Supernova neutrinos are weakly interactive elementary particles produced during a core-collapse supernova explosion. [1] A massive star collapses at the end of its life, emitting on the order of 1058 neutrinos and antineutrinos in all lepton flavors. [2] The luminosity of different neutrino and antineutrino species are roughly the same. [3] They carry away about 99% of the gravitational energy of the dying star as a burst lasting tens of seconds. [4] [5] The typical supernova neutrino energies are 10 to 20 MeV . [6] Supernovae [lower-alpha 1] are considered the strongest and most frequent source of cosmic neutrinos in the MeV energy range.
Since neutrinos are generated in the core of a supernova, they play a crucial role in the star's collapse and explosion. [7] Neutrino heating is believed to be a critical factor in supernova explosions. [1] Therefore, observation of neutrinos from supernovae provides detailed information about core collapse and the explosion mechanism. [8] Further, neutrinos undergoing collective flavor conversions in a supernova's dense interior offers opportunities to study neutrino-neutrino interactions. [9] The only supernova neutrino event detected so far is SN 1987A. [lower-alpha 2] Nevertheless, with current detector sensitivities, it is expected that thousands of neutrino events from a galactic core-collapse supernova would be observed. [11] The next generation of experiments are designed to be sensitive to neutrinos from supernova explosions as far as Andromeda or beyond. [12] The observation of supernovae will broaden our understanding of various astrophysical and particle physics phenomena. [13] Further, coincident detection of supernova neutrino in different experiments would provide an early alarm to astronomers about a supernova. [14]
Stirling A. Colgate and Richard H. White, [16] and independently W. David Arnett, [17] identified the role of neutrinos in core collapse, which resulted in the subsequent development of the theory of supernova explosion mechanism. [6] In February 1987, the observation of supernova neutrinos experimentally verified the theoretical relationship between neutrinos and supernovae. The Nobel Prize-winning event, [6] known as SN 1987A, was the collapse of a blue supergiant star Sanduleak -69° 202, in the Large Magellanic Cloud outside our Galaxy, 51 kpc away. [18] About 1058 lightweight weakly-interacting neutrinos were produced, carrying away almost all of the energy of the supernova. [19] Two kiloton-scale water Cherenkov detectors, Kamiokande II and IMB, along with a smaller Baksan Observatory, detected a total of 25 neutrino-events [19] over a period of about 13 seconds. [6] Only electron-type neutrinos were detected because neutrino energies were below the threshold of muon or tau production. [19] The SN 1987A neutrino data, although sparse, confirmed the salient features of the basic supernova model of gravitational collapse and associated neutrino emission. [19] It put strong constraints on neutrino properties such as charge and decay rate. [19] [20] The observation is considered a breakthrough in the field of supernova and neutrino physics. [15]
Neutrinos are fermions, i.e. elementary particles with a spin of 1/2. They interact only through weak interaction and gravity. [21] A core-collapse supernova emits a burst of ~ neutrinos and antineutrinos on a time scale of tens of seconds. [2] [lower-alpha 3] Supernova neutrinos carry away about 99% of the gravitational energy of the dying star in the form of kinetic energy. [5] [lower-alpha 4] Energy is divided roughly equally between the three flavors of neutrinos and three flavors of antineutrinos. [22] Their average energy is of the order 10 MeV. [23] The neutrino luminosity of a supernova is typically on the order of or . [24] The core-collapse events are the strongest and most frequent source of cosmic neutrinos in the MeV energy range. [6]
During a supernova, neutrinos are produced in enormous numbers inside the core. Therefore, they have a fundamental influence on the collapse and supernova explosions. [25] Neutrino heating is predicted to be responsible for the supernova explosion. [1] Neutrino oscillations during the collapse and explosion generate the gravitational wave bursts. [26] Furthermore, neutrino interactions set the neutron-to-proton ratio, determining the nucleosynthesis outcome of heavier elements in the neutrino driven wind. [27]
