Neutrino

Neutrino

Overview
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

 with a half-integer spin, chirality
Chirality (physics)
A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...

 and a disputed but small non-zero mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

. It is able to pass through ordinary matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 almost unaffected. The neutrino (meaning "small neutral one" in Italian) is denoted by the Greek letter ν (nu
Nu (letter)
Nu , is the 13th letter of the Greek alphabet. In the system of Greek numerals it has a value of 50...

).

Neutrinos do not carry electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons.
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Encyclopedia
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

 with a half-integer spin, chirality
Chirality (physics)
A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...

 and a disputed but small non-zero mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

. It is able to pass through ordinary matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 almost unaffected. The neutrino (meaning "small neutral one" in Italian) is denoted by the Greek letter ν (nu
Nu (letter)
Nu , is the 13th letter of the Greek alphabet. In the system of Greek numerals it has a value of 50...

).

Neutrinos do not carry electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are affected only by the weak sub-atomic force
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...

, of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale, and are therefore able to travel great distances through matter without being affected by it.

Neutrinos are created as a result of certain types of radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

, or nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...

s such as those that take place in the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, in nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

s, or when cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s hit atoms. There are three types, or "flavors", of neutrinos: electron neutrino
Electron neutrino
The electron neutrino is a subatomic lepton elementary particle which has no net electric charge. Together with the electron it forms the first generation of leptons, hence its name electron neutrino...

s, muon neutrino
Muon neutrino
The muon neutrino is a subatomic lepton elementary particle which has the symbol and no net electric charge. Together with the muon it forms the second generation of leptons, hence its name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by...

s and tau neutrinos. Each type also has a corresponding antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...

, called an antineutrino with an opposite chirality.

Most neutrinos passing through the Earth emanate from the Sun. About 65 billion solar neutrino
Solar neutrino
Electron neutrinos are produced in the Sun as a product of nuclear fusion. By far the largest fraction of neutrinos passing through the Earth are Solar neutrinos....

s per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.

In September 2011, neutrinos apparently moving faster than light were detected (see OPERA neutrino anomaly
OPERA neutrino anomaly
The OPERA neutrino anomaly is the detection of apparently faster-than-light neutrinos by the OPERA experiment as publicly announced in September 2011. The detection is anomalous because speeds exceeding that of light in a vacuum are generally thought to violate special relativity, a prevailing...

). Since then the experiment has undergone extensive critique and efforts to replicate the results because confirming the results would change our understanding of the theory of relativity
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

. In November 2011, the experiment was refined and yielded the same result. (See Speed below)

Pauli's proposal


The neutrinoMore specifically, the electron neutrino. was first postulated in 1930 by Wolfgang Pauli
Wolfgang Pauli
Wolfgang Ernst Pauli was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after being nominated by Albert Einstein, he received the Nobel Prize in Physics for his "decisive contribution through his discovery of a new law of Nature, the exclusion principle or...

 to preserve the conservation of energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...

, conservation of momentum, and conservation of angular momentum (spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

) in beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

. This was done by adding an undetected particle that Pauli termed a "neutron" to the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 already known to be products of beta decay:Niels Bohr
Niels Bohr
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...

 was notably opposed to this interpretation of beta decay and was ready to accept that energy, momentum and angular momentum were not conserved quantities.
→ + +


He theorized that an undetected particle was carrying away the observed difference between the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

, momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

, and angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 of the initial and final particles.

Pauli originally named his proposed light particle a neutron. When James Chadwick
James Chadwick
Sir James Chadwick CH FRS was an English Nobel laureate in physics awarded for his discovery of the neutron....

 discovered a much more massive nuclear particle in 1932 and also named it a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

, this left two particles with the same name. Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

, who developed the theory of beta decay, coined the term neutrino (the Italian
Italian language
Italian is a Romance language spoken mainly in Europe: Italy, Switzerland, San Marino, Vatican City, by minorities in Malta, Monaco, Croatia, Slovenia, France, Libya, Eritrea, and Somalia, and by immigrant communities in the Americas and Australia...

 equivalent of "little neutral one") in 1933 as a way to resolve the confusion.
It is a pun on the Italian word for neutron, neutrone, the -one ending being (though not in this case) an augmentative in Italian, so neutrone could be read as the "large neutral one".
Fermi's paper, written in 1934, unified Pauli's neutrino with Paul Dirac
Paul Dirac
Paul Adrien Maurice Dirac, OM, FRS was an English theoretical physicist who made fundamental contributions to the early development of both quantum mechanics and quantum electrodynamics...

's positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

 and Werner Heisenberg
Werner Heisenberg
Werner Karl Heisenberg was a German theoretical physicist who made foundational contributions to quantum mechanics and is best known for asserting the uncertainty principle of quantum theory...

's neutron-proton model and gave a solid theoretical basis for future experimental work. However the journal Nature rejected Fermi's paper, saying that the theory was "too remote from reality". He submitted the paper to an Italian journal, which accepted it, but the general lack of interest in his theory at that early date caused him to switch to experimental physics.

Direct detection


In 1942 Kan-Chang Wang
Kan-Chang Wang
Wang Ganchang was a nuclear physicist from China. He was one of the initiators of research in China in nuclear physics, cosmic rays and particle physics. Wang figured among the top leaders, pioneers and scientists of the Chinese nuclear deterrent program...

 first proposed the use of beta-capture to experimentally detect neutrinos. In the July 20, 1956 issue of Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

, Clyde Cowan
Clyde Cowan
Clyde Lorrain Cowan Jr was the co-discoverer of the neutrino, along with Frederick Reines. The discovery was made in 1956, detected in the neutrino experiment....

, Frederick Reines
Frederick Reines
Frederick Reines was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment, and may be the only scientist in history "so intimately associated with the discovery of an elementary particle and the...

, F. B. Harrison, H. W. Kruse, and A. D. McGuire published confirmation that they had detected the neutrino, a result that was rewarded almost forty years later with the 1995 Nobel Prize
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

.

