A neutrino (Italian pronunciation: [neuˈtriːno], meaning "small neutral one"; English pronunciation: /njuːˈtriːnoʊ/) is an elementary particle that usually travels close to the speed of light, is electrically neutral, and is able to pass through ordinary matter almost unaffected. This makes neutrinos extremely difficult to detect. Neutrinos have a very small, but nonzero mass. They are denoted by the Greek letter ν (nu).
Neutrinos are similar to the more familiar electron, with one crucial difference: neutrinos do not carry electric charge. Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons. Neutrinos are affected only by a "weak" sub-atomic force of much shorter range than electromagnetism, and are therefore able to pass through great distances within matter without being affected by it. Neutrinos also interact gravitationally with other particles.
Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Each type also has a corresponding antiparticle, called an antineutrino. Electron neutrinos (or antineutrinos) are generated whenever protons change into neutrons, or vice versa—the two forms of beta decay. Interactions involving neutrinos are mediated by the weak interaction.
Most neutrinos passing through the Earth emanate from the Sun. Every second, in the region of the Earth, about 65 billion (6.5×1010 ) solar neutrinos pass through every square centimeter perpendicular to the direction of the sun.[1]
History
Proposal of neutrino existence, from conservation arguments
The first use of a hydrogen bubble chamber to detect neutrinos, on November 13, 1970. A neutrino hit a proton in a hydrogen atom. The collision occurred at the point where three tracks emanate on the right of the photograph.
The neutrino[nb 1] was first postulated in 1930 by Wolfgang Pauli to preserve the conservation of energy, conservation of momentum, and conservation of angular momentum in beta decay—the decay of an atomic nucleus (not known to contain or involve the neutron at the time) into a proton, an electron and an antineutrino.[nb 2][2]
n0 → p+ + e− + νe
He theorized that an undetected particle was carrying away the observed difference between the energy, momentum, and angular momentum of the initial and final particles.
Pauli originally named his proposed light particle a neutron. When James Chadwick discovered a much more massive nuclear particle in 1932 and also named it a neutron, this left the two particles with the same name. Enrico Fermi, who developed the theory of beta decay, coined the term neutrino in 1934 as a way to resolve the confusion. It is the Italian equivalent of "little neutral one".[3]
Direct detection from induced beta decay
In 1942 Kan-Chang Wang first proposed the use of beta-capture to experimentally detect neutrinos.[4] In the July 20, 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,[5][6] a result that was rewarded almost forty years later with the 1995 Nobel Prize.[7]
In this experiment, now known as the Cowan–Reines neutrino experiment, neutrinos created in a nuclear reactor by beta decay were shot into protons producing neutrons and positrons.
νe + 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.
It is now known that both the proposed and the observed particles were antineutrinos.
Experimental demonstration of neutrino flavors
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[8]), which earned them the 1988 Nobel Prize. 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 neutrino. The first detection of tau neutrino interactions was announced in summer of 2000 by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed; its existence had already been inferred by both theoretical consistency and experimental data from the Large Electron–Positron Collider.
The solar neutrino number discrepancy 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. This discrepancy, which became known as the solar neutrino problem, remained unresolved for some thirty years. The Standard Model of particle physics assumes that neutrinos are massless and cannot change flavor. However, if neutrinos had mass, they could change flavor (or oscillate between flavors).
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.
Direct detection of flavor oscillation in solar neutrinos
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.[9]
Detection of supernova neutrinos
See also: Supernova Early Warning System
Raymond Davis Jr. and Masatoshi Koshiba were jointly awarded the 2002 Nobel Prize in Physics; Davis for his pioneer work on cosmic neutrinos and Koshiba for the first real time observation of supernova neutrinos. The detection of solar neutrinos, and of neutrinos of the SN 1987A supernova in 1987 marked the beginning of neutrino astronomy.