Supernova neutrinos are produced when a massive star collapses at the end of its life, ejecting its outer mantle in an explosion. [6] Wilson's delayed neutrino explosion mechanism has been used for 30 years to explain core collapse supernova. [1]
Near the end of life, a massive star is made up of onion-layered shells of elements with an iron core. During the early stage of the collapse, electron neutrinos are created through electron-capture on protons bound inside iron-nuclei: [15]
The above reaction produces neutron-rich nuclei, leading to neutronization of the core. Therefore, this is known as the neutronization phase. Some of these nuclei undergo beta-decay and produce anti-electron neutrinos: [15]
The above processes reduce the core energy and its lepton density. Hence, the electron degeneracy pressure is unable to stabilize the stellar core against the gravitational force, and the star collapses. [15] When the density of the central region of collapse exceeds 1012 g/cm3, the diffusion time of neutrinos exceeds the collapse time. Therefore, the neutrinos become trapped inside the core. When the central region of the core reaches nuclear densities (~ 1014 g/cm3), the nuclear pressure causes the collapse to deaccelerate. [28] This generates a shock wave in the outer core (region of iron core), which triggers the supernova explosion. [15] The trapped electron neutrinos are released in the form of neutrino burst in the first tens of milliseconds. [3] [29] It is found from simulations that the neutrino burst and iron photo-disintegration weaken the shock wave within milliseconds of propagation through the iron core. [1] The weakening of the shock wave results in mass infall, which forms a neutron star. [lower-alpha 5] This is known as the accretion phase and lasts between few tens to few hundreds of milliseconds. [3] The high-density region traps neutrinos. [15] When the temperature reaches 10 MeV, thermal photons generate electron–positron pairs. Neutrinos and antineutrinos are created through weak-interaction of electron–positron pairs: [19]
The luminosity of electron flavor is significantly higher than for non-electrons. [3] As the neutrino temperature rises in the compressionally heated core, neutrinos energize the shock wave through charged current reactions with free nucleons: [1]
When the thermal pressure created by neutrino heating increases above the pressure of the infalling material, the stalled shock wave is rejuvenated, and neutrinos are released. The neutron star cools down as the neutrino-pair production and neutrino release continues. Therefore, it is known as the cooling phase. [15] The Luminosities of different neutrino and antineutrino species are roughly the same. [3] The supernova neutrino luminosity drops significantly after several tens of seconds. [15]
The knowledge of flux and flavor content of the neutrinos behind the shock wave is essential to implement the neutrino-driven heating mechanism in computer simulations of supernova explosions. [30] Neutrino oscillations in dense matter is an active field of research. [31]
Neutrinos undergo flavor conversions after they thermally decouple from the proto-neutron star. Within the neutrino-bulb model, neutrinos of all flavors decouple at a single sharp surface near the surface of the star. [32] Also, the neutrinos travelling in different directions are assumed to travel the same path length in reaching a certain distance R from the center. This assumption is known as single angle approximation, which along with spherical symmetricity of the supernova, allows us to treat neutrinos emitted in the same flavor as an ensemble and describe their evolution only as a function of distance. [22]
The flavor evolution of neutrinos for each energy mode is described by the density matrix: [22]
Here, is the initial neutrino luminosity at the surface of a proto-neutron star which drops exponentially. Assuming decay time by , the total energy emitted per unit time for a particular flavor can be given by . represents average energy. Therefore, the fraction gives the number of neutrinos emitted per unit of time in that flavor. is normalized energy distribution for the corresponding flavor.