In this experiment, now known as the Cowan–Reines neutrino experiment, antineutrinos created in a nuclear reactor by beta decay reacted with protons producing neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s and positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

s:
+ → +


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 a gamma ray. The coincidence of both events – positron annihilation and neutron capture – gives a unique signature of an antineutrino interaction.

Neutrino flavor


In 1962 Leon M. Lederman
Leon M. Lederman
Leon Max Lederman is an American experimental physicist and Nobel Prize in Physics laureate for his work with neutrinos. He is Director Emeritus of Fermi National Accelerator Laboratory in Batavia, Illinois, USA...

, Melvin Schwartz
Melvin Schwartz
Melvin Schwartz 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.He grew up in...

 and Jack Steinberger
Jack Steinberger
Jack Steinberger is a German-American physicist currently residing near Geneva, Switzerland. He co-discovered the muon neutrino, along with Leon Lederman and Melvin Schwartz, for which they were given the 1988 Nobel Prize in Physics.-Life:...

 showed that more than one type of neutrino exists by first detecting interactions of the muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...

 neutrino (already hypothesised with the name neutretto), which earned them the 1988 Nobel Prize
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

. When the third type of lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...

, the tau, was discovered in 1975 at the Stanford Linear Accelerator Center
Stanford Linear Accelerator Center
The SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S...

, it too was expected to have an associated neutrino (the tau neutrino). First evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay leading to the discovery of the neutrino. The first detection of tau neutrino interactions was announced in summer of 2000 by the DONUT collaboration
DONUT
DONUT was an experiment at Fermilab dedicated to the search for tau neutrino interactions. Even though the detector operated only during a few months in the summer of 1997, it was largely successful. By detecting the tau neutrino, it confirmed the existence of the last lepton predicted by the...

 at Fermilab
Fermilab
Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...

, making it the latest particle of the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 to have been directly observed; its existence had already been inferred by both theoretical consistency and experimental data from the Large Electron–Positron Collider.

Solar neutrino problem


Starting in the late 1960s, several experiments found that the number of electron neutrinos arriving from the Sun was between one third and one half the number predicted by the Standard Solar Model
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...

. This discrepancy, which became known as the solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...

, remained unresolved for some thirty years. It was resolved by discovery of neutrino oscillation and mass. (The Standard Model of particle physics
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 had assumed that neutrinos are massless and cannot change flavor. However, if neutrinos had mass, they could change flavor
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

, or oscillate between flavors).

Oscillation


A practical method for investigating neutrino oscillations was first suggested by Bruno Pontecorvo
Bruno Pontecorvo
Bruno Pontecorvo was an Italian-born nuclear physicist, an early assistant of Enrico Fermi and then the author of numerous studies in high energy physics, especially on neutrinos. According to Oleg Gordievsky and Pavel Sudoplatov , Pontecorvo was also a Soviet agent...

 in 1957 using an analogy with kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...

 oscillations; over the subsequent 10 years he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 Stanislav Mikheyev
Stanislav Mikheyev
Stanislav Pavlovich Mikheyev was a Russian physicist known for a co-discovering of the MSW effect.-Education and research:Stanislav Mikheyev graduated from Faculty of Physics of Moscow State University in 1965. Then he became a researcher at Lebedev Physical Institute...

 and Alexei Smirnov (expanding on 1978 work by Lincoln Wolfenstein
Lincoln Wolfenstein
Lincoln Wolfenstein is an American particle physicist who studies the weak interaction. Wolfenstein was born in 1923 and obtained his PhD in 1949 from the University of Chicago. He retired from Carnegie Mellon University in 2000 after being a faculty member for 52 years, but still lectures there...

) noted that flavor oscillations can be modified when neutrinos propagate through matter. This so-called Mikheyev–Smirnov–Wolfenstein effect (MSW effect) is important to understand because many neutrinos emitted by fusion in the Sun pass through the dense matter in the solar core
Solar core
The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radius. It is the hottest part of the Sun and of the Solar System. It has a density of up to 150 g/cm³ and a temperature of close to 15,000,000 kelvin...

 (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
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...

 and Sudbury Neutrino Observatory
Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a neutrino observatory located 6,800 feet underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 1999,...

). This resolved the solar neutrino problem: the electron neutrinos produced in the Sun had partly changed into other flavors which the experiments could not detect.

Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations. Especially relevant in this context are the reactor experiment KamLAND and the accelerator experiments such as MINOS
MINOS
MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande experiment in 1998...

. 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.

Supernova neutrinos


Raymond Davis Jr.
Raymond Davis Jr.
Raymond Davis, Jr. was an American chemist, physicist, and Nobel Prize in Physics laureate.-Early life and education:...

 and Masatoshi Koshiba
Masatoshi Koshiba
is a Japanese physicist. He jointly won the Nobel Prize in Physics in 2002.He graduated from the University of Tokyo in 1951 and received a Ph.D. in physics at the University of Rochester, New York, in 1955...

 were jointly awarded the 2002 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

; Davis for his pioneer work on cosmic neutrinos and Koshiba for the first real time observation of supernova neutrinos. The detection of solar neutrino
Solar neutrino
Electron neutrinos are produced in the Sun as a product of nuclear fusion. By far the largest fraction of neutrinos passing through the Earth are Solar neutrinos....

s, and of neutrinos of the SN 1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...

 supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 in 1987 marked the beginning of neutrino astronomy
Neutrino astronomy
Neutrino astronomy is the branch of astronomy that observes astronomical objects with neutrino detectors in special observatories. Nuclear reactions in stars and supernova explosions produce very large numbers of neutrinos, a very few of which may be detected by a neutrino telescope...

.

Properties and reactions


The neutrino has half-integer spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

 (½ħ) and is therefore a fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....