Properties and reactions
The neutrino has half-integer spin (½ħ) and is therefore a fermion. Neutrinos interact primarily through the weak force. The discovery of neutrino flavor oscillations implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment[10] of the order of 10−19
μB, allowing the possibility that neutrinos may interact electromagnetically as well. An experiment done by C. S. Wu at Columbia University showed that neutrinos always have left-handed chirality.
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.
Analogies to the index of refraction and the 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, but gives rise to the MSW effect. Only a small fraction of the neutrino's energy is transferred to the material.
Neutrinos can interact with a heavier 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.
Neutrinos might affect nuclear decay rate
A Russian study suggests that the decay rate of radioactive isotopes is not constant as is commonly believed,[11] and a recent study[12] also finds this, and says it appears to be affected by the rate of neutrinos emitted by the Sun. If true, this finding casts doubt on the absolute reliability of radiometric dating if the neutrino flux from the Sun has not been constant throughout history.
Theoretical reactions not yet observed: neutrino induced fission
Very much like neutrons do in nuclear reactors, neutrinos can induce fission reactions within heavy nuclei.[13] 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.[14]
Types of neutrinos
Neutrinos in the Standard Model
of elementary particles Fermion Symbol Mass[nb 3]
Generation 1
Electron neutrino ν
e < 2.2 eV
Electron antineutrino ν
e < 2.2 eV
Generation 2
Muon neutrino ν
μ < 170 keV
Muon antineutrino ν
μ < 170 keV
Generation 3
Tau neutrino ν
τ < 15.5 MeV
Tau antineutrino ν
τ < 15.5 MeV
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[nb 4] 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.[10] 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. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
The possibility of sterile neutrinos—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 experiment. However, the currently running MiniBooNE experiment suggested, until recently, that sterile neutrinos are not required to explain the experimental data,[15] 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.[16] A recent re-analysis of reference electron spectra data from the ILL[17] has also hinted at a fourth, sterile neutrino.[18]
Recently analyzed data from the Wilkinson Microwave Anisotropy Probe of the cosmic background radiation 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.[19]
Antineutrinos
Antineutrinos are the antiparticles of neutrinos, which are neutral particles produced in nuclear beta decay. These are emitted in beta particle emissions, where a neutron turns into a proton. They have a spin of ½, and they are part of the lepton 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 only through the gravitational and weak forces, making them very difficult to detect experimentally. Neutrino oscillation experiments indicate that antineutrinos have mass, 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.
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.[20][21][22]
Antineutrinos were first detected as a result of their interaction with cadmium nuclei 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
Main article: Neutrino oscillation
Neutrinos are most often created or detected with a well defined flavor (electron, muon, tau). However, in a phenomenon known as neutrino flavor oscillation, 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's core failing to match the expected numbers, dubbed as the "solar neutrino problem". In the Standard Model 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.
It is possible that the neutrino and antineutrino are in fact the same particle, a hypothesis first proposed by the Italian physicist Ettore Majorana. The neutrino could transform into an antineutrino (and vice versa) by flipping the orientation of its spin state.[23]
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 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 the idea of neutrino oscillations came up, it was generally assumed that neutrinos travel at the speed of light. The question of neutrino velocity is closely related to their mass. According to relativity, if neutrinos are massless, they must travel at the speed of light. However, if they carry a mass, they cannot reach the speed of light.
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 has been repeated using the MINOS detectors, which found the speed of 3 GeV neutrinos to be 1.000051(29) c. While the central value is higher than the speed of light, the uncertainty is great enough that it is very likely that the true velocity is not greater than the speed of light. This measurement set an upper bound on the mass of the muon neutrino of 50 MeV at 99% confidence.[24]
The same observation was made, on a somewhat larger scale, with supernova 1987a. The neutrinos 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.
Mass
The Standard Model 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 requires neutrinos to have nonzero masses.[15] This was originally conceived by Bruno Pontecorvo in the 1950s.
