# Neutrino

Composition The first use of a hydrogen bubble chamber to detect neutrinos, on 13 November 1970, at Argonne National Laboratory. Here a neutrino hits a proton in a hydrogen atom; the collision occurs at the point where three tracks emanate on the right of the photograph. Elementary particle Fermionic First, second and third Weak interaction and gravitation νe, νμ, ντ, νe, νμ, ντ Opposite chirality from particle νe (Electron neutrino): Wolfgang Pauli (1930) νμ (Muon neutrino): Late 1940s ντ (Tau neutrino): Mid 1970s νe: Clyde Cowan, Frederick Reines (1956) νμ: Leon Lederman, Melvin Schwartz and Jack Steinberger (1962) ντ: DONUT collaboration (2000) 3 – electron neutrino, muon neutrino and tau neutrino <0.120 eV/c2 (95% confidence level, sum of 3 flavors)[1] 0 e 1/2 LH: +1/2, RH: 0 LH: −1, RH: 0 −1 −3

A neutrino (/nˈtrn/ or /njˈtrn/) (denoted by the Greek letter ν) is a fermion (an elementary particle with half-integer spin) that interacts only via the weak subatomic force and gravity.[2][3] The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. The mass of the neutrino is much smaller than that of the other known elementary particles.[1] The weak force has a very short range, the gravitational interaction is extremely weak, and neutrinos, as leptons, do not participate in the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.[2][3]

Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos (
ν
e
),
muon neutrinos (
ν
μ
), or tau neutrinos (
ν
τ
), in association with the corresponding charged lepton.[4] Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values, but they do not correspond uniquely to the three flavors. A neutrino created with a specific flavor has an associated specific quantum superposition of all three mass states. As a result, neutrinos oscillate between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino.[5][6] Although only differences of squares of the three mass values are known as of 2016,[7] cosmological observations imply that the sum of the three masses must be less than one millionth that of the electron.[1][8]

For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has half-integer spin and no electric charge. They are distinguished from the neutrinos by having opposite signs of lepton number and chirality. To conserve total lepton number, in nuclear beta decay, electron neutrinos appear together with only positrons (anti-electrons) or electron-antineutrinos, and electron antineutrinos with electrons or electron neutrinos.[9][10]

Neutrinos are created by various radioactive decays, including in beta decay of atomic nuclei or hadrons, nuclear reactions such as those that take place in the core of a star or artificially in nuclear reactors, nuclear bombs or particle accelerators, during a supernova, in the spin-down of a neutron star, or when accelerated particle beams or cosmic rays strike atoms. The majority of neutrinos in the vicinity of the Earth are from nuclear reactions in the Sun. In the vicinity of the Earth, about 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun.[11][12]

For study, neutrinos can be created artificially with nuclear reactors and particle accelerators. There is intense research activity involving neutrinos, with goals that include the determination of the three neutrino mass values, the measurement of the degree of CP violation in the leptonic sector (leading to leptogenesis); and searches for evidence of physics beyond the Standard Model of particle physics, such as neutrinoless double beta decay, which would be evidence for violation of lepton number conservation. Neutrinos can also be used for tomography of the interior of the earth.[13][14]

## History

### Pauli's proposal

The neutrino[a] was postulated first by Wolfgang Pauli in 1930 to explain how beta decay could conserve energy, momentum, and angular momentum (spin). In contrast to Niels Bohr, who proposed a statistical version of the conservation laws to explain the observed continuous energy spectra in beta decay, Pauli hypothesized an undetected particle that he called a "neutron", using the same -on ending employed for naming both the proton and the electron. He considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay.[15][b]

James Chadwick discovered a much more massive neutral nuclear particle in 1932 and named it a neutron also, leaving two kinds of particles with the same name. Earlier (in 1930) Pauli had used the term "neutron" for both the neutral particle that conserved energy in beta decay, and a presumed neutral particle in the nucleus; initially he did not consider these two neutral particles as distinct from each other.[15] The word "neutrino" entered the scientific vocabulary through Enrico Fermi, who used it during a conference in Paris in July 1932 and at the Solvay Conference in October 1933, where Pauli also employed it. The name (the Italian equivalent of "little neutral one") was jokingly coined by Edoardo Amaldi during a conversation with Fermi at the Institute of Physics of via Panisperna in Rome, in order to distinguish this light neutral particle from Chadwick's heavy neutron.[16]

In Fermi's theory of beta decay, Chadwick's large neutral particle could decay to a proton, electron, and the smaller neutral particle (now called an electron antineutrino):

n0

p+
+
e
+
ν
e

Fermi's paper, written in 1934, unified Pauli's neutrino with Paul Dirac's positron and Werner Heisenberg's neutron–proton model and gave a solid theoretical basis for future experimental work. 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.[17]:24[18]