. Neutrinos interact primarily through the weak force. The discovery of neutrino flavor oscillations
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

 implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment of the order of , allowing the possibility that neutrinos may interact electromagnetically as well. An experiment done by C. S. Wu at Columbia University
Columbia University
Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...

 showed that neutrinos always have left-handed chirality
Chirality (physics)
A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...

.
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 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, don't need to be considered for the detection experiment. Within a cubic metre 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.

MSW effect



Neutrinos traveling through matter, in general, undergo a process analogous to light traveling through a transparent material. This process is not directly observable because it doesn't produce ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

, but gives rise to the MSW effect. Only a small fraction of the neutrino's energy is transferred to the material.

Nuclear reactions


Neutrinos can interact with a nucleus, changing it to another nucleus. This process is used in radiochemical neutrino detector
Neutrino detector
A neutrino detector is a physics apparatus designed to study neutrinos. Because neutrinos are only weakly interacting with other particles of matter, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground to isolate...

s. In this case, the energy levels and spin states within the target nucleus have to be taken into account to estimate the probability for an interaction. In general the interaction probability increases with the number of neutrons and protons within a nucleus.

Alteration of nuclear decay rate


A Russian study suggests that the decay rate of radioactive isotopes is not constant as is commonly believed, and a recent study also finds this, and says it appears to be affected by the rate of neutrinos emitted by the Sun.

Induced fission


Very much like neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s do in nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

s, neutrinos can induce fission reactions within heavy nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

. So far, this reaction has not been measured in a laboratory, but is predicted to happen within stars and supernovae. The process affects the abundance of isotopes seen in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

.

Types

Neutrinos in the Standard Model
of elementary particles
Fermion Symbol MassSince neutrino flavor eigenstates are not the same as neutrino mass eigenstates (see neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

), the given masses are actually mass expectation values. If the mass of a neutrino could be measured directly, the value would always be that of one of the three mass eigenstates: ν1, ν2, and ν3. In practice, the mass cannot be measured directly. Instead it is measured by looking at the shape of the endpoint of the energy spectrum in particle decay
Particle decay
Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...

s. This sort of measurement directly measures the expectation value of the mass; it is not sensitive to any of the mass eigenstates separately.
Generation 1
Electron neutrino < 2.2 eV
Electron antineutrino < 2.2 eV
Generation 2
Muon neutrino < 170 keV
Muon antineutrino < 170 keV
Generation 3
Tau neutrino < 15.5 MeV
Tau antineutrino < 15.5 MeV


There are three known types (flavors) of neutrinos: electron neutrino , muon neutrino and tau neutrino , named after their partner lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...

s in the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 (see table at right). The current best measurement of the number of neutrino types comes from observing the decay of the Z boson
W and Z bosons
The W and Z bosons are the elementary particles that mediate the weak interaction; their symbols are , and . The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own...

. This particle can decay into any light neutrino and its antineutrino, and the more types of light neutrinosIn this context, "light neutrino" means neutrinos with less than half the mass of the Z boson. available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types is 3. The correspondence between the six quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...

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. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.

The possibility of sterile neutrinos
Sterile neutrino
Sterile neutrinosIn scientific literature, these particles are also variously referred to as right-handed neutrinos, inert neutrinos, heavy neutrinos, or neutral heavy leptons . are a hypothetical type of neutrino that do not interact via any of the fundamental interactions of the Standard Model...

—relatively light neutrinos which do not participate in the weak interaction but which could be created through flavor oscillation (see below)—is unaffected by these Z-boson-based measurements, and the existence of such particles is in fact hinted by experimental data from the LSND
LSND
The Liquid Scintillator Neutrino Detector was a scintillation counter at Los Alamos National Laboratory that measured the number of neutrinos being produced by an accelerator neutrino source...

 experiment. However, the currently running MiniBooNE
MiniBooNE
MiniBooNE is an experiment at Fermilab designed to observe neutrino oscillations . A neutrino beam consisting primarily of muon neutrinos is directed at a detector filled with 800 tons of mineral oil and lined with 1,280 photomultiplier tubes...

 experiment suggested, until recently, that sterile neutrinos are not required to explain the experimental data, although the latest research into this area is on-going and anomalies in the MiniBooNE data may allow for exotic neutrino types, including sterile neutrinos. A recent re-analysis of reference electron spectra data from the Institut Laue-Langevin
Institut Laue-Langevin
The Institut Laue–Langevin, or ILL, is an internationally-financed scientific facility, situated in Grenoble, France. It is one of the world centres for research using neutrons...

 has also hinted at a fourth, sterile neutrino.

Recently analyzed data from the Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 of the cosmic background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

 is compatible with either three or four types of neutrinos. It is hoped that the addition of two more years of data from the probe will resolve this uncertainty.

Antineutrinos



Antineutrinos are the antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...

s of neutrinos, which are neutral
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 particles produced in nuclear
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...

 beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

. These are emitted in beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...

 emissions, where a neutron turns into a proton. They have a spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

 of ½, and are part of the lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...

 family of particles. The antineutrinos observed so far all have right-handed helicity (i.e. only one of the two possible spin states has ever been seen), while the neutrinos are left-handed. Antineutrinos, like neutrinos, interact with other matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 only through the gravitational and weak forces, making them very difficult to detect experimentally. Neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

 experiments indicate that antineutrinos have mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

, but beta decay experiments constrain that mass to be very small. A neutrino-antineutrino interaction has been suggested in attempts to form a composite photon with the neutrino theory of light
Neutrino theory of light
The neutrino theory of light is the proposal that the photon is a composite particle formed of a neutrino-antineutrino pair. It is based on the idea that emission and absorption of a photon corresponds to the creation and annihilation of a particle-antiparticle pair...

.

Because antineutrinos and neutrinos are neutral particles it is possible that they are actually the same particle. Particles which have this property are known as Majorana particles. If neutrinos are indeed Majorana particles then the neutrinoless double beta decay process is allowed. Several experiments have been proposed to search for this process.

Researchers around the world have begun to investigate the possibility of using antineutrinos for reactor monitoring in the context of preventing the proliferation of nuclear weapons
Nuclear proliferation
Nuclear proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as "Nuclear Weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the...

