICARUS team published results directly contradicting the results of OPERA.[4]

Later, in July 2012, the apparent anomalous super-luminous propagation of neutrinos was traced to a faulty element of the fibre optic timing system in Gran-Sasso. After it was corrected the neutrinos appeared to travel with the speed of light within the errors of the experiment.[5]

See also


  1. ^ G. Danby; J.-M. Gaillard; K. Goulianos; L. M. Lederman; N. B. Mistry; M. Schwartz; J. Steinberger (1962). "Observation of high-energy neutrino reactions and the existence of two kinds of neutrinos". Physical Review Letters. 9 (1): 36. Bibcode:1962PhRvL...9...36D. doi:10.1103/PhysRevLett.9.36.
  2. ^ I.V. Anicin (2005). "The Neutrino - Its Past, Present and Future". arXiv:physics/0503172.
  3. ^ "The Nobel Prize in Physics 1988". The Nobel Foundation. Retrieved 2010-02-11.
  4. ^ Anicin, Ivan V.; Aprili, P.; Baiboussinov, B.; Baldo Ceolin, M.; Benetti, P.; Calligarich, E.; Canci, N.; Centro, S.; Cesana, A.; Cieślik, K.; Cline, D.B.; Cocco, A.G.; Dabrowska, A.; Dequal, D.; Dermenev, A.; Dolfini, R.; Farnese, C.; Fava, A.; Ferrari, A.; Fiorillo, G.; Gibin, D.; Gigli Berzolari, A.; Gninenko, S.; Guglielmi, A.; Haranczyk, M.; Holeczek, J.; Ivashkin, A.; Kisiel, J.; Kochanek, I.; et al. (2012). "Measurement of the neutrino velocity with the ICARUS detector at the CNGS beam". Physics Letters B. 713 (1): 17–22. arXiv:1203.3433. Bibcode:2012PhLB..713...17A. doi:10.1016/j.physletb.2012.05.033.
  5. ^ "OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso (UPDATE 8 June 2012)". CERN press office. 8 June 2012. Retrieved 19 April 2013.

Further reading

Bruno Pontecorvo

Bruno Pontecorvo (Italian: [ponteˈkɔrvo]; Russian: Бру́но Макси́мович Понтеко́рво, Bruno Maksimovich Pontecorvo; 22 August 1913 – 24 September 1993) was an Italian nuclear physicist, an early assistant of Enrico Fermi and the author of numerous studies in high energy physics, especially on neutrinos. A convinced communist, he defected to the Soviet Union in 1950, where he continued his research on the decay of the muon and on neutrinos. The prestigious Pontecorvo Prize was instituted in his memory in 1995.

The fourth of eight children of a wealthy Jewish-Italian family, Pontecorvo studied physics at the University of Rome La Sapienza, under Fermi, becoming the youngest of his Via Panisperna boys. In 1934 he participated in Fermi's famous experiment showing the properties of slow neutrons that led the way to the discovery of nuclear fission. He moved to Paris in 1934, where he conducted research under Irène and Frédéric Joliot-Curie. Influenced by his cousin, Emilio Sereni, he joined the French Communist Party, as did his sisters Giuliana and Laura and brother Gillo. The Italian Fascist regime's 1938 racial laws against Jews caused his family members to leave Italy for Britain, France and the United States.

When the German Army closed in on Paris during the Second World War, Pontecorvo, his brother Gillo, cousin Emilio Sereni and Salvador Luria fled the city on bicycles. He eventually made his way to Tulsa, Oklahoma, where he applied his knowledge of nuclear physics to prospecting for oil and minerals. In 1943, he joined the British Tube Alloys team at the Montreal Laboratory in Canada. This became part of the Manhattan Project to develop the first atomic bombs. At Chalk River Laboratories, he worked on the design of the nuclear reactor ZEEP, the first reactor outside of the United States that went critical in 1945, followed by the NRX reactor in 1947. He also looked into cosmic rays, the decay of muons, and what would become his obsession, neutrinos. He moved to Britain in 1949, where he worked for the Atomic Energy Research Establishment at Harwell.

After his defection to the Soviet Union in 1950, he worked at the Joint Institute for Nuclear Research (JINR) in Dubna. He had proposed using chlorine to detect neutrinos. In a 1959 paper, he argued that the electron neutrino (νe) and the muon neutrino (νμ) were different particles. Solar neutrinos were detected by the Homestake Experiment, but only between one third and one half of the predicted number. In response to this solar neutrino problem, he proposed a phenomenon known as neutrino oscillation, whereby electron neutrinos became muon neutrinos. The existence of the oscillations was finally established by the Super-Kamiokande experiment in 1998. He also predicted in 1958 that supernovae would produce intense bursts of neutrinos, which was confirmed in 1987 when Supernova SN1987A was detected by neutrino detectors.