By 1934 there was experimental evidence against Bohr's idea that energy conservation is invalid for beta decay: At the Solvay conference of that year, measurements of the energy spectra of beta particles (electrons) were reported, showing that there is a strict limit on the energy of electrons from each type of beta decay. Such a limit is not expected if the conservation of energy is invalid, in which case any amount of energy would be statistically available in at least a few decays. The natural explanation of the beta decay spectrum as first measured in 1934 was that only a limited (and conserved) amount of energy was available, and a new particle was sometimes taking a varying fraction of this limited energy, leaving the rest for the beta particle. Pauli made use of the occasion to publicly emphasize that the still-undetected "neutrino" must be an actual particle.[17]:25

### Direct detection

Clyde Cowan conducting the neutrino experiment circa 1956

In 1942, Wang Ganchang first proposed the use of beta capture to experimentally detect neutrinos.[19] In the 20 July 1956 issue of Science, Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published confirmation that they had detected the neutrino,[20][21] a result that was rewarded almost forty years later with the 1995 Nobel Prize.[22]

In this experiment, now known as the Cowan–Reines neutrino experiment, antineutrinos created in a nuclear reactor by beta decay reacted with protons to produce neutrons and positrons:

ν
e
+
p+

n0
+
e+

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.

In February 1965, the first neutrino found in nature was identified in one of South Africa's gold mines by a group which included Friedel Sellschop.[23] The experiment was performed in a specially prepared chamber at a depth of 3 km in the ERPM mine near Boksburg. A plaque in the main building commemorates the discovery. The experiments also implemented a primitive neutrino astronomy and looked at issues of neutrino physics and weak interactions.[24]

### Neutrino flavor

The antineutrino discovered by Cowan and Reines is the antiparticle of the electron neutrino.

In 1962, Leon M. Lederman, Melvin Schwartz and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of the muon neutrino (already hypothesised with the name neutretto),[25] which earned them the 1988 Nobel Prize in Physics.

When the third type of lepton, the tau, was discovered in 1975 at the Stanford Linear Accelerator Center, it 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 electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the DONUT collaboration at Fermilab; its existence had already been inferred by both theoretical consistency and experimental data from the Large Electron–Positron Collider.[26]

### Solar neutrino problem

In the 1960s, the now-famous Homestake experiment made the first measurement of the flux of electron neutrinos arriving from the core of the Sun and found a value that was between one third and one half the number predicted by the Standard Solar Model. This discrepancy, which became known as the solar neutrino problem, remained unresolved for some thirty years, while possible problems with both the experiment and the solar model were investigated, but none could be found. Eventually it was realized that both were actually correct, and that the discrepancy between them was due to neutrinos being more complex than was previously assumed. It was postulated that the three neutrinos had nonzero and slightly but indistinguishably different masses, and could therefore oscillate into undetectable flavors on their flight to the Earth. This hypothesis was investigated by a new series of experiments, thereby opening a new major field of research that still continues. Eventual confirmation of the phenomenon of neutrino oscillation led to two Nobel prizes, to Raymond Davis, Jr., who conceived and led the Homestake experiment, and to Art McDonald, who led the SNO experiment, which could detect all of the neutrino flavors and found no deficit.[27]

### Oscillation

A practical method for investigating neutrino oscillations was first suggested by Bruno Pontecorvo in 1957 using an analogy with kaon oscillations; over the subsequent 10 years he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 Stanislav Mikheyev and Alexei Smirnov (expanding on 1978 work by Lincoln Wolfenstein) noted that flavor oscillations can be modified when neutrinos propagate through matter. This so-called Mikheyev–Smirnov–Wolfenstein effect (MSW effect) is important to understand because many neutrinos emitted by fusion in the Sun pass through the dense matter in the solar core (where essentially all solar fusion takes place) on their way to detectors on Earth.

Starting in 1998, experiments began to show that solar and atmospheric neutrinos change flavors (see Super-Kamiokande and Sudbury Neutrino Observatory). This resolved the solar neutrino problem: 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. 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.[28] Takaaki Kajita of Japan and Arthur B. McDonald of Canada received the 2015 Nobel Prize for Physics for their landmark finding, theoretical and experimental, that neutrinos can change flavors.

### Cosmic neutrinos

Raymond Davis, Jr. and Masatoshi Koshiba were jointly awarded the 2002 Nobel Prize in Physics. Both conducted pioneering work on solar neutrino detection, and Koshiba's work also resulted in the first real-time observation of neutrinos from the SN 1987A supernova in the nearby Large Magellanic Cloud. These efforts marked the beginning of neutrino astronomy.[29]

SN 1987A represents the only verified detection of neutrinos from a supernova. However, many stars have gone supernova in our galaxy, leaving a theorized diffuse supernova neutrino background. The same concept extends to the entire universe, giving the cosmic neutrino background, or relic neutrinos.

## Properties and reactions

The neutrino has half-integer spin (½ ħ) and is therefore a fermion. Also being leptons, neutrinos have been observed to interact through only the weak force, although it is assumed that they also interact gravitationally.