.

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)

Flavor oscillations



Neutrinos are most often created or detected with a well defined flavor (electron, muon, tau). However, in a phenomenon known as neutrino flavor oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

, neutrinos are able to oscillate between the three available flavors while they propagate through space. Specifically, this occurs because the neutrino flavor eigenstates are not the same as the neutrino mass eigenstates (simply called 1, 2, 3). This allows for a neutrino that was produced as an electron neutrino at a given location to have a calculable probability to be detected as either a muon or tau neutrino after it has traveled to another location. This quantum mechanical effect was first hinted by the discrepancy between the number of electron neutrinos detected from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's core failing to match the expected numbers, dubbed as the "solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...

". In the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 the existence of flavor oscillations implies nonzero differences between the neutrino masses, because the amount of mixing between neutrino flavors at a given time depends on the differences in their squared-masses. There are other possibilities in which neutrino can oscillate even if they are massless. If Lorentz invariance is not an exact symmetry, neutrinos can experience Lorentz-violating oscillations
Lorentz-violating neutrino oscillations
Lorentz-violating neutrino oscillation refers to the quantum phenomenon of neutrino oscillations described in a framework that allows the breakdown of Lorentz invariance...

.

It is possible that the neutrino and antineutrino are in fact the same particle, a hypothesis first proposed by the Italian physicist Ettore Majorana
Ettore Majorana
Ettore Majorana was an Italian theoretical physicist who began work on neutrino masses. He disappeared suddenly in mysterious circumstances. He is noted for the eponymous Majorana equation and for Majorana fermions.-Gifted in mathematics:Majorana was born in Catania, Sicily...

. The neutrino could transform into an antineutrino (and vice versa) by flipping the orientation of its spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

 state.

This change in spin would require the neutrino and antineutrino to have nonzero mass, and therefore travel slower than light, because such a spin flip, caused only by a change in point of view, can take place only if inertial frames of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

 exist that move faster than the particle: such a particle has a spin of one orientation when seen from a frame which moves slower than the particle, but the opposite spin when observed from a frame that moves faster than the particle.

Speed


Before 2011, experimental evidence could be interpreted to mean that neutrinos traveled at or below the speed of light. Following the results of the September 2011 observation of faster-than-light
Faster-than-light
Faster-than-light communications and travel refer to the propagation of information or matter faster than the speed of light....

 neutrino velocities in the OPERA experiment
OPERA neutrino anomaly
The OPERA neutrino anomaly is the detection of apparently faster-than-light neutrinos by the OPERA experiment as publicly announced in September 2011. The detection is anomalous because speeds exceeding that of light in a vacuum are generally thought to violate special relativity, a prevailing...

, the value of the neutrino velocity is the subject of theoretical and experimental studies.

Theory


Before the idea of neutrino oscillations came up, it was generally assumed that neutrinos travel below or at the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

.

According to the theory of special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, the question of neutrino velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

 is closely related to their mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

. If neutrinos are massless, they must travel at the speed of light. However, if they have mass, they cannot reach the speed of light.

Within the framework of the Standard-Model Extension
Standard-Model Extension
Standard-Model Extension is an effective field theory that contains the Standard Model, General Relativity, and all possible operators that break Lorentz symmetry.Violations of this fundamental symmetry can be studied within this general framework...

 (SME theory), various theorists have suggested that the neutrino might have a tachyon
Tachyon
A tachyon is a hypothetical subatomic particle that always moves faster than light. In the language of special relativity, a tachyon would be a particle with space-like four-momentum and imaginary proper time. A tachyon would be constrained to the space-like portion of the energy-momentum graph...

ic nature,
while others have disputed the possibility.

In some theories of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...

, such as the superfluid vacuum theory, superluminal propagation of particles, including neutrinos, is allowed.
According to this theory, at very high velocities the behavior of the particles becomes distinct from the relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 one - they can reach the speed of light limit at finite energy; also, the faster-than-light propagation is possible without requiring moving objects to have imaginary mass.

Experiment


In the early 1980s, first measurements of neutrino speed were done using pulsed pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....

 beams (produced by pulsed proton beams hitting a target). The pions decayed producing neutrinos, and the neutrino interactions observed within a time window in a detector at a distance were consistent with the speed of light. This measurement has been repeated in 2007 using the MINOS
MINOS
MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande experiment in 1998...

 detectors, which found the speed of neutrinos to be at 68% confidence level, and at 99% confidence level a range between to . The central value is higher than the speed of light and is consistent with superluminal velocity; however, the uncertainty is great enough that the result also does not rule out speeds less than or equal to light at this high confidence level. This measurement set an upper bound on the mass of the muon neutrino of at 99% confidence
Confidence interval
In statistics, a confidence interval is a particular kind of interval estimate of a population parameter and is used to indicate the reliability of an estimate. It is an observed interval , in principle different from sample to sample, that frequently includes the parameter of interest, if the...

.
The detectors for the project are being upgraded, and new results are not expected until at least 2012.

The same observation was made, on a somewhat larger scale, with supernova 1987A (SN 1987A). 10-MeV antineutrinos from the supernova were detected within a time window that was consistent with a speed of light for the neutrinos. So far, the question of neutrino masses cannot be decided based on measurements of the neutrino speed.

In September 2011, the OPERA collaboration
OPERA Experiment
The Oscillation Project with Emulsion-tRacking Apparatus is a scientific experiment for detecting tau neutrinos from muon neutrino oscillations. It is a collaboration between CERN in Geneva, Switzerland, and the Laboratori Nazionali del Gran Sasso in Gran Sasso, Italy and uses the CERN Neutrinos...

 released calculations showing velocities of 17-GeV and 28-GeV neutrinos exceeding the speed of light in their experiments. In November 2011, OPERA repeated its experiment with changes so that the speed could be determined individually for each detected neutrino. The results showed the same faster-than-light speed. More analyses and studies are underway.(See: OPERA neutrino anomaly
OPERA neutrino anomaly
The OPERA neutrino anomaly is the detection of apparently faster-than-light neutrinos by the OPERA experiment as publicly announced in September 2011. The detection is anomalous because speeds exceeding that of light in a vacuum are generally thought to violate special relativity, a prevailing...