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.[citation needed] 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. These indicate that the sum of the neutrino masses must be less than 0.3 eV.[25]
In 1998, research results at the Super-Kamiokande neutrino detector determined that neutrinos do indeed flavor oscillate, and therefore have mass.[26] 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.[27] The best estimate of the difference in the squares of the masses of mass eigenstates 1 and 2 was published by KamLAND in 2005: Δm2
21 = 0.000079 eV2.[28] 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 |Δm2
32| = 0.0027 eV2, consistent with previous results from Super-Kamiokande.[29] 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 0.04 eV.[30]
In 2009 lensing data of a galaxy cluster were analyzed to predict a neutrino mass of about 1.5 eV.[31] All neutrino masses are then nearly equal, with neutrino oscillations of order meV. They lie below the Mainz-Troitsk[clarification needed] upper bound of 2 eV for the electron anti-neutrino. The latter will be tested in 2015 in the KATRIN experiment, that searches for a mass between 0.2 eV and 2 eV. If it is found around 1.5 eV, then the Cold Dark Matter particle likely does not exist.
Currently 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) or neutrinoless double beta decay (e.g. GERDA, CUORE/Cuoricino, NEMO-3 and others).
In May 2010, it was reported that physicists from CERN and the Italian National Institute for Nuclear Physics' Gran Sasso National Laboratory had observed for the first time a transformation in neutrinos; evidence that they have mass.[32][33]
In July 2010 the 3-D MegaZ experiment suggested the upper limit of the combined mass of the neutrino to be less than 0.28 eV, a disagreement with the astronomical evidence that requires resolution.[34]
Handedness
Experimental results show that (nearly) all produced and observed neutrinos have left-handed helicities (spins antiparallel to momenta), 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 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. 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 100 keV–1 MeV, so the fraction of neutrinos with "wrong" helicity among them cannot exceed 10−10
).[35][36]
Neutrino sources
Artificially produced neutrinos
Nuclear reactors are the major source of human-generated neutrinos. Anti-neutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. Generally, the four main isotopes contributing to the anti-neutrino flux are 235
U, 238
U, 239
Pu and 241
Pu (i.e. the anti-neutrinos emitted during beta-minus decay of their respective fission fragments). The average nuclear fission releases about 200 MeV of energy, of which roughly 4.5% (or about 9 MeV)[37] is radiated away as anti-neutrinos. For a typical nuclear reactor with a thermal power of 4,000 MW, meaning that the core produces this much heat, and an electrical power generation of 1,300 MW, the total power production from fissioning atoms is actually 4,185 MW, of which 185 MW is radiated away as anti-neutrino 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 the anti-neutrinos penetrate all building materials essentially tracelessly, and disappear.[38]
The anti-neutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission anti-neutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the detectable anti-neutrinos from fission have a peak energy between about 3.5 and 4 MeV, with a maximal energy of about 10 MeV.[39] There is no established experimental method to measure the flux of low energy anti-neutrinos. Only anti-neutrinos with an energy above threshold of 1.8 MeV can be uniquely identified (see neutrino detection below). An estimated 3% of all anti-neutrinos from a nuclear reactor carry an energy above this threshold. An average nuclear power plant may generate over 1020
anti-neutrinos per second above this threshold, and a much larger number which cannot be seen with present detector technology.
Some particle accelerators have been used to make neutrino beams. The technique is to smash protons into 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.[40] Such a setup is generally known as a neutrino factory.
Nuclear bombs 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 having been recommended as a better alternative by Los Alamos physics division leader J.M.B. Kellogg.[41]
Geologically produced neutrinos
Neutrinos are part of 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 anti-neutrinos. 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 anti-neutrinos coming from reactors. Several future experiments aim at improving the geoneutrino measurement and these will necessarily have to be far away from reactors.
Solar neutrinos (proton-proton chain) in the Standard Solar Model
Atmospheric neutrinos
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.
Solar neutrinos
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. Every second, about 65 billion (6.5×1010
) solar neutrinos pass through every square centimeter on the part of the Earth that faces the Sun.[1] 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.