### Flavor, mass, and their mixing

Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos (
ν
e
), muon neutrinos (
ν
μ
), or tau neutrinos (
ν
τ
), in association with the corresponding electron, muon, and tau charged leptons, respectively.[30]

Although neutrinos were long believed to be massless, it is now known that there are also three discrete neutrino masses, but they do not correspond uniquely to the three flavors. Although only differences of squares of the three mass values are known as of 2016,[7] experiments have shown that these masses are tiny in magnitude. From cosmological measurements, it has been calculated that the sum of the three neutrino masses must be less than one millionth that of the electron.[1][8]

More formally, neutrino flavor eigenstates are not the same as the neutrino mass eigenstates (simply labelled 1, 2, 3). As of 2016, it is not known which of these three is the heaviest. In analogy with the mass hierarchy of the charged leptons, the configuration with mass 2 being lighter than mass 3 is conventionally called the "normal hierarchy", while in the "inverted hierarchy", the opposite would hold. Several major experimental efforts are underway to help establish which is correct.[31]

A neutrino created in a specific flavor eigenstate is in an associated specific quantum superposition of all three mass eigenstates. This is possible because the three masses differ so little that they cannot be experimentally distinguished within any practical flight path, due to the uncertainty principle. The proportion of each mass state in the produced pure flavor state has been found to depend strongly on that flavor. The relationship between flavor and mass eigenstates is encoded in the PMNS matrix. Experiments have established values for the elements of this matrix.[7]

The existence of a neutrino mass allows the possibility of a tiny neutrino magnetic moment, in which case neutrinos could interact electromagnetically as well; no such interaction has been discovered.[32]

### Flavor oscillations

Neutrinos oscillate between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino, as defined by the flavor of the charged lepton produced in the detector. This oscillation occurs because the three mass state components of the produced flavor travel at slightly different speeds, so that their quantum mechanical wave packets develop relative phase shifts that change how they combine to produce a varying superposition of three flavors. Each flavor component thereby oscillates sinusoidally as the neutrino travels, with the flavors varying in relative strengths. The relative flavor proportions when the neutrino interacts represent the relative probabilities for that flavor of interaction to produce the corresponding flavor of charged lepton.[5][6]

There are other possibilities in which neutrino could oscillate even if they were massless. If Lorentz symmetry were not an exact symmetry, neutrinos could experience Lorentz-violating oscillations.[33]

### Mikheyev–Smirnov–Wolfenstein 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 does not produce ionizing radiation, but gives rise to the MSW effect. Only a small fraction of the neutrino's energy is transferred to the material.[34]

### Antineutrinos

For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has no electric charge and half-integer spin. They are distinguished from the neutrinos by having opposite signs of lepton number and opposite chirality. As of 2016, no evidence has been found for any other difference. In all observations so far of leptonic processes (despite extensive and continuing searches for exceptions), there is no overall change in lepton number; for example, if total lepton number is zero in the initial state, electron neutrinos appear in the final state together with only positrons (anti-electrons) or electron-antineutrinos, and electron antineutrinos with electrons or electron neutrinos.[9][10]

Antineutrinos are produced in nuclear beta decay together with a beta particle, in which, e.g., a neutron decays into a proton, electron, and antineutrino. All antineutrinos observed thus far possess right-handed helicity (i.e. only one of the two possible spin states has ever been seen), while neutrinos are left-handed. Nevertheless, as neutrinos have mass, their helicity is frame-dependent, so it is the related frame-independent property of chirality that is relevant here.

Antineutrinos were first detected as a result of their interaction with protons in a large tank of water. This was installed next to a nuclear reactor as a controllable source of the antineutrinos (See: Cowan–Reines neutrino experiment). Researchers around the world have begun to investigate the possibility of using antineutrinos for reactor monitoring in the context of preventing the proliferation of nuclear weapons.[35][36][37]

### Majorana mass

Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Particles that have this property are known as Majorana particles, named after the Italian physicist Ettore Majorana who first proposed the concept. For the case of neutrinos this theory has gained popularity as it can be used, in combination with the seesaw mechanism, to explain why neutrino masses are so small compared to those of the other elementary particles, such as electrons or quarks. Majorana neutrinos have the property that the neutrino and antineutrino could be distinguished only by chirality; what experiments observe as a difference between the neutrino and antineutrino could simply be due to one particle with two possible chiralities.

It is not yet known whether neutrinos are Majorana or Dirac particles; it is possible to test this property experimentally. For example, if neutrinos are indeed Majorana particles, then lepton-number violating processes such as neutrinoless double beta decay would be allowed, while they would not if neutrinos are Dirac particles. Several experiments have been and are being conducted to search for this process, e.g. GERDA,[38] EXO,[39] and SNO+.[40] The cosmic neutrino background is also a probe of whether neutrinos are Majorana particles, since there should be a different number of cosmic neutrinos detected in either the Dirac or Majorana case.[41]

### Nuclear reactions

Neutrinos can interact with a nucleus, changing it to another nucleus. This process is used in radiochemical neutrino detectors. In this case, the energy levels and spin states within the target nucleus have to be taken into account to estimate the probability for an interaction. In general the interaction probability increases with the number of neutrons and protons within a nucleus.[27][42]

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.

