A neutrino ( ; denoted by the Greek letter Nu (Greek)|) is an elementary particle that interacts via the weak interaction and gravity.
The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the electromagnetic interaction or the strong interaction.
Consequently, neutrinos typically pass through normal matter unimpeded and with no detectable effect.
Weak interactions create neutrinos in one of three leptonic flavors: electron neutrino, ; muon neutrino, and tau neutrino, . Each flavor is associated with the correspondingly named charged lepton. Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different values (all tiny, the smallest of which could be zero), but the three masses do not uniquely correspond to the three flavors: A neutrino created with a specific flavor is a specific mixture of all three mass states (a quantum superposition). Similar to some other neutral particles, neutrinos oscillate between different flavors in flight as a consequence. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. The three mass values are not yet known as of 2024, but laboratory experiments and cosmological observations have determined the differences of their squares, an upper limit on their sum (< ), Neutrinos are fermions, which have spin of .
For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has spin of and no electric charge. Antineutrinos are distinguished from neutrinos by having opposite-signed lepton number and weak isospin, and right-handed instead of left-handed chirality. To conserve total lepton number (in nuclear beta decay), electron neutrinos only appear together with positrons (anti-electrons) or electron-antineutrinos, whereas electron antineutrinos only appear with electrons or electron neutrinos.
Neutrinos are created by various radioactive decays. These include the beta decay of atomic nuclei or hadrons and natural nuclear reactions such as those that take place in the core of a star. Other mechanisms include artificial nuclear reactions in nuclear reactors, nuclear bombs, or particle accelerators, supernovas, during the spin-down of a neutron star and when cosmic rays or accelerated particle beams strike atoms
The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun. At the surface of the Earth, the flux is about 66 billion () solar neutrinos, per second per square centimeter. Neutrinos can be used for tomography of the interior of the Earth.
History
Pauli's proposal
The neutrino 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 of electrons 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 beta particles (electrons or positrons) and had a mass similar to the electron.
James Chadwick discovered a much more massive neutral nuclear particle in 1932 and named it a neutron also, leaving two kinds of particles with the same name. The word "neutrino" entered the scientific vocabulary through Enrico Fermi, who used it during a conference in Paris in July 1932 and at the Solvay Conference in October 1933, where Pauli also employed it. The name (the Italian equivalent of "little neutral one") was jokingly coined by Edoardo Amaldi during a conversation with Fermi at the Institute of Physics of via Panisperna in Rome, in order to distinguish this light neutral particle from Chadwick's heavy neutron.
In Fermi's theory of beta decay, Chadwick's large neutral particle could decay to a proton, electron, and the smaller neutral particle (now called an electron antineutrino):
:
Fermi's paper, written in 1934,<!---->
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.
Direct detection
thumb|upright=1.1|Fred Reines and Clyde Cowan conducting the neutrino experiment c. 1956
In 1942, Wang Ganchang first proposed the use of beta capture to experimentally detect neutrinos. In the 20 July 1956 issue of Science, Clyde Cowan, Frederick Reines, Francis B. "Kiko" Harrison, Herald W. Kruse, and Austin D. McGuire published confirmation that they had detected the neutrino, a result that was rewarded almost forty years later with the 1995 Nobel Prize.
In this experiment, now known as the Cowan–Reines neutrino experiment, antineutrinos created in a nuclear reactor by beta decay reacted with protons to produce neutrons and positrons:
:
The positron quickly finds an electron, and they annihilate each other. The two resulting gamma rays (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events—positron annihilation and neutron capture—gives a unique signature of an antineutrino interaction.
In February 1965, the first neutrino found in nature was identified by a group including Frederick Reines and Friedel Sellschop. The experiment was performed in a specially prepared chamber at a depth of 3 km in the East Rand ("ERPM") gold mine near Boksburg, South Africa. A plaque in the main building commemorates the discovery. The experiments also implemented a primitive neutrino astronomy and looked at issues of neutrino physics and weak interactions.
Neutrino flavor <span class="anchor" id="Neutrino_flavors_anchor"></span>
The antineutrino discovered by Clyde Cowan and Frederick Reines was the antiparticle of the electron neutrino.
In 1962, Leon M. Lederman, Melvin Schwartz, and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of the muon neutrino (already hypothesised with the name neutretto), which earned them the 1988 Nobel Prize in Physics.