Eugene W. Beier

Eugene William Beier (born 30 January 1940 in Harvey, Illinois) is an American physicist.

Beier received in 1961 his bachelor's degree from Stanford University and in 1963 his M.S. and in 1966 his Ph.D., with advisor Louis J. Koester Jr., from the University of Illinois at Urbana–Champaign with thesis A search for heavy leptons using a differential Cherenkov counter. He became in 1967 an assistant professor and in 1979 a full professor at the University of Pennsylvania.

Beier has worked, since the end of the 1970s, on neutrino physics, first at Brookhaven National Laboratory (Experiment 734) and then, starting in 1984, on the science team of Kamiokande II.

In 1984 Professor Beier joined with other scientists from the University of Pennsylvania and from a group of Japanese institutions in the Kamiokande II experiment. The goal of this work was to use the sun as a source of neutrinos for studying their fundamental properties. This collaboration was quite successful, resulting in 1) observation of neutrinos from the supernova SN1987a, 2) the first direct measurement of neutrinos emitted by the sun, and 3) observation of an unexpected result in the ratio of electron neutrino to muon neutrino interactions from cosmic ray neutrinos produced in the earth's atmosphere. In 1998, the Super-Kamiokande collaboration determined that the atmospheric neutrino effect was due to neutrino oscillations.

In 1987 Beier joined the science team at the Sudbury Neutrino Observatory (SNO). He was co-spokesperson for the United States collaborators (along with R. G. H. Robertson of the University of Washington) working on the Sudbury Neutrino Observatory. The SNO science team provided strong evidence for solar neutrino flavor transformation. This flavor transformation implies that neutrinos have non-zero masses. The total flux of all neutrino flavors measured by SNO agrees well with the best theoretical models of the sun.His current research deals with the question of whether neutrinos are their own anti-particles; the investigation involves searching for the rare (and perhaps entirely hypothetical) neutrino-less double beta decay occurring within atomic nuclei.In 2008 Beier received the Panofsky Prize. Also, he was Chair of the Division of Particles and Fields of the American Physical Society in 2000. He was a member of the International Committee for Future Accelerators 1998–2000. For the academic year 1998–1999 he was a Guggenheim Fellow. He is a Fellow of the American Physical Society. In 1989 the Bruno Rossi Prize was awarded to the Kamiokande II team (and the Irvine-Michigan-Brookhaven team).

The Kamiokande II work and especially the observation from Supernova 1987a led to the award of the 2002 Nobel Prize in Physics to Masatoshi Koshiba. The Kamiokande II work (i.e. observation of an unexpected result in the ratio of electron neutrino to muon neutrino interactions from cosmic ray neutrinos produced in the earth's atmosphere) extended by the 1998 work by SuperKamiokande, along with the work of the science team in the Sudbury Neutrino Observatory, led to the 2015 award of the Nobel Prize in Physics to Takaaki Kajita and Arthur B. McDonald.

IceCube Neutrino Observatory

The IceCube Neutrino Observatory (or simply IceCube) is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica.

Its thousands of sensors are located under the Antarctic ice, distributed over a cubic kilometre.

Similar to its predecessor, the Antarctic Muon And Neutrino Detector Array (AMANDA), IceCube consists of spherical optical sensors called Digital Optical Modules (DOMs), each with a photomultiplier tube (PMT) and a single-board data acquisition computer which sends digital data to the counting house on the surface above the array. IceCube was completed on 18 December 2010.DOMs are deployed on strings of 60 modules each at depths between 1,450 to 2,450 meters into holes melted in the ice using a hot water drill. IceCube is designed to look for point sources of neutrinos in the TeV range to explore the highest-energy astrophysical processes.

In November 2013 it was announced that IceCube had detected 28 neutrinos that likely originated outside the Solar System.

Jack Steinberger

Jack Steinberger (born May 25, 1921) is an American physicist who, along with Leon M. Lederman and Melvin Schwartz, received the 1988 Nobel Prize in Physics for the discovery of the muon neutrino.