### Induced fission

Very much like neutrons do in nuclear reactors, neutrinos can induce fission reactions within heavy nuclei.[43] 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.[42] Neutrino fission of deuterium nuclei has been observed in the Sudbury Neutrino Observatory, which uses a heavy water detector.

### No self interaction

Observations of the cosmic microwave background suggest that neutrinos do not interact with themselves.[44]

### Types

Neutrinos in the Standard Model of elementary particles
Fermion Symbol
Generation 1
Electron neutrino
ν
e
Electron antineutrino
ν
e
Generation 2
Muon neutrino
ν
μ
Muon antineutrino
ν
μ
Generation 3
Tau neutrino
ν
τ
Tau antineutrino
ν
τ

There are three known types (flavors) of neutrinos: electron neutrino
ν
e
, muon neutrino
ν
μ
and tau neutrino
ν
τ
, named after their partner leptons in the Standard Model (see table at right). The current best measurement of the number of neutrino types comes from observing the decay of the Z boson. This particle can decay into any light neutrino and its antineutrino, and the more types of light neutrinos[c] available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino flavors that couple to the Z is 3.[30] The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicists' intuition that there should be exactly three types of neutrino. Proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.

## Research

There are several active research areas involving the neutrino. Some are concerned with testing predictions of neutrino behavior. Other research is focused on measurement of unknown properties of neutrinos; there is special interest in experiments that determine their masses and rates of CP violation, which cannot be predicted from current theory.

### Detectors near artificial neutrino sources

International scientific collaborations install large neutrino detectors near nuclear reactors or in neutrino beams from particle accelerators to better constrain the neutrino masses and the values for the magnitude and rates of oscillations between neutrino flavors. These experiments are thereby searching for the existence of CP violation in the neutrino sector; that is, whether or not the laws of physics treat neutrinos and antineutrinos differently.[7]

The KATRIN experiment in Germany has begun to acquire data in June 2018[45] to determine the value of the mass of the electron neutrino, with other approaches to this problem in the planning stages.[1]

### Tests of neutrino oscillation

On 19 July 2013, the results from the T2K experiment presented at the European Physical Society Conference on High Energy Physics in Stockholm, Sweden, confirmed neutrino oscillation theory.[46][47]

### Gravitational effects

Despite their tiny masses, neutrinos are so numerous that their gravitational force can influence other matter in the universe.

The three known neutrino flavors are the only established elementary particle candidates for dark matter, specifically hot dark matter, although that possibility appears to be largely ruled out by observations of the cosmic microwave background. If heavier sterile neutrinos exist, they might serve as warm dark matter, which still seems plausible.[48]

### Sterile neutrino searches

Other efforts search for evidence of a sterile neutrino – a fourth neutrino flavor that does not interact with matter like the three known neutrino flavors.[49][50][51][52] The possibility of sterile neutrinos is unaffected by the Z-boson decay measurements described above: If their mass is greater than half the Z-boson's mass, they would not be a decay product. Therefore, heavy sterile neutrinos would have a mass of at least 45.6 GeV.

The existence of such particles is in fact hinted by experimental data from the LSND experiment. On the other hand, the currently running MiniBooNE experiment suggested that sterile neutrinos are not required to explain the experimental data,[53] 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.[54] A recent re-analysis of reference electron spectra data from the Institut Laue-Langevin[55] has also hinted at a fourth, sterile neutrino.[56]

According to an analysis published in 2010, data from the Wilkinson Microwave Anisotropy Probe of the cosmic background radiation is compatible with either three or four types of neutrinos.[57]

### Neutrinoless double-beta decay searches

Another hypothesis concerns "neutrinoless double-beta decay", which, if it exists, would violate lepton number conservation and imply a minuscule splitting (or difference) between the physical masses of what are conventionally called a "neutrino" and its corresponding "antineutrino" having the opposite sign in its lepton number. Searches for this mechanism are underway but have not yet found strong evidence for it. If they were to, then what are now called antineutrinos could not be true antiparticles. The resulting six distinct neutrinos would have no distinct antiparticle partner.[58] Cosmic ray neutrino experiments detect neutrinos from space to study both the nature of neutrinos and the cosmic sources producing them.[59]

### Speed

Before neutrinos were found to oscillate, they were generally assumed to be massless, propagating at the speed of light. According to the theory of special relativity, the question of neutrino velocity is closely related to their mass: if neutrinos are massless, they must travel at the speed of light, and if they have mass they cannot reach the speed of light. Due to their tiny mass, the predicted speed is extremely close to the speed of light in all experiments, and current detectors are not sensitive to the expected difference.

Also some Lorentz-violating variants of quantum gravity might allow faster-than-light neutrinos. A comprehensive framework for Lorentz violations is the Standard-Model Extension (SME).