When the third type of lepton, the tau, was discovered in 1975 at the Stanford Linear Accelerator Center, it was also expected to have an associated neutrino (the tau neutrino). The first evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to 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.
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 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, one to R. Davis, who conceived and led the Homestake experiment and Masatoshi Koshiba of Kamiokande, whose work confirmed it, and one to Takaaki Kajita of Super-Kamiokande and A.B. McDonald of Sudbury Neutrino Observatory for their joint experiment, which confirmed the existence of all three neutrino flavors and found no deficit. 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
As well as specific sources, a general background level of neutrinos is expected to pervade the universe, theorized to occur due to two main sources: the Big Bang and supernova.
Around 1 second after the Big Bang, neutrinos decoupled, giving rise to a background level of neutrinos known as the cosmic neutrino background (CNB). A second diffuse neutrino background is supernova-originated.
SN 1987A represents the only verified detection of neutrinos from a supernova. However, many stars have exploded as supernovae in the universe, leaving a theorized diffuse supernova neutrino background.
Properties and reactions
Neutrinos have half-integer spin (); therefore they are fermions. Neutrinos are leptons; therefore they are colorless fermions that cannot interact with the gluons of the strong force. They have only been observed to interact through the weak force, although it is assumed that they also interact gravitationally. Since they have non-zero mass, some theories permit, but do not require, neutrinos to interact magnetically; as yet there is no experimental evidence for a non-zero magnetic moment in neutrinos.
Flavor, mass, and their mixing <span id="neutrino_flavor_anchor" class="anchor"></span>
<!-- "Neutrino flavor" redirects here -->
Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos (), muon neutrinos (), or tau neutrinos (), associated with the corresponding charged leptons, the electron (), muon (), and tau (), respectively.
Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses; each neutrino flavor state is a linear combination of the three distinct mass eigenstates. From calculations based on cosmological models, the sum of the three neutrino masses must be below . This large ratio suggests the possibility that the mass-creation mechanism for neutrinos differs from that of other fermions.
|-
| Electron anti-neutrino || < ||   0  ||
A neutrino created in a specific flavor eigenstate is in an associated specific quantum superposition of all three mass eigenstates. The three masses differ so little that they cannot possibly be distinguished experimentally within any practical flight path. The proportion of each mass state in the pure flavor states produced has been found to depend profoundly on the flavor. The relationship between flavor and mass eigenstates is encoded in the PMNS matrix. Experiments have established moderate- to low-precision values for the elements of this matrix, with the single complex phase in the matrix being only poorly known, as of 2016.
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 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.
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 Mikheyev–Smirnov–Wolfenstein effect. Only a small fraction of the neutrino's energy is transferred to the material.
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 (and consequently opposite-sign weak isospin). As of 2016, no evidence has been found for any other difference.
So far, despite extensive and continuing searches for exceptions, in all observed leptonic processes there has never been any change in total lepton number; for example, if the total lepton number is zero in the initial state, then the final state has only matched lepton and anti-lepton pairs: electron neutrinos appear in the final state together with only positrons (anti-electrons) or electron antineutrinos, and electron antineutrinos with electrons or electron neutrinos.
Majorana mass
Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Rather than conventional Dirac fermions, neutral particles can be another type of spin particle called 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 would 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 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, SNO+, and CUORE. 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.
Nuclear reactions
Neutrinos can interact with a nucleus, changing it to a different 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.
Types
{| class="wikitable floatright"
|+Neutrinos in the Standard Model of elementary particles
|-
! Fermion
! Symbol
|-
!colspan="2" style="background:#ffdead;"| Generation 1
|-
|style="background:#efefef;"| Electron neutrino
|style="text-align:center;"|
|-
|style="background:#efefef;"| Electron antineutrino
|style="text-align:center;"|
|-
!colspan="3" style="background:#ffdead;"| Generation 2
|-
|style="background:#efefef;"| Muon neutrino
|style="text-align:center;"|
|-
|style="background:#efefef;"| Muon antineutrino
|style="text-align:center;"|
|-
!colspan="3" style="background:#ffdead;"| Generation 3
|-
|style="background:#efefef;"| Tau neutrino
|style="text-align:center;"|
|-
|style="background:#efefef;"| Tau antineutrino
|style="text-align:center;"|
|}
There are three known types (flavors) of neutrinos: electron neutrino , muon neutrino , and tau neutrino , 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 available types of light neutrinos, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that three light neutrino flavors couple to the Z. to determine the value of the mass of the electron neutrino, with other approaches to this problem in the planning stages.