K2K experiment

The K2K experiment (KEK to Kamioka) was a neutrino experiment that ran from June 1999 to November 2004. It used muon neutrinos from a well-controlled and well-understood beam to verify the oscillations previously observed by Super-Kamiokande using atmospheric neutrinos. This was the first positive measurement of neutrino oscillations in which both the source and detector were fully under experimenters' control. Previous experiments relied on neutrinos from the Sun or from cosmic sources. The experiment found oscillation parameters which were consistent with those measured by Super-Kamiokande.


In particle physics, a lepton is an elementary particle of half-integer spin (spin ​1⁄2) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the electron.

There are six types of leptons, known as flavours, grouped in three generations. The first-generation leptons, also called electronic leptons, comprise the electron (e−) and the electron neutrino (νe); the second are the muonic leptons, comprising the muon (μ−) and the muon neutrino (νμ); and the third are the tauonic leptons, comprising the tau (τ−) and the tau neutrino (ντ). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).

Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, the weak interaction, and to electromagnetism, of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos.

For every lepton flavor there is a corresponding type of antiparticle, known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. According to certain theories, neutrinos may be their own antiparticle. It is not currently known whether this is the case.

The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by J. J. Thomson. The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, which was classified as a meson at the time. After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particle to be proposed. The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay. It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956. The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory. The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery.Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons. Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium.

Lepton number

In particle physics, lepton number (historically also called lepton charge) is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction. Lepton number is an additive quantum number, so its sum is preserved in interactions (as opposed to multiplicative quantum numbers such as parity, where the product is preserved instead). Mathematically, the lepton number is defined by , where is the number of leptons and is the number of antileptons.

Lepton number was introduced in 1953 to explain the absence of reactions such as in the Cowan–Reines neutrino experiment, which instead observed . This process, inverse beta decay, conserves lepton number, as the incoming antineutrino has lepton number –1, while the outgoing positron (antielectron) also has lepton number –1.


Main injector neutrino oscillation search (MINOS) was a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande (Super-K) experiment in 1998. Neutrinos produced by the NuMI ("Neutrinos at Main Injector") beamline at Fermilab near Chicago are observed at two detectors, one very close to where the beam is produced (the near detector), and another much larger detector 735 km away in northern Minnesota (the far detector).

The MINOS experiment started detecting neutrinos from the NuMI beam in February 2005. On 30 March 2006, the MINOS collaboration announced that the analysis of the initial data, collected in 2005, is consistent with neutrino oscillations, with the oscillation parameters which are consistent with Super-K measurements.

MINOS received the last neutrinos from the NUMI beam line at midnight on 30 April 2012. It was upgraded to MINOS+ which started taking data in 2013. The experiment was shut down on June 29, 2016, and the far detector has been dismantled and removed.

Magnetic horn

A magnetic horn or neutrino horn (also known as the Van der Meer horn) is a high-current, pulsed focusing device, invented by the Dutch physicist Simon van der Meer in CERN, that selects pions and focuses them into a sharp beam. The original application of the magnetic horn was in the context of neutrino physics, where beams of pions have to be tightly focused. When the pions then decay into muons and neutrinos or antineutrinos, an equally well-focused neutrino beam is obtained.

Melvin Schwartz

Melvin Schwartz (; November 2, 1932 – August 28, 2006) was an American physicist. He shared the 1988 Nobel Prize in Physics with Leon M. Lederman and Jack Steinberger for their development of the neutrino beam method and their demonstration of the doublet structure of the leptons through the discovery of the muon neutrino.


The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 1/2, but with a much greater mass. It is classified as a lepton. As is the case with other leptons, the muon is not believed to have any sub-structure—that is, it is not thought to be composed of any simpler particles.

The muon is an unstable subatomic particle with a mean lifetime of 2.2 μs, much longer than many other subatomic particles. As with the decay of the non-elementary neutron (with a lifetime around 15 minutes), muon decay is slow (by subatomic standards) because the decay is mediated by the weak interaction exclusively (rather than the more powerful strong interaction or electromagnetic interaction), and because the mass difference between the muon and the set of its decay products is small, providing few kinetic degrees of freedom for decay. Muon decay almost always produces at least three particles, which must include an electron of the same charge as the muon and two neutrinos of different types.

Like all elementary particles, the muon has a corresponding antiparticle of opposite charge (+1 e) but equal mass and spin: the antimuon (also called a positive muon). Muons are denoted by μ− and antimuons by μ+. Muons were previously called mu mesons, but are not classified as mesons by modern particle physicists (see § History), and that name is no longer used by the physics community.