In the early 1980s, first measurements of neutrino speed were done using pulsed pion 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 was repeated in 2007 using the MINOS detectors, which found the speed of GeV neutrinos to be, at the 99% confidence level, in the range between 0.999976 c and 1.000126 c. The central value of 1.000051c is higher than the speed of light but is also consistent with a velocity of exactly c or even slightly less. This measurement set an upper bound on the mass of the muon neutrino of 50 MeV at 99% confidence.[60][61] After the detectors for the project were upgraded in 2012, MINOS refined their initial result and found agreement with the speed of light, with the difference in the arrival time of neutrinos and light of −0.0006% (±0.0012%).[62]

A similar observation was made, on a much larger scale, with supernova 1987A (SN 1987A). 10 MeV antineutrinos from the supernova were detected within a time window that was consistent with the speed of light for the neutrinos. So far, all measurements of neutrino speed have been consistent with the speed of light.[63][64]

In September 2011, the OPERA collaboration released calculations showing velocities of 17 GeV and 28 GeV neutrinos exceeding the speed of light in their experiments (see Faster-than-light neutrino anomaly). 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. In February 2012, reports came out that the results may have been caused by a loose fiber optic cable attached to one of the atomic clocks which measured the departure and arrival times of the neutrinos. An independent recreation of the experiment in the same laboratory by ICARUS found no discernible difference between the speed of a neutrino and the speed of light.[65]

In June 2012, CERN announced that new measurements conducted by all four Gran Sasso experiments (OPERA, ICARUS, Borexino and LVD) found agreement between the speed of light and the speed of neutrinos, finally refuting the initial OPERA claim.[66]

### Mass

 Unsolved problem in physics:Can we measure the neutrino masses? Do neutrinos follow Dirac or Majorana statistics?(more unsolved problems in physics)

The Standard Model of particle physics assumed that neutrinos are massless. The experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavour states with neutrino mass states (analogously to CKM mixing), requires neutrinos to have nonzero masses.[67] Massive neutrinos were originally conceived by Bruno Pontecorvo in the 1950s. Enhancing the basic framework to accommodate their mass is straightforward by adding a right-handed Lagrangian.

Providing for neutrino mass can be done in two ways, and some proposals use both:

1. If, like other fundamental Standard Model particles, mass is generated by the Dirac mechanism, then the framework would require an SU(2) singlet. This particle would have the Yukawa interactions with the neutral component of the Higgs doublet, but otherwise would have no interactions with Standard Model particles, so is called a "sterile" neutrino.
2. Or, mass can be generated by the Majorana mechanism, which would require the neutrino and antineutrino to be the same particle.

The strongest upper limit on the masses of neutrinos comes from cosmology: the Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total energy of all three types of neutrinos exceeded an average of 50 eV per neutrino, there would be so much mass in the universe that it would collapse.[68] This limit can be circumvented by assuming that the neutrino is unstable, but 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. These indicate that the summed masses of the three neutrinos must be less than 0.3 eV.[69]

The Nobel prize in Physics 2015 was awarded to both Takaaki Kajita and Arthur B. McDonald for their experimental discovery of neutrino oscillations, which demonstrates that neutrinos have mass.[70][71]

In 1998, research results at the Super-Kamiokande neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass.[72] 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.[73] The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by KamLAND in 2005: m2
21
| = 0.000079 eV2.
[74] In 2006, the MINOS 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 m2
32
| = 0.0027 eV2
, consistent with previous results from Super-Kamiokande.[75] Since |Δm2
32
| 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 0.05 eV.[76]

In 2009, lensing data of a galaxy cluster were analyzed to predict a neutrino mass of about 1.5 eV.[77] This surprisingly high value requires that the three neutrino masses be nearly equal, with neutrino oscillations on the order of milli electron-Volts. In 2016 this was updated to a mass of 1.85 eV.[78] It predicts 3 sterile neutrinos of the same mass, stems with the Planck dark matter fraction and the non-observation of neutrinoless double beta decay. The masses lie below the Mainz-Troitsk upper bound of 2.2 eV for the electron antineutrino.[79] The latter is being tested since June 2018 in the KATRIN experiment, that searches for a mass between 0.2 eV and 2 eV.[45]

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 and MARE).

On 31 May 2010, OPERA researchers observed the first tau neutrino candidate event in a muon neutrino beam, the first time this transformation in neutrinos had been observed, providing further evidence that they have mass.[80]

In July 2010, the 3-D MegaZ DR7 galaxy survey reported that they had measured a limit of the combined mass of the three neutrino varieties to be less than 0.28 eV.[81] A tighter upper bound yet for this sum of masses, 0.23 eV, was reported in March 2013 by the Planck collaboration,[82] whereas a February 2014 result estimates the sum as 0.320 ± 0.081 eV based on discrepancies between the cosmological consequences implied by Planck's detailed measurements of the Cosmic Microwave Background and predictions arising from observing other phenomena, combined with the assumption that neutrinos are responsible for the observed weaker gravitational lensing than would be expected from massless neutrinos.[83]

If the neutrino is a Majorana particle, the mass may be calculated by finding the half-life of neutrinoless double-beta decay of certain nuclei. The current lowest upper limit on the Majorana mass of the neutrino has been set by KamLAND-Zen: 0.060–0.161 eV.[84]