.)

Mass


The Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 of particle physics assumed that neutrinos are massless, although adding massive neutrinos to the basic framework is not difficult. Indeed, the experimentally established phenomenon of neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...

 requires neutrinos to have nonzero masses. This was originally conceived by Bruno Pontecorvo
Bruno Pontecorvo
Bruno Pontecorvo was an Italian-born nuclear physicist, an early assistant of Enrico Fermi and then the author of numerous studies in high energy physics, especially on neutrinos. According to Oleg Gordievsky and Pavel Sudoplatov , Pontecorvo was also a Soviet agent...

 in the 1950s.

The strongest upper limit on the masses of neutrinos comes from cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

: the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 model predicts that there is a fixed ratio between the number of neutrinos and the number of photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s in the cosmic microwave background
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

. If the total energy of all three types of neutrinos exceeded an average of per neutrino, there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable; however, there are limits within the Standard Model that make this difficult. A much more stringent constraint comes from a careful analysis of cosmological data, such as the cosmic microwave background radiation, galaxy surveys, and the Lyman-alpha forest
Lyman-alpha forest
In astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....

. These indicate that the combined mass of the three neutrino varieties must be less than .

In 1998, research results at the Super-Kamiokande
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...

 neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass. 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. The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by KamLAND in 2005: Δm = . In 2006, the MINOS
MINOS
MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande experiment in 1998...

 experiment measured oscillations from an intense muon neutrino beam, determining the difference in the squares of the masses between neutrino mass eigenstates 2 and 3. The initial results indicate |Δm| = , consistent with previous results from Super-Kamiokande. Since |Δm| is the difference of two squared masses, at least one of them has to have a value which is at least the square root of this value. Thus, there exists at least one neutrino mass eigenstate with a mass of at least .

In 2009 lensing data of a galaxy cluster were analyzed to predict a neutrino mass of about . All neutrino masses are then nearly equal, with neutrino oscillations of order meV. They lie below the Mainz-Troitsk upper bound of for the electron antineutrino. The latter will be tested in 2015 in the KATRIN
KATRIN
KATRIN is an experiment to measure the mass of the electron antineutrino with sub-eV precision by examining the spectrum of electrons emitted from the beta decay of tritium...

 experiment, that searches for a mass between and . If it is found around , then the Cold Dark Matter
Cold dark matter
Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...

 particle likely does not exist.

A number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments. The methods applied involve nuclear beta decay (KATRIN
KATRIN
KATRIN is an experiment to measure the mass of the electron antineutrino with sub-eV precision by examining the spectrum of electrons emitted from the beta decay of tritium...

 and MARE
Mare
Female horses are called mares.Mare is the Latin word for "sea".The word may also refer to:-People:* Ahmed Marzooq, also known as Mare, a footballer and Secretary General of Maldives Olympic Committee* Mare Winningham, American actress and singer...

) or neutrinoless double beta decay (e.g. GERDA
Gerda
Gerda may refer to:* 122 Gerda, an asteroid* Gerðr or Gerda, giantess wife of the Norse god Freyr* Gerda, a character of Hans Christian Andersen's Snow Queen and its adaptations.* Gerda, a character on the children's television show Sesamstraat...

, CUORE
Cuore
Cuore is Italian term for "heart". It may refer to:* Heart * Heart * Daihatsu Cuore* Cryogenic Underground Observatory for Rare Events, particle physics facility in the Laboratori Nazionali del Gran Sasso...

/Cuoricino, NEMO-3
Neutrino Ettore Majorana Observatory
The Neutrino Ettore Majorana Observatory is an international collaboration of scientists searching for neutrinoless double beta decay. The observation of neutrinoless double beta decay would be evidence that neutrinos are Majorana particles and could be used to measure the neutrino mass...

 and others).

In May 2010, it was reported that physicists from CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...

 and the Italian National Institute for Nuclear Physics
Istituto Nazionale di Fisica Nucleare
The Istituto Nazionale di Fisica Nucleare is the coordinating institution for nuclear, particle and astroparticle physics in Italy. It was founded on the 8th of August 1951, to further the nuclear physics research tradition initiated by Enrico Fermi in Rome, in the 1930s...

' Gran Sasso National Laboratory
Laboratori Nazionali del Gran Sasso
Laboratori Nazionali del Gran Sasso is a particle physics laboratory of the INFN, situated near the Gran Sasso mountain in Italy, between the towns of L'Aquila and Teramo, about 120 km from Rome. In addition to a surface portion of the laboratory, there are extensive underground facilities...

 had observed for the first time a transformation in neutrinos, evidence that they have mass.

In July 2010 the 3-D MegaZ experiment reported that they had measured the upper limit of the combined mass of the three neutrino varieties to be less than .

Handedness


Experimental results show that (nearly) all produced and observed neutrinos have left-handed helicities (spins antiparallel to momenta
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

), and all antineutrinos have right-handed helicities, within the margin of error. In the massless limit, it means that only one of two possible chiralities
Chirality (physics)
A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...

 is observed for either particle. These are the only chiralities included in the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 of particle interactions.

It is possible that their counterparts (right-handed neutrinos and left-handed antineutrinos) simply do not exist. If they do, their properties are substantially different from observable neutrinos and antineutrinos. It is theorized that they are either very heavy (on the order of GUT scale—see Seesaw mechanism
Seesaw mechanism
In theoretical physics, the seesaw mechanism is a mechanism within grand unification theory, and in particular in theories of neutrino masses and neutrino oscillation, where it can be used to explain the smallness of observed neutrino masses relative to those of quarks and leptons.There are several...