Supernova neutrinos
SN 1987A
Neutrinos are an important product of Types Ib, Ic and II (core-collapse) supernovae. In such events, the density at the core becomes 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 important 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.[42] 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 were detected. The water-based detectors Kamiokande II and IMB detected 11 and 8 antineutrinos of thermal origin,[42] respectively, while the scintillator-based Baksan detector found 5 neutrinos (lepton number = 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 stars. 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 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.
High energy cosmic neutrinos
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 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.
Cosmic background radiation neutrinos
Main article: Cosmic neutrino background
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: 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 move at speeds close to the speed of light. Thus, 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.
Further, these same galaxies and groups of galaxies appear to be surrounded by dark matter which is not fast enough to escape from those galaxies. Presumably this matter provided the gravitational nucleus for formation. 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 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, 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.
Neutrino detection
Main article: Neutrino detector
Because neutrinos are very weakly interacting, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground in order 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. The resulting positron annihilations with electrons 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 uses a solid plastic scintillator coupled to photomultiplier tubes, while Borexino uses a liquid pseudocumene scintillator also watched by photomultiplier tubes and the proposed NOνA detector will use 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.
Motivation for scientific interest in the neutrino
The neutrino is of scientific interest because it can make an exceptional probe for environments that are typically concealed from the standpoint of other observation techniques, such as optical and radio observation.
The first such use of neutrinos was proposed in the early 20th century for observation of the core of the Sun. Direct optical observation of the solar core is impossible due to the diffusion of electromagnetic radiation by the huge amount of matter surrounding the core. On the other hand, neutrinos generated in stellar fusion reactions interact very weakly with matter, and pass through the Sun with few interactions. While photons emitted by the solar core may require some 40,000 years to diffuse to the outer layers of the Sun, neutrinos are virtually unimpeded and cross this distance at nearly the speed of light.[43][44]
Neutrinos are also useful for probing astrophysical sources beyond our solar system. Neutrinos 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 not able to travel more than about 100 megaparsecs due to the GZK cutoff. Neutrinos can travel this and greater distances with very little attenuation.
The galactic core of the Milky Way is completely obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core will be measurable by Earth-based neutrino telescopes in the next decade.
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 almost unimaginably 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 energy in a quick (10-second) burst of neutrinos. As a result, neutrinos are a very useful probe for these important events.
Determining the mass of the neutrino (see above) is also an important test of cosmology (see Dark matter). Many other important uses of the neutrino may be imagined in the future. It is clear that the astrophysical significance of the neutrino as an observational technique is comparable with all other known techniques, and is therefore a major focus of study in astrophysical communities.
In particle physics the main virtue of studying neutrinos is that they are typically the lowest mass, and hence lowest energy examples of particles theorized in extensions of the Standard Model of particle physics. For example, one would expect that if there is a fourth class of fermions beyond the electron, muon, and tau generations of particles, that the fourth generation neutrino would be the easiest to generate in a particle accelerator.
Neutrinos could also be used for studying quantum gravity effects. Because they are not affected by either the strong interaction or electromagnetism (unless they have a magnetic moment), and because they are not normally found in composite particles (unlike quarks) or prone to near instantaneous decay (like many other standard model particles) it might be possible to isolate and measure gravitational effects on neutrinos at a quantum level.
Book: Particles of the Standard Model
List of neutrino experiments
Cowan–Reines neutrino experiment
Neutrino astronomy
Neutrino detector
Neutrino oscillations
Seesaw mechanism
Sterile neutrino
Supernova Early Warning System
References
Notes
^ More specifically, the electron neutrino.
^ Niels Bohr was notably opposed to this interpretation of beta decay and was ready to accept that energy, momentum and angular momentum were not conserved quantities.
^ Since neutrino flavor eigenstates are not the same as neutrino mass eigenstates (see neutrino oscillation), 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 decays. This sort of measurement directly measures the expectation value of the mass; it is not sensitive to any of the mass eigenstates separately.
^ In this context, "light neutrino" means neutrinos with less than half the mass of the Z boson.
References
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