The same formula holds for antineutrinos too. [22]
Neutrino luminosity is found by the following relation: [22]
The integral is multiplied by 6 because the released binding energy is divided equally between the three flavors of neutrinos and three flavors of antineutrinos. [22]
The evolution of the density operator is given by Liouville's equation: [22]
The Hamiltonian covers vacuum oscillations, charged current interaction of neutrinos from electrons and protons, [33] as well as neutrino–neutrino interactions. [34] Neutrino self-interactions are non-linear effects that result in collective flavor conversions. They are significant only when interaction frequency exceeds vacuum oscillation frequency. Typically, they become negligible after a few hundred kilometers from the center. Thereafter, Mikheyev–Smirnov–Wolfenstein resonances with the matter in the stellar envelope can describe the neutrino evolution. [33]
There are several different ways to observe supernova neutrinos. Almost all of them involves the inverse beta decay reaction for the detection of neutrinos. The reaction is a charged current weak interaction, where an electron antineutrino interacts with a proton produces a positron and a neutron: [35]
The positron retains most of the energy of the incoming neutrino. It produces a cone of Cherenkov light, which is detected by photomultiplier tubes (PMT's) arrayed on the walls of the detector. [35] Neutrino oscillations in the Earth matter may affect the supernova neutrino signals detected in experimental facilities. [36]
Water Cherenkov detector | Super-Kamiokande, Hyper-Kamiokande, IceCube, KM3NeT, Baikal |
Scintillator detector | Baksan, LVD, Borexino, KamLAND, JUNO, SNO+, NOνA |
Lead-based detector | HALO |
Liquid noble dark matter detector | ArDM, Xenon |
Liquid argon time projection chamber detector | DUNE |
Other detector | nEXO |
With current detector sensitivities, it is expected that thousands of neutrino events from a galactic core-collapse supernova would be observed. [11] Large-scale detectors such as Hyper-Kamiokande or IceCube can detect up to events. [37] Unfortunately, SN 1987A is the only supernova neutrino event detected so far. [lower-alpha 2] There have not been any galactic supernova in the Milky Way in the last 120 years, [38] despite the expected rate of 0.8-3 per century. [39] Nevertheless, a supernova at 10 kPc distance will enable a detailed study of the neutrino signal, providing unique physics insights. [13] Additionally, the next generation of underground experiments, like Hyper-Kamiokande, are designed to be sensitive to neutrinos from supernova explosions as far as Andromeda or beyond. [12] Further they are speculated to have good supernova pointing capability too. [14]
Since supernova neutrinos originate deep inside the stellar core, they are a relatively reliable messenger of the supernova mechanism. [3] Due to their weakly interacting nature, the neutrino signals from a galactic supernova can give information about the physical conditions at the center of core collapse, which would be otherwise inaccessible. [8] Furthermore, they are the only source of information for core-collapse events which don't result in a supernova or when the supernova is in a dust-obscured region. [14] Future observations of supernova neutrinos will constrain the different theoretical models of core collapse and explosion mechanism, by testing them against the direct empirical information from the supernova core. [8]
Due to their weakly interacting nature, near light speed neutrinos emerge promptly after the collapse. In contrast, there may be a delay of hours or days before the photon signal emerges from the stellar envelope. Therefore, a supernova will be observed in neutrino observatories before the optical signal, even after travelling millions of light years. The coincident detection of neutrino signals from different experiments would provide an early alarm to astronomers to direct telescopes to the right part of the sky to capture the supernova's light. The Supernova Early Warning System is a project which aims to connect neutrino detectors around the world, and trigger the electromagnetic counterpart experiments in case of a sudden influx of neutrinos in the detectors. [14]
The flavor evolution of neutrinos, propagating through the dense and turbulent interior of the supernova, is dominated by the collective behavior associated with neutrino-neutrino interactions. Therefore, supernova neutrinos offer an opportunity to examine neutrino flavor mixing under high-density conditions. [9] Being sensitive to neutrino mass ordering and mass hierarchy, they can provide information about neutrino properties. [40] Further, they can act as a standard candle to measure cosmic distance as the neutronization burst signal does not depend on its progenitor. [41]
The Diffuse Supernova Neutrino Background (DSNB) is a cosmic background of (anti)neutrinos formed by the accumulation of neutrinos emitted from all past core-collapse supernovae. [1] Their existence was predicted even before the observation of supernova neutrinos. [42] DSNB can be used to study physics on the cosmological scale. [43] They provide an independent test of the supernova rate. [8] They can also give information about neutrino emission properties, stellar dynamics and failed progenitors. [44] Super-Kamiokande has put the observational upper limit on the DSNB flux as above 19.3 MeV of neutrino energy. [45] The theoretically estimated flux is only half this value. [46] Therefore, the DSNB signal is expected to be detected in the near future with detectors like JUNO and SuperK-Gd. [8]
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle, transforming into an isobar of that nuclide. For example, beta decay of a neutron transforms it into a proton by the emission of an electron accompanied by an antineutrino; or, conversely a proton is converted into a neutron by the emission of a positron with a neutrino in what is called positron emission. Neither the beta particle nor its associated (anti-)neutrino exist within the nucleus prior to beta decay, but are created in the decay process. By this process, unstable atoms obtain a more stable ratio of protons to neutrons. The probability of a nuclide decaying due to beta and other forms of decay is determined by its nuclear binding energy. The binding energies of all existing nuclides form what is called the nuclear band or valley of stability. For either electron or positron emission to be energetically possible, the energy release or Q value must be positive.