), do not participate in weak interaction (so-called sterile neutrino
Sterile neutrino
Sterile neutrinosIn scientific literature, these particles are also variously referred to as right-handed neutrinos, inert neutrinos, heavy neutrinos, or neutral heavy leptons . are a hypothetical type of neutrino that do not interact via any of the fundamental interactions of the Standard Model...

s), or both.

The existence of nonzero neutrino masses somewhat complicates the situation. Neutrinos are produced in weak interactions as chirality eigenstates. However, chirality of a massive particle is not a constant of motion; helicity is, but the chirality operator does not share eigenstates with the helicity operator. Free neutrinos propagate as mixtures of left- and right-handed helicity states, with mixing amplitudes on the order of mν/E. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small. For example, most solar neutrinos have energies on the order of –, so the fraction of neutrinos with "wrong" helicity among them cannot exceed .

Artificial


Nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

s are the major source of human-generated neutrinos. Antineutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the antineutrino flux are , , and (i.e. via the antineutrinos emitted during beta-minus decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

 of their respective fission fragments). The average nuclear fission releases about of energy, of which roughly 4.5% (or about ) is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of , meaning that the core produces this much heat, and an electrical power generation of , the total power production from fissioning atoms is actually , of which is radiated away as antineutrino radiation and never appears in the engineering. This is to say, of fission energy is lost from this reactor and does not appear as heat available to run turbines, since the antineutrinos penetrate all building materials essentially without any trace, and disappear.

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 , with a maximal energy of about . There is no established experimental method to measure the flux of low energy antineutrinos. Only antineutrinos with an energy above threshold of can be uniquely identified (see neutrino detection below). An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above this threshold. An average nuclear power plant may generate over antineutrinos per second above this threshold, and a much larger number which cannot be seen with present detector technology.

Some particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

s have been used to make neutrino beams. The technique is to smash protons into a fixed target, producing charged pions or kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...

s. These unstable particles are then magnetically focused into a long tunnel where they decay while in flight. Because of the relativistic boost of the decaying particle the neutrinos are produced as a beam rather than isotropically. Efforts to construct an accelerator facility where neutrinos are produced through muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...

 decays are ongoing. Such a setup is generally known as a neutrino factory.

Nuclear bombs also produce very large quantities of neutrinos. Fred Reines and Clyde Cowan
Clyde Cowan
Clyde Lorrain Cowan Jr was the co-discoverer of the neutrino, along with Frederick Reines. The discovery was made in 1956, detected in the neutrino experiment....

 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.

Geologic


Neutrinos are part of the natural background radiation
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....

. In particular, the decay chains of and isotopes, as well as, include beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

s 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. KamLAND's main background in the geoneutrino measurement are the antineutrinos coming from reactors. Several future experiments aim at improving the geoneutrino measurement and these will necessarily have to be far away from reactors.

Atmospheric


Atmospheric neutrinos result from the interaction of cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s with atomic nuclei in the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, 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
Tata Institute of Fundamental Research
The Tata Institute of Fundamental Research is a research institution in India dedicated to basic research in mathematics and the sciences. It is a Deemed University and works under the umbrella of the Department of Atomic Energy of the Government of India. It is located at Navy Nagar, Colaba, Mumbai...

 (India), Osaka City University
Osaka City University
, abbreviated to , is a public university in Japan. It is located in Sumiyoshi-ku, Osaka City.- History :OCU's predecessor was founded in 1880, as the Osaka Commercial Training Institute with donations by local merchants. It developed into Osaka Commercial School in 1885, then was municipalized in...

 (Japan) and Durham University
Durham University
The University of Durham, commonly known as Durham University, is a university in Durham, England. It was founded by Act of Parliament in 1832 and granted a Royal Charter in 1837...

 (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in Kolar Gold Fields
Kolar Gold Fields
Kolar Gold Fields is a town in Bangarpet Taluk, in the Kolar District of Karnataka state, India. It includes the township of the same name, viz. KGF, where reside mainly the families of the employees of BGML. To the east of KGF is a ridge of hills of which Dod Betta hill, 3195 feet above sea...

 in India in 1965.

Solar


Solar neutrinos originate from the nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 powering the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 and other stars.
The details of the operation of the Sun are explained by the Standard Solar Model
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...

. In short: when four protons fuse to become one helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 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. Every second, about 65 billion
1000000000 (number)
1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....

  solar neutrinos pass through every square centimeter on the part of the Earth that faces the Sun. 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



Neutrinos are an important product of Type Ib and Ic
Type Ib and Ic supernovae
Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...

 and Type II
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...

 (core-collapse) supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e. In such events, the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 at the core becomes so high that the degeneracy of electrons is not enough to prevent protons and electrons from combining to form a neutron and an electron neutrino. A second and more important neutrino source is the thermal energy (100 billion kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s) of the newly formed neutron core, which is dissipated via the formation of neutrino-antineutrino pairs of all flavors. Most of the energy produced in supernovas is thus radiated away in the form of an immense burst of neutrinos. The first experimental evidence of this phenomenon came in 1987, when neutrinos from supernova 1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...

 were detected. The water-based detectors Kamiokande II and IMB
Irvine-Michigan-Brookhaven (detector)
IMB, the Irvine-Michigan-Brookhaven detector, was a nucleon decay experiment and neutrino observatory located in a Morton Salt company's Fairport mine on the shore of Lake Erie in the United States. It was a joint venture of the University of California, Irvine, the University of Michigan, and...

 detected 11 and 8 antineutrinos of thermal origin, respectively, while the scintillator-based Baksan
Baksan Neutrino Observatory
The Baksan Neutrino Observatory is a scientific laboratory of INR RAS located in the Baksan gorge in the Caucasus mountains in Russia. It started operations in 1977, becoming the first such observatory in the USSR...

 detector found 5 neutrinos (lepton number
Lepton number
In particle physics, the lepton number is the number of leptons minus the number of antileptons.In equation form,so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0...