### Size

Standard Model neutrinos are fundamental point-like particles, without any width or volume. Since the neutrino is an elementary particle it does not have a size in the same sense as everyday objects.[85] An effective size can be defined using their electroweak cross section (apparent size in electroweak interaction). The characteristic areas for the electroweak interaction are measured in units called nanobarns (nb) which are 10−33 cm² or 10−37 m², roughly the area of a disc a little more than 0.3 attometer in diameter, or about 1 billionth of the size of a uranium nucleus. The electron neutrino cross section is 3.2  nanobarns the muon neutrino cross section is 1.7 nanobarns, and the tau neutrino 1.0 nanobarn. These scattering cross sections depend on no other properties than the masses of the corresponding charged leptons.[86] This size is relevant only to the probability of scattering. Properties associated with conventional "size" are absent: neutrinos cannot be condensed to form a separate uniform substance and they have no minimal distance between them.

### Chirality

Experimental results show that within the margin of error, all produced and observed neutrinos have left-handed helicities (spins antiparallel to momenta), and all antineutrinos have right-handed helicities.[87] In the massless limit, that means that only one of two possible chiralities is observed for either particle. These are the only chiralities included in the Standard Model 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), do not participate in weak interaction (so-called sterile neutrinos), or both.

The existence of nonzero neutrino masses somewhat complicates the situation. Neutrinos are produced in weak interactions as chirality eigenstates. 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 ​mνE. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small. Effectively, they travel so quickly and time passes so slowly in their rest-frames that they do not have enough time to change over any observable path. For example, most solar neutrinos have energies on the order of 0.100 MeV1 MeV, so the fraction of neutrinos with "wrong" helicity among them cannot exceed 10−10.[88][89]

### GSI anomaly

An unexpected series of experimental results for the rate of decay of heavy highly charged radioactive ions circulating in a storage ring has provoked theoretical activity in an effort to find a convincing explanation. The rates of weak decay of two radioactive species with half lives of about 40 s and 200 s are found to have a significant oscillatory modulation, with a period of about 7 s.[90] The observed phenomenon is known as the GSI anomaly, as the storage ring is a facility at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt Germany. As the decay process produces an electron neutrino, some of the proposed explanations for the observed oscillation rate invoke neutrino properties. Initial ideas related to flavour oscillation were met with skepticism.[91] A more recent proposal involves mass differences between neutrino mass eigenstates.[92]

## Sources

### Artificial

#### Reactor neutrinos

Nuclear reactors are the major source of human-generated neutrinos. The majority of energy in a nuclear reactor is generated by fission (the four main fissile isotopes in nuclear reactors are 235
U
, 238
U
, 239
Pu
and 241
Pu
), the resultant neutron-rich daughter nuclides rapidly undergo additional beta decays, each converting one neutron to a proton and an electron and releasing an electron antineutrino (
n

p
+
e
+
ν
e
). Including these subsequent decays, the average nuclear fission releases about 200 MeV of energy, of which roughly 95.5% is retained in the core as heat, and roughly 4.5% (or about 9 MeV)[93] is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of 4000 MW,[d] the total power production from fissioning atoms is actually 4185 MW, of which 185 MW is radiated away as antineutrino radiation and never appears in the engineering. This is to say, 185 MW of fission energy is lost from this reactor and does not appear as heat available to run turbines, since antineutrinos penetrate all building materials practically without interaction.

The antineutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission antineutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the detectable antineutrinos from fission have a peak energy between about 3.5 and 4 MeV, with a maximum energy of about 10 MeV.[94] There is no established experimental method to measure the flux of low-energy antineutrinos. Only antineutrinos with an energy above threshold of 1.8 MeV can trigger inverse beta decay and thus be unambiguously identified (see § Detection below). An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above this threshold. Thus, an average nuclear power plant may generate over 1020 antineutrinos per second above this threshold, but also a much larger number (97%/3% ≈ 30 times this number) below the energy threshold, which cannot be seen with present detector technology. The ND280 detector has been proposed as a viable safeguard unit.[95]

#### Accelerator neutrinos

Some particle accelerators have been used to make neutrino beams. The technique is to collide protons with a fixed target, producing charged pions or kaons. 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 decays are ongoing.[96] Such a setup is generally known as a "neutrino factory".

#### Nuclear weapons

Nuclear weapons also produce very large quantities of neutrinos. Fred Reines and Clyde Cowan considered the detection of neutrinos from a bomb prior to their search for reactor neutrinos; a fission reactor was recommended as a better alternative by Los Alamos physics division leader J.M.B. Kellogg.[97] Fission weapons produce antineutrinos (from the fission process), and fusion weapons produce both neutrinos (from the fusion process) and antineutrinos (from the initiating fission explosion).

### Geologic

Neutrinos are produced together with the natural background radiation. In particular, the decay chains of 238
U
and 232
Th
isotopes, as well as40
K
, include beta decays which emit antineutrinos. These so-called geoneutrinos can provide valuable information on the Earth's interior. A first indication for geoneutrinos was found by the KamLAND experiment in 2005, updated results have been presented by KamLAND[98] and Borexino.[99] The main background in the geoneutrino measurements are the antineutrinos coming from reactors.