Sterile neutrino searches
Other efforts search for evidence of a sterile neutrino – a fourth neutrino flavor that would not interact with matter like the three known neutrino flavors. 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 could not be a decay product. Therefore, to be consistent with not having been detected in Z boson decays, heavy sterile neutrinos would need to 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, although the latest research into this area is on-going and anomalies in the MiniBooNE data may allow for exotic neutrino types, including sterile neutrinos. A re-analysis of reference electron spectra data from the Institut Laue-Langevin in 2011 has also hinted at a fourth, light sterile neutrino. Triggered by the 2011 findings, several experiments at very short distances from nuclear reactors have searched for sterile neutrinos since then. While most of them were able to rule out the existence of a light sterile neutrino, the combined results are ambiguous.
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.
Neutrinoless double-beta decay searches
Another hypothesis concerns "neutrinoless double-beta decay", which, if it exists, would violate lepton number conservation. Searches for this mechanism are underway but have not yet found evidence for it. If they were to, then what are now called antineutrinos could not be true antiparticles.
Cosmic ray neutrinos
Cosmic ray neutrino experiments detect neutrinos from space to study both the nature of neutrinos and the cosmic sources producing them.
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, there are some Lorentz-violating variants of quantum gravity which might allow faster-than-light neutrinos. A comprehensive framework for Lorentz violations is the Standard-Model Extension (SME).
The first measurements of neutrino speed were made in the early 1980s 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 neutrinos to be, at the 99% confidence level, in the range between and . The central value of is higher than the speed of light but, with uncertainty taken into account, is also consistent with a velocity of exactly or slightly less. This measurement set an upper bound on the mass of the muon neutrino at with 99% confidence. 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 .
A similar observation was made, on a much larger scale, with supernova 1987A (SN 1987A). Antineutrinos with an energy of 10 MeV 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.
Superluminal neutrino glitch
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. 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. The experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavor states with neutrino mass states (analogously to CKM mixing), requires neutrinos to have nonzero masses. 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:
- If, like other fundamental Standard Model fermions, mass is generated by the Dirac mechanism, then the framework would require an additional right-chiral component which is an SU(2) singlet. This component would have the conventional Yukawa interactions with the neutral component of the Higgs doublet; but, otherwise, would have no interactions with Standard Model particles.
- Or, else, mass can be generated by the Majorana mechanism, which would require the neutrino and antineutrino to be the same particle.
A hard 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 mass of all three types of neutrinos exceeded an average of per neutrino, there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable, 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. Analysis of data from the WMAP microwave space telescope found that the sum of the masses of the three neutrino species must be less than . In 2018, the Planck collaboration published a stronger bound of , which was derived by combining their CMB total intensity, polarization and gravitational lensing observations with baryon acoustic oscillation measurements from galaxy surveys and supernova measurements from Pantheon. A 2021 reanalysis that adds redshift space distortion measurements from the SDSS-IV eBOSS survey gets an even tighter upper limit of . However, several ground-based telescopes with similarly sized error bars as Planck prefer higher values for the neutrino mass sum, indicating some tension in the data sets.
The Nobel prize in Physics 2015 was awarded to Takaaki Kajita and Arthur B. McDonald for their experimental discovery of neutrino oscillations, which demonstrates that neutrinos have mass.
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. Since June 2018 the KATRIN experiment searches for a mass between and in tritium decays.
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.
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. 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 exist, 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 . 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 to ; consequently, the fraction of neutrinos with "wrong" helicity among them cannot exceed .
Sources
Artificial
Reactor neutrinos
A nuclear fission reactor produced around 10<sup>20</sup> electron antineutrinos per second. Fission of , as well as , and produce neutron-rich daughter nuclides that rapidly undergo a series of additional beta decays, yielding about six electron antineutrino per fission. Including these subsequent decays, the average nuclear fission releases about of energy, of which roughly 95.5% remains in the core as heat, and roughly 4.5% (or about ) is radiated away as antineutrinos.
The antineutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission antineutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the detectable antineutrinos from fission have a peak energy between about 3.5 and , with a maximum energy of about . There is no established experimental method to measure the flux of low-energy antineutrinos, though experiments to demonstrate the capacity of low-energy neutrino detection via the threshold-less CEνNS interaction are ongoing. Only antineutrinos with an energy above threshold of can trigger inverse beta decay and thus be unambiguously identified (see ' below).
An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above that threshold. Thus, an average nuclear power plant may generate over antineutrinos per second above the threshold, but also a much larger number ( times this number) below the energy threshold; these lower-energy antineutrinos are invisible to present detector technology.
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 design an accelerator facility where neutrinos are produced through muon decays are ongoing.
Nuclear weapons
Fred Reines and Clyde Cowan suspected that nuclear weapons produce very large quantities of neutrinos but concluded that short timescale of the blast would make detection impossible. They instead turned to nuclear reactors as a possible source; a fission reactor was recommended as a better alternative by Los Alamos physics division leader J.M.B. Kellogg. 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
thumb|upright=1.5|AGM2015: A worldwide v̄<sub>e</sub> flux map combining [[geoneutrinos from natural Uranium-238 and Thorium-232 decay in the Earth's crust and mantle as well as reactor-v̄<sub>e</sub> emitted by power plant reactors worldwide.]]
Neutrinos are produced together with the natural background radiation. In particular, the decay chains of and isotopes, as well as , 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,
The Sun sends enormous numbers of neutrinos in all directions. Each second, about 65 billion () solar neutrinos pass through every square centimeter on the part of the Earth orthogonal to the direction of the Sun. Since neutrinos are insignificantly absorbed by the mass of the Earth, the surface area on the side of the Earth opposite the Sun receives about the same number of neutrinos as the side facing the Sun.
Supernovae
thumb|[[SN 1987A]]
Colgate & White (1966)
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), 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 3,200 kilometers in diameter for SN 1987A.
The number of neutrinos counted was also consistent with a total neutrino energy of , which was estimated to be nearly all of the total energy of the supernova.
Antineutrinos with an energy above the threshold of caused charged current interactions with the protons in the water, an interaction usually known as inverse beta decay, producing positrons and neutrons. This is very much like decay, where energy is used to convert a proton into a neutron, a positron () and an electron neutrino () is emitted:
In the Cowan and Reines experiment, instead of an outgoing neutrino, an incoming antineutrino () from a nuclear reactor interacts with a proton:
The resulting positron annihilation with electrons in the detector material created photons with an energy of about . Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about that were detected a few microseconds after the photons from a positron annihilation event.
Since then, various detection methods have been used. Super Kamiokande is a large volume of water surrounded by photomultiplier tubes that watch for the Cherenkov radiation emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but used 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, such as the one used in the Homestake Experiment, 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 of the Antarctic ice sheet near the South Pole with photomultiplier tubes distributed throughout the volume. Another modern detection method is The Liquid Argon Time Projection Chamber (LArTPC), which consists of a large volume of liquid argon with a high voltage field applied to drift the ionized electrons to a series of charge collection planes, allowing for 3D reconstruction of particle tracks. Several experiments have used this technology, including MicroBooNE, the Short-Baseline Near Detector, and the upcoming Deep Underground Neutrino Experiment.thumb|Diagram of coherent elastic neutrino-nucleus scattering|213x213px
More exotically, some experiments (such as COHERENT and CONUS) leverage the neutral current interaction of neutrinos with a whole nucleus, the Coherent elastic neutrino-nucleus scattering (CEνNS) interaction, to detect neutrinos below the threshold of inverse beta decay. These experiments, which overwhelmingly use crystal-based detectors very similar to the solid-state detectors in use for direct detection of dark matter experiments, are some of the most sensitive particle detectors in modern physics, boasting thresholds as low as 20 eV deposited in the detector. This is necessary as the heavier nuclei, selected for the high probability of interaction, will retain very little of the energy in an elastic scattering, being much more massive than the neutrino.
Other ways neutrinos might affect their environment, such as the MSW effect, do not produce traceable radiation, and are not predicted to be detectable.
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 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.
In June 2023, astronomers reported using a new technique to detect, for the first time, the release of neutrinos from the galactic plane of the Milky Way galaxy.
See also
Notes
References
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Further reading
External links
- (Pauli's letter stating the hypothesis of the neutrino: online and analyzed; for English version translated by John Moran, click 'The Neutrinos saga').