Muons have a mass of 105.66 MeV/c2, which is about 207 times that of the electron. Due to their greater mass, muons are not as sharply accelerated when they encounter electromagnetic fields, and do not emit as much bremsstrahlung (deceleration radiation). This allows muons of a given energy to penetrate far more deeply into matter than electrons since the deceleration of electrons and muons is primarily due to energy loss by the bremsstrahlung mechanism. As an example, so-called "secondary muons", generated by cosmic rays hitting the atmosphere, can penetrate to the Earth's surface, and even into deep mines.

Because muons have a very large mass and energy compared with the decay energy of radioactivity, they are never produced by radioactive decay. They are, however, produced in copious amounts in high-energy interactions in normal matter, in certain particle accelerator experiments with hadrons, or naturally in cosmic ray interactions with matter. These interactions usually produce pi mesons initially, which most often decay to muons.

As with the case of the other charged leptons, the muon has an associated muon neutrino, denoted by νμ, which is not the same particle as the electron neutrino, and does not participate in the same nuclear reactions.


A neutrino ( or ) (denoted by the Greek letter ν) is a fermion (an elementary particle with half-integer spin) that interacts only via the weak subatomic force and gravity. The neutrino 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 mass of the neutrino is much smaller than that of the other known elementary particles. The weak force has a very short range, the gravitational interaction is extremely weak, and neutrinos, as leptons, do not participate in the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos (νe), muon neutrinos (νμ), or tau neutrinos (ντ), in association with the corresponding 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, but they do not correspond uniquely to the three flavors. A neutrino created with a specific flavor has an associated specific quantum superposition of all three mass states. As a result, neutrinos oscillate between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. Although only differences of squares of the three mass values are known as of 2016, cosmological observations imply that the sum of the three masses must be less than one millionth that of the electron.For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has half-integer spin and no electric charge. They are distinguished from the neutrinos by having opposite signs of lepton number and chirality. To conserve total lepton number, in nuclear beta decay, electron neutrinos appear together with only positrons (anti-electrons) or electron-antineutrinos, and electron antineutrinos with electrons or electron neutrinos.Neutrinos are created by various radioactive decays, including in beta decay of atomic nuclei or hadrons, nuclear reactions such as those that take place in the core of a star or artificially in nuclear reactors, nuclear bombs or particle accelerators, during a supernova, in the spin-down of a neutron star, or when accelerated particle beams or cosmic rays strike atoms. The majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. In the vicinity of the Earth, about 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun.For study, neutrinos can be created artificially with nuclear reactors and particle accelerators. There is intense research activity involving neutrinos, with goals that include the determination of the three neutrino mass values, the measurement of the degree of CP violation in the leptonic sector (leading to leptogenesis); and searches for evidence of physics beyond the Standard Model of particle physics, such as neutrinoless double beta decay, which would be evidence for violation of lepton number conservation. Neutrinos can also be used for tomography of the interior of the earth.

Neutrino oscillation

Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton family number ("lepton flavor": electron, muon, or tau) can later be measured to have a different lepton family number. The probability of measuring a particular flavor for a neutrino varies between 3 known states, as it propagates through space.First predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by a multitude of experiments in several different contexts. Notably, the existence of neutrino oscillation resolved the long-standing solar neutrino problem.

Neutrino oscillation is of great theoretical and experimental interest, as the precise properties of the process can shed light on several properties of the neutrino. In particular, it implies that the neutrino has a non-zero mass, which requires a modification to the Standard Model of particle physics. The experimental discovery of neutrino oscillation, and thus neutrino mass, by the Super-Kamiokande Observatory and the Sudbury Neutrino Observatories was recognized with the 2015 Nobel Prize for Physics.

Ni Guangjiong

Ni Guangjiong (Chinese: 倪光炯; born December 29, 1934 in Ningbo, Zhejiang) is a Chinese physicist and science writer. He began studies in physics about 1950, and became a Doctor of Philosophy in 1955. He married Su Qing, a physics professor, in 1960. He published his first book in 1978. He holds a Chair in Physics at Fudan University, Shanghai. He is the director of Modern Physics Institute and the head of the Division for Theoretical Physics.

He is a specialist in quantum mechanics, field theory, and particle physics. His books include Modern Physics (1979), Methods of Mathematical Physics (1989), Levinson Theorem, Anomaly and Phase Transition of Vacuum (1995), Physics Changing the World (1998), and Advanced Quantum Mechanics (2000).