Solar neutrinos (proton–proton chain) in the Standard Solar Model

### Atmospheric

Atmospheric neutrinos result from the interaction of cosmic rays with atomic nuclei in the Earth's atmosphere, creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from Tata Institute of Fundamental Research (India), Osaka City University (Japan) and Durham University (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in Kolar Gold Fields in India in 1965.[100]

### Solar

Solar neutrinos originate from the nuclear fusion powering the Sun and other stars. The details of the operation of the Sun are explained by the Standard Solar Model. In short: when four protons fuse to become one helium nucleus, two of them have to convert into neutrons, and each such conversion releases one electron neutrino.

The Sun sends enormous numbers of neutrinos in all directions. Each second, about 65 billion (6.5×1010) solar neutrinos pass through every square centimeter on the part of the Earth orthogonal to the direction of the Sun.[12] 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

In 1966, Colgate and White[101] calculated that neutrinos carry away most of the gravitational energy released by the collapse of massive stars, events now categorized as Type Ib and Ic and Type II supernovae. When such stars collapse, matter densities at the core become so high (1017 kg/m3) 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 profuse neutrino source is the thermal energy (100 billion kelvins) of the newly formed neutron core, which is dissipated via the formation of neutrino–antineutrino pairs of all flavors.[102]

Colgate and White's theory of supernova neutrino production was confirmed in 1987, when neutrinos from Supernova 1987A were detected. The water-based detectors Kamiokande II and IMB detected 11 and 8 antineutrinos (lepton number = −1) of thermal origin,[102] respectively, while the scintillator-based Baksan detector found 5 neutrinos (lepton number = +1) of either thermal or electron-capture origin, in a burst less than 13 seconds long. The neutrino signal from the supernova arrived at earth several hours before the arrival of the first electromagnetic radiation, as expected from the evident fact that the latter emerges along with the shock wave. The exceptionally feeble interaction with normal matter allowed the neutrinos to pass through the churning mass of the exploding star, while the electromagnetic photons were slowed.

Because neutrinos interact so little with matter, it is thought that a supernova's neutrino emissions carry information about the innermost regions of the explosion. Much of the visible light comes from the decay of radioactive elements produced by the supernova shock wave, and even light from the explosion itself is scattered by dense and turbulent gases, and thus delayed. The neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays, or radio waves. The exact time delay of the electromagnetic waves' arrivals depends on the velocity of the shock wave and on the thickness of the outer layer of the star. For a Type II supernova, astronomers expect the neutrino flood to be released seconds after the stellar core collapse, while the first electromagnetic signal may emerge hours later, after the explosion shock wave has had time to reach the surface of the star. The Supernova Early Warning System project uses a network of neutrino detectors to monitor the sky for candidate supernova events; the neutrino signal will provide a useful advance warning of a star exploding in the Milky Way.

Although neutrinos pass through the outer gases of a supernova without scattering, they provide information about the deeper supernova core with evidence that here, even neutrinos scatter to a significant extent. In a supernova core the densities are those of a neutron star (which is expected to be formed in this type of supernova),[103] becoming large enough to influence the duration of the neutrino signal by delaying some neutrinos. The 13 second-long neutrino signal from SN 1987A lasted far longer than it would take for unimpeded neutrinos to cross through the neutrino-generating core of a supernova, expected to be only 3200 kilometers in diameter for SN 1987A.

The number of neutrinos counted was also consistent with a total neutrino energy of 2.2×1046 joules, which was estimated to be nearly all of the total energy of the supernova.[104]

For an average supernova, approximately 10+57 (an octodecillion) neutrinos are released, but the actual number detected at a terrestrial detector ${\displaystyle N}$ will be far smaller, at the level of

${\displaystyle N\sim 10^{4}\left({\frac {M}{25\mathrm {kton} }}\right)\left({\frac {d}{10\mathrm {kpc} }}\right)^{-2}}$,

where ${\displaystyle M}$ is the mass of the detector (with e.g. Super Kamiokande having a mass of 50 kton) and ${\displaystyle d}$ is the distance to the supernova.[105] Hence in practice it will only be possible to detect neutrino bursts from supernovae within or nearby the Milky Way (our own galaxy). In addition to the detection of neutrinos from individual supernovae, it should also be possible to detect the diffuse supernova neutrino background, which originates from all supernovae in the Universe.[106]

### Supernova remnants

The energy of supernova neutrinos ranges from a few to several tens of MeV. The sites where cosmic rays are accelerated are expected to produce neutrinos that are at least one million times more energetic, produced from turbulent gaseous environments left over by supernova explosions: the supernova remnants. The origin of the cosmic rays was attributed to supernovas by Walter Baade and Fritz Zwicky; 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, 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, AMANDA, IceCube, ANTARES, NEMO and Nestor. Related information is provided by very-high-energy gamma ray observatories, such as VERITAS, HESS and MAGIC. 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 or with the dedicated experiment named ANITA.