A neutrino is a fermion that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. The rest mass of the neutrino is much smaller than that of the other known elementary particles. The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the electromagnetic interaction or the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.
A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.
Super-Kamiokande is a neutrino observatory located under Mount Ikeno near the city of Hida, Gifu Prefecture, Japan. It is operated by the Institute for Cosmic Ray Research, University of Tokyo with the help of an international team. It is located 1,000 m (3,300 ft) underground in the Mozumi Mine in Hida's Kamioka area. The observatory was designed to detect high-energy neutrinos, to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way Galaxy.
SN 1987A was a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. It occurred approximately 51.4 kiloparsecs from Earth and was the closest observed supernova since Kepler's Supernova in 1604. Light and neutrinos from the explosion reached Earth on February 23, 1987 and was designated "SN 1987A" as the first supernova discovered that year. Its brightness peaked in May of that year, with an apparent magnitude of about 3.
Neutrino astronomy is the branch of astronomy that gathers information about astronomical objects by observing and studying neutrinos emitted by them with the help of neutrino detectors in special Earth observatories. It is an emerging field in astroparticle physics providing insights into the high-energy and non-thermal processes in the universe.
Neutrino oscillation is a quantum mechanical phenomenon in which a neutrino created with a specific lepton family number can later be measured to have a different lepton family number. The probability of measuring a particular flavor for a neutrino varies between three known states, as it propagates through space.
In particle physics, lepton number is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction. Lepton number is an additive quantum number, so its sum is preserved in interactions. The lepton number is defined by where
Neutrinoless double beta decay (0νββ) is a commonly proposed and experimentally pursued theoretical radioactive decay process that would prove a Majorana nature of the neutrino particle. To this day, it has not been found.
The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an electron antineutrino detector at the Kamioka Observatory, an underground neutrino detection facility in Hida, Gifu, Japan. The device is situated in a drift mine shaft in the old KamiokaNDE cavity in the Japanese Alps. Although located in the Kamioka Observatory, which is part of the University of Tokyo, this project is conducted by a team at Tohoku University. The site is surrounded by 53 Japanese commercial nuclear reactors. Nuclear reactors produce electron antineutrinos () during the decay of radioactive fission products in the nuclear fuel. Like the intensity of light from a light bulb or a distant star, the isotropically-emitted flux decreases at 1/R2 per increasing distance R from the reactor. The device is sensitive up to an estimated 25% of antineutrinos from nuclear reactors that exceed the threshold energy of 1.8 megaelectronvolts (MeV) and thus produces a signal in the detector.
The cosmic neutrino background is the universe's background particle radiation composed of neutrinos. They are sometimes known as relic neutrinos.
The Mikheyev–Smirnov–Wolfenstein effect is a particle physics process which modifies neutrino oscillations in matter of varying density. The MSW effect is broadly analogous to the differential retardation of sound waves in density-variable media, however it also involves the propagation dynamics of three separate quantum fields which experience distortion.
In nuclear and particle physics, inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction involving an electron antineutrino scattering off a proton, creating a positron and a neutron. This process is commonly used in the detection of electron antineutrinos in neutrino detectors, such as the first detection of antineutrinos in the Cowan–Reines neutrino experiment, or in neutrino experiments such as KamLAND and Borexino. It is an essential process to experiments involving low-energy neutrinos such as those studying neutrino oscillation, reactor neutrinos, sterile neutrinos, and geoneutrinos.