OPERA experiment

The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) was an instrument used in a scientific experiment for detecting tau neutrinos from muon neutrino oscillations. The experiment is a collaboration between CERN in Geneva, Switzerland, and the Laboratori Nazionali del Gran Sasso (LNGS) in Gran Sasso, Italy and uses the CERN Neutrinos to Gran Sasso (CNGS) neutrino beam.

The process started with protons from the Super Proton Synchrotron (SPS) at CERN being fired in pulses at a carbon target to produce pions and kaons. These particles decay to produce muons and neutrinos.The beam from CERN was stopped on 3 December 2012, ending data taking, but the analysis of the collected data has continued.


SciBar Booster Neutrino Experiment (SciBooNE), was a neutrino experiment located at the Fermi National Accelerator Laboratory (Fermilab) in the USA. It observed neutrinos of the Fermilab Booster Neutrino Beam (BNB) that are produced when protons from the Fermilab Booster-accelerator were made to hit a beryllium target; this led to the production of many short-lived particles that decayed into neutrinos. The SciBooNE detector was located some 100 meters downrange from the beryllium target, with a 50 meter decay-volume (where the particle decay into neutrinos) and absorber combined with 50 meters of solid ground between the target and the detector to absorb other particles than neutrinos. The neutrino-beam continued through SciBooNE and ground to the MiniBooNE-detector, located some 540 meters downrange from the target.

SciBooNE was designed to make precise measurements of neutrino and antineutrino cross-sections on carbon and iron nuclei, and combine with MiniBooNE to improve neutrino oscillation searches for sterile neutrinos. The cross section measurements have been used by the T2K experiment which began running in Japan in 2009.

The SciBooNE detector had three subsystems: SciBar, the EC (electron catcher) and the MRD (muon range detector). They can be seen in the event display of SciBooNE's first neutrino event. [1] Many of the components of SciBooNE were recycled from other experiments; thus the budget of SciBooNE was as low as 1.2 million dollars.

SciBooNE took data from June 2007 to August 2008. The operation consisted of 3 data runs; run 1 and 3 were antineutrino studies and run 2 was neutrino study. Data analysis and results were published after 2008. In total, SciBooNE published eight peer-reviewed journal articles, garnering over 711 citations, and many more articles in conference proceedings. Highlights include results about muon neutrino disappearance and muon antineutrino disappearance, which were world-leading at the time of publication. In Fermilab's records, the SciBooNE experiment status is listed as "Completed: Aug. 1, 2013".The SciBooNE collaboration was a group of approximately 60 scientists from 17 institutions in five countries (Italy, Japan, Spain, UK and USA). [2] SciBooNE is led by Tsuyoshi Nakaya (Kyoto University) and Morgan Wascko (Imperial College, London).

The SciBooNE experiment hall has since been taken over by the ANNIE experiment.

T2K experiment

T2K (Tokai to Kamioka, Japan) is a particle physics experiment that is a collaboration between several countries, including Japan, Canada, France, Germany, Italy, South Korea, Poland, Russia, Spain, Switzerland, the United States, and the United Kingdom. It is the second generation follow up to the K2K experiment, a similar long baseline neutrino oscillation experiment.

The J-PARC facility produces an intense off-axis beam of muon neutrinos. The beam is directed towards the Super-Kamiokande detector, which is 295 km away. The main goal of T2K is to measure the oscillation of νμ to νe and to measure the value of θ13, one of the parameters of the Pontecorvo–Maki–Nakagawa–Sakata matrix.

On June 15, 2011, the T2K collaboration announced the observation of six electron neutrino-like events compared to an expected background of 1.5, a significance of 2.5 standard deviations.On July 19, 2013, at the European Physical Society meeting in Stockholm, the international T2K collaboration announced a definitive observation of muon neutrino to electron neutrino transformation.On August 4, 2017, Mark Hartz revealed at a KEK seminar that the latest data from T2K hinted at CP violation, rejecting the hypothesis that neutrinos and antineutrinos oscillate with the same probability at the 95% confidence (2σ) level.

Tau neutrino

The tau neutrino or tauon neutrino is a subatomic elementary particle which has the symbol ντ and no net electric charge. Together with the tau, it forms the third generation of leptons, hence the name tau neutrino. Its existence was immediately implied after the tau particle was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC–LBL group. The discovery of the tau neutrino was announced in July 2000 by the DONUT collaboration (Direct Observation of the Nu Tau).

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