### Big Bang

It is thought that, just like the cosmic microwave background radiation left over from the Big Bang, there is a background of low-energy neutrinos in our Universe. In the 1980s it was proposed that these may be the explanation for the dark matter thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: it is known that they exist. This idea also has serious problems.

From particle experiments, it is known that neutrinos are very light. This means that they easily move at speeds close to the speed of light. For this reason, dark matter made from neutrinos is termed "hot dark matter". The problem is that being fast moving, the neutrinos would tend to have spread out evenly in the universe before cosmological expansion made them cold enough to congregate in clumps. This would cause the part of dark matter made of neutrinos to be smeared out and unable to cause the large galactic structures that we see.

These same galaxies and groups of galaxies appear to be surrounded by dark matter that is not fast enough to escape from those galaxies. Presumably this matter provided the gravitational nucleus for formation. This implies that neutrinos cannot make up a significant part of the total amount of dark matter.

From cosmological arguments, relic background neutrinos are estimated to have density of 56 of each type per cubic centimeter and temperature 1.9 K (1.7×10−4 eV) if they are massless, much colder if their mass exceeds 0.001 eV. Although their density is quite high, they have not yet been observed in the laboratory, as their energy is below thresholds of most detection methods, and due to extremely low neutrino interaction cross-sections at sub-eV energies. In contrast, boron-8 solar neutrinos—which are emitted with a higher energy—have been detected definitively despite having a space density that is lower than that of relic neutrinos by some 6 orders of magnitude.

## Detection

Neutrinos cannot be detected directly, because they do not ionize the materials they are passing through (they do not 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 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 to isolate the detector from cosmic rays and other background radiation.

Antineutrinos were first detected in the 1950s near a nuclear reactor. Reines and Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrinos with an energy above the threshold of 1.8 MeV caused charged current interactions with the protons in the water, producing positrons and neutrons. This is very much like
β+
decay, where energy is used to convert a proton into a neutron, a positron (
e+
) and an electron neutrino (
ν
e
) is emitted:

From known
β+
decay:

Energy +
p

n
+
e+
+
ν
e

In the Cowan and Reines experiment, instead of an outgoing neutrino, you have an incoming antineutrino (
ν
e
) from a nuclear reactor:

Energy (>1.8 MeV) +
p
+
ν
e

n
+
e+

The resulting positron annihilation with electrons in the detector material created photons with an energy of about 0.5 MeV. 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 8 MeV 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 emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but uses heavy water 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 or gallium which are periodically checked for excesses of argon or germanium, respectively, which are created by electron-neutrinos interacting with the original substance. MINOS used a solid plastic scintillator coupled to photomultiplier tubes, while Borexino uses a liquid pseudocumene scintillator also watched by photomultiplier tubes and the NOνA detector uses liquid scintillator watched by avalanche photodiodes. The IceCube Neutrino Observatory uses 1 km3 of the Antarctic ice sheet near the south pole with photomultiplier tubes distributed throughout the volume. The University of Liverpool ND280 detector employs the novel use of gadolinium encased light detectors in a temperature controlled magnetic field capturing double light pulse events. The T2K experiment developed the technology and practical experiments were successful in both Japan and at Wylfa power station.[107]

## 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 in the mid-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.[108][109]

Neutrinos are also useful for probing astrophysical sources beyond the Solar System because they are the only known particles that are not significantly attenuated 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 (GZK cutoff). Neutrinos, in contrast, can travel even greater distances barely attenuated.

The galactic core of the Milky Way is fully obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core might be measurable by Earth-based neutrino telescopes.[17]

Another important use of the neutrino is in the observation of supernovae, 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.[110] These neutrinos are a very useful probe for core collapse studies.

The rest mass of the neutrino is an important test of cosmological and astrophysical theories (see Dark matter). 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.[111]

The study of neutrinos is important in particle physics because neutrinos typically have the lowest mass, and hence are examples of the lowest-energy particles theorized in extensions of the Standard Model of particle physics.

In November 2012, American scientists used a particle accelerator to send a coherent neutrino message through 780 feet of rock. This marks the first use of neutrinos for communication, and future research may permit binary neutrino messages to be sent immense distances through even the densest materials, such as the Earth's core.[112]

In July 2018, the IceCube Neutrino Observatory announced that they have traced an extremely-high-energy neutrino that hit their Antarctica-based research station in September 2017 back to its point of origin in the blazar TXS 0506 +056 located 3.7 billion light-years away in the direction of the constellation Orion. This is the first time that a neutrino detector has been used to locate an object in space and that a source of cosmic rays has been identified.[113][114][115]

## Notes

1. ^ More specifically, Pauli postulated what is now called the electron neutrino. Two other types were discovered later: see Neutrino flavor below.
2. ^ Niels Bohr was notably opposed to this interpretation of beta decay – he was ready to accept that energy, momentum, and angular momentum were not conserved quantities at the atomic level.
3. ^ In this context, "light neutrino" means neutrinos with less than half the mass of the Z boson.
4. ^ Like all thermal power plants, only about one third of the heat generated can be converted to electricity, so a 4000 MW reactor would produce only 1300 MW of electric power, with 2700 MW being waste heat.

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