 = 1) of either thermal or electron-capture origin, in a burst lasting less than 13 seconds. It is thought that neutrinos would also be produced from other events such as the collision of neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

s. 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. Neutrinos, on the other hand, pass through these gases, providing information about the supernova core (where the densities were large enough to influence the neutrino signal). Furthermore, the neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays or radio waves. The exact time delay depends on the velocity of the shock wave and on the thickness of the outer layer of the star. For a Type 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. The SNEWS
Supernova Early Warning System
The SuperNova Early Warning System is a network of neutrino detectors designed to give early warning to astronomers in the event of a supernova in the Milky Way galaxy or a nearby galaxy such as the Large Magellanic Cloud or the Canis Major Dwarf Galaxy. Enormous numbers of neutrinos are produced...

 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.

Supernova remnants


The energy of supernova neutrinos ranges from a few to several tens of MeV. However, 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: the supernova remnant
Supernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...

s. The origin of the cosmic rays was attributed to supernovas by Walter Baade
Walter Baade
Wilhelm Heinrich Walter Baade was a German astronomer who worked in the USA from 1931 to 1959.-Biography:He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the...

 and Fritz Zwicky
Fritz Zwicky
Fritz Zwicky was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy.- Biography :Fritz Zwicky was born in Varna, Bulgaria to a Swiss father....

; this hypothesis was refined by Vitaly L. Ginzburg and Sergei I. 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 percent. 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
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

, 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
Baikal Deep Underwater Neutrino Telescope
Since 1993, neutrino research has been conducted at the Baikal Deep Underwater Neutrino Telescope which is located 1.1 km below the surface of Lake Baikal....

, AMANDA
Antarctic Muon And Neutrino Detector Array
The Antarctic Muon And Neutrino Detector Array is a neutrino telescope located beneath the Amundsen-Scott South Pole Station. In 2005, after nine years of operation, AMANDA officially became part of its successor project, the IceCube Neutrino Observatory.AMANDA consists of optical modules, each...

, IceCube, ANTARES
ANTARES (telescope)
ANTARES is the name of a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon, France. It is designed to be used as a directional Neutrino Telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the...

, NEMO and Nestor
Nestor Project
The NESTOR Project is an international scientific collaboration whose target is the deployment of a neutrino telescope on the sea floor off Pylos, Greece.-Neutrino:...

. Related information is provided by very high energy gamma ray observatories, such as VERITAS
VERITAS
VERITAS is a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV - TeV energy range. The telescope design is based on the design of the existing 10m gamma-ray telescope of the Fred Lawrence Whipple Observatory...

, HESS
High Energy Stereoscopic System
High Energy Stereoscopic System or H.E.S.S. is a next-generation system of Imaging Atmospheric Cherenkov Telescopes for the investigation of cosmic gamma rays in the 100 GeV and TeV energy range...

 and MAGIC
MAGIC (telescope)
MAGIC is a system of two Imaging Atmospheric Cherenkov telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands, at about 2200 m above sea level...

. Indeed, the collisions of cosmic rays are supposed to produce charged pions, whose decay give the neutrinos, and also neutral pions, whose decay give gamma rays: the environment of a supernova remnant 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
Pierre Auger Observatory
The Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: single sub-atomic particles with energies beyond 1020 eV...

 or with the dedicated experiment named ANITA
ANtarctic Impulse Transient Antenna
The AA experiment has been designed to study ultra-high-energy cosmic neutrinos by detecting the radio pulses emitted by their interacting with the Antarctic ice sheet...

.

Big Bang



It is thought that, just like the cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

 left over from the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, 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
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

 thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: we know they exist. However, they also have 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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

. Thus, dark matter made from neutrinos is termed "hot dark matter
Hot dark matter
Hot dark matter is a hypothetical form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best candidate for the identity of hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the...

". The problem is that being fast moving, the neutrinos would tend to have spread out evenly in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 before cosmological expansion made them cold enough to congregate in clumps. This would cause the part of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

 made of neutrinos to be smeared out and unable to cause the large galactic
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 structures that we see.

Further, these same galaxies and groups of galaxies
Galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe...

 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 formation
Galaxy formation and evolution
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...

. This implies that neutrinos make up only a small part of the total amount of dark matter.

From cosmological arguments, relic background neutrinos are estimated to have density of 56 of each type per cubic centimeter and temperature if they are massless, much colder if their mass exceeds . Although their density is quite high, due to extremely low neutrino cross-sections at sub-eV energies, the relic neutrino background has not yet been observed in the laboratory. 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 6 orders of magnitude.

Detection


Neutrinos cannot be detected directly, because they don't ionize the materials they are passing through (they don't carry electric charge and other proposed effects, like the MSW effect, do not produce traceable radiation). A unique reaction to identify antineutrinos, sometimes referred to as inverse beta decay, as applied by Reines and Cowan (see below), requires a very large detector in order to detect a significant number of neutrinos. All detection methods require the neutrinos to carry a minimum threshold energy. So far, there is no detection method for low energy neutrinos, in the sense that potential neutrino interactions (for example by the MSW effect) cannot be uniquely distinguished from other causes. Neutrino detectors are often built underground in order to isolate the detector from cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s and other background radiation.

Antineutrinos were first detected in the 1950s near a nuclear reactor. Reines
Frederick Reines
Frederick Reines was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment, and may be the only scientist in history "so intimately associated with the discovery of an elementary particle and the...

 and Cowan
Clyde Cowan
Clyde Lorrain Cowan Jr was the co-discoverer of the neutrino, along with Frederick Reines. The discovery was made in 1956, detected in the neutrino experiment....

 used two targets containing a solution of cadmium chloride
Cadmium chloride
Cadmium chloride is a white crystalline compound of cadmium and chlorine, with the formula CdCl2. It is a hygroscopic solid that is highly soluble in water and slightly soluble in alcohol. Although it is considered to be ionic, it has considerable covalent character to its bonding...

 in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrinos with an energy above the threshold of caused charged current interactions with the protons in the water, producing positrons and neutrons. This is very much like decay, where energy is used to convert a proton into a neutron, a positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

  and an electron neutrino
Electron neutrino
The electron neutrino is a subatomic lepton elementary particle which has no net electric charge. Together with the electron it forms the first generation of leptons, hence its name electron neutrino...

  is emitted:

From known decay:
Energy + → + +


In the Cowan and Reines experiment, instead of an outgoing neutrino, you have an incoming antineutrino from a nuclear reactor:
Energy (>) + + → +


The resulting positron annihilation with electrons in the detector material created photons with an energy of about . Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about that were detected a few microseconds after the photons from a positron annihilation event.