A Type II supernova or SNII results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion. Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies; those are generally composed of older, low-mass stars, with few of the young, very massive stars necessary to cause a supernova.
Borexino is a deep underground particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter and is protected by 3,800 meters of water-equivalent depth. The scintillator is pseudocumene and PPO which is held in place by a thin nylon sphere. It is placed within a stainless steel sphere which holds the photomultiplier tubes (PMTs) used as signal detectors and is shielded by a water tank to protect it against external radiation. Outward pointing PMT's look for any outward facing light flashes to tag incoming cosmic muons that manage to penetrate the overburden of the mountain above. Neutrino energy can be determined through the number of photoelectrons measured in the PMT's. While the position can be determined by extrapolating the difference in arrival times of photons at PMT's throughout the chamber.
Lorentz-violating neutrino oscillation refers to the quantum phenomenon of neutrino oscillations described in a framework that allows the breakdown of Lorentz invariance. Today, neutrino oscillation or change of one type of neutrino into another is an experimentally verified fact; however, the details of the underlying theory responsible for these processes remain an open issue and an active field of study. The conventional model of neutrino oscillations assumes that neutrinos are massive, which provides a successful description of a wide variety of experiments; however, there are a few oscillation signals that cannot be accommodated within this model, which motivates the study of other descriptions. In a theory with Lorentz violation, neutrinos can oscillate with and without masses and many other novel effects described below appear. The generalization of the theory by incorporating Lorentz violation has shown to provide alternative scenarios to explain all the established experimental data through the construction of global models.
p-nuclei (p stands for proton-rich) are certain proton-rich, naturally occurring isotopes of some elements between selenium and mercury inclusive which cannot be produced in either the s- or the r-process.
Double Chooz was a short-baseline neutrino oscillation experiment in Chooz, France. Its goal was to measure or set a limit on the θ13 mixing angle, a neutrino oscillation parameter responsible for changing electron neutrinos into other neutrinos. The experiment used the Chooz Nuclear Power Plant reactors as a neutrino source and measured the flux of neutrinos from them. To accomplish this, Double Chooz had a set of two detectors situated 400 meters and 1050 meters from the reactors. Double Chooz was a successor to the Chooz experiment; one of its detectors occupies the same site as its predecessor. Until January 2015 all data had been collected using only the far detector. The near detector was completed in September 2014, after construction delays, and started taking data at the beginning of 2015. Both detectors stopped taking data in late December 2017.
In nuclear and particle physics, a geoneutrino is a neutrino or antineutrino emitted during the decay of naturally-occurring radionuclides in the Earth. Neutrinos, the lightest of the known subatomic particles, lack measurable electromagnetic properties and interact only via the weak nuclear force when ignoring gravity. Matter is virtually transparent to neutrinos and consequently they travel, unimpeded, at near light speed through the Earth from their point of emission. Collectively, geoneutrinos carry integrated information about the abundances of their radioactive sources inside the Earth. A major objective of the emerging field of neutrino geophysics involves extracting geologically useful information from geoneutrino measurements. Analysts from the Borexino collaboration have been able to get to 53 events of neutrinos originating from the interior of the Earth.
The diffuse supernova neutrino background(DSNB) is a theoretical population of neutrinos (and anti-neutrinos) cumulatively originating from all core-collapse supernovae events throughout the history of the universe. Though it has not yet been directly detected, the DSNB is theorized to be isotropic and consists of neutrinos with typical energies on the scale of 107 eV. Current detection efforts are limited by the influence of background noise in the search for DSNB neutrinos and are therefore limited to placing limits on the parameters of the DSNB, namely the neutrino flux. Restrictions on these parameters have gotten more strict in recent years, but many researchers are looking to make direct observations in the near future with next generation detectors. The DSNB is not to be confused with the cosmic neutrino background (CNB), which is comprised by relic neutrinos that were produced during the Big Bang and have much lower energies (10−4 to 10−6 eV).
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