Since then, various detection methods have been used. Super Kamiokande is a large volume of water surrounded by photomultiplier tubes that watch for the Cherenkov radiation
Cherenkov radiation
Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium...

 emitted when an incoming neutrino creates an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 or muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...

 in the water. The Sudbury Neutrino Observatory
Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a neutrino observatory located 6,800 feet underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 1999,...

 is similar, but uses heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

 as the detecting medium, which uses the same effects, but also allows the additional reaction any-flavor 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
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 or gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...

 which are periodically checked for excesses of argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 or germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

, respectively, which are created by electron-neutrinos interacting with the original substance. MINOS
MINOS
MINOS is a particle physics experiment designed to study the phenomena of neutrino oscillations, first discovered by a Super-Kamiokande experiment in 1998...

 uses a solid plastic scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...

 coupled to photomultiplier tubes, while Borexino
Borexino
Borexino is a particle physics experiment to study low energy solar neutrinos. The primary aim of the experiment is to make a precise measurement of the beryllium-7 neutrino flux from the sun and comparing it to the Standard solar model prediction...

 uses a liquid pseudocumene scintillator also watched by photomultiplier tubes and the proposed NOνA detector will use liquid scintillator watched by avalanche photodiode
Avalanche photodiode
An avalanche photodiode is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint,...

s. The IceCube Neutrino Observatory uses of the Antarctic ice sheet
Antarctic ice sheet
The Antarctic ice sheet is one of the two polar ice caps of the Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth. It covers an area of almost 14 million square km and contains 30 million cubic km of ice...

 near the south pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...

 with photomultiplier tubes distributed throughout the volume.

Motivation for scientific interest


Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.

Using neutrinos as a probe was first proposed early in the 20th century as a way to detect conditions at the core of the Sun. The solar core cannot be imaged directly because electromagnetic radiation (such as light) is diffused by the great amount and density of matter surrounding the core. On the other hand, neutrinos pass through the Sun with few interactions. Whereas photons emitted from the solar core may require 40,000 years to diffuse to the outer layers of the Sun, neutrinos generated in stellar fusion reactions at the core cross this distance practically unimpeded at nearly the speed of light.

Neutrinos are also useful for probing astrophysical sources beyond our solar system because they are the only known particles that are not significantly attenuated
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

 by their travel through the interstellar medium. Optical photons can be obscured or diffused by dust, gas, and background radiation. High-energy cosmic rays, in the form of swift protons and atomic nuclei, are unable to travel more than about 100 megaparsecs due to the Greisen–Zatsepin–Kuzmin limit
Greisen–Zatsepin–Kuzmin limit
The Greisen–Zatsepin–Kuzmin limit is a theoretical upper limit on the energy of cosmic rays coming from "distant" sources. The limit is 5×1019 eV, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances...

 (GZK cutoff). Neutrinos, in contrast, can travel even greater distances barely attenuated.

The galactic core of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 is fully obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core should be measurable by Earth-based neutrino telescopes in the next decade.

Another important use of the neutrino is in the observation of supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e, the explosions that end the lives of highly massive stars. The core collapse phase of a supernova is an extremely dense and energetic event. It is so dense that no known particles are able to escape the advancing core front except for neutrinos. Consequently, supernovae are known to release approximately 99% of their radiant energy in a short (10-second) burst of neutrinos. These neutrinos are a very useful probe for core collapse studies.

The rest mass of the neutrino (see above) is an important test of cosmological and astrophysical theories (see Dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

). The neutrino's significance in probing cosmological phenomena is as great as any other method, and is thus a major focus of study in astrophysical communities.

The study of neutrinos is important in particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

 because neutrinos typically have the lowest mass, and hence are examples of the lowest energy particles theorized in extensions of the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 of particle physics. For example, one would expect that if there is a fourth class of fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....

s beyond the electron, muon, and tau generations of particles, then the fourth generation neutrino would be the easiest to generate in a particle accelerator.

See also



  • List of neutrino experiments
  • Cowan–Reines neutrino experiment
  • Neutrino astronomy
    Neutrino astronomy
    Neutrino astronomy is the branch of astronomy that observes astronomical objects with neutrino detectors in special observatories. Nuclear reactions in stars and supernova explosions produce very large numbers of neutrinos, a very few of which may be detected by a neutrino telescope...

  • Neutrino oscillations
  • Seesaw mechanism
    Seesaw mechanism
    In theoretical physics, the seesaw mechanism is a mechanism within grand unification theory, and in particular in theories of neutrino masses and neutrino oscillation, where it can be used to explain the smallness of observed neutrino masses relative to those of quarks and leptons.There are several...

  • Sterile neutrino
    Sterile neutrino
    Sterile neutrinosIn scientific literature, these particles are also variously referred to as right-handed neutrinos, inert neutrinos, heavy neutrinos, or neutral heavy leptons . are a hypothetical type of neutrino that do not interact via any of the fundamental interactions of the Standard Model...

  • Supernova Early Warning System
    Supernova Early Warning System
    The SuperNova Early Warning System is a network of neutrino detectors designed to give early warning to astronomers in the event of a supernova in the Milky Way galaxy or a nearby galaxy such as the Large Magellanic Cloud or the Canis Major Dwarf Galaxy. Enormous numbers of neutrinos are produced...

  • IceCube Neutrino Observatory

External links