Neutrino astronomy is the branch of astronomy that observes astronomical objects with neutrino detectors in special observatories. 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. Due to their weak interactions with matter, neutrinos offer a unique opportunity to observe processes that are inaccessible to optical telescopes.
A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only weakly interact with other particles of matter, neutrino detectors must be very large to detect a significant number of neutrinos. Neutrino detectors are often built underground, to isolate the detector from cosmic rays and other background radiation. The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources so far as of 2018 are the Sun and the supernova 1987A in the nearby Large Magellenic Cloud. Another likely source is the blazar TXS 0506+056 about 3.7 billion light years away. Neutrino observatories will "give astronomers fresh eyes with which to study the universe."
Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce one or more nuclides that are different from the nuclide(s) that began the process. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.
Neutrinos were first recorded in 1956 by Clyde Cowan and Frederick Reines in an experiment employing a nearby nuclear reactor as a neutrino source.Their discovery was acknowledged with a Nobel Prize for physics in 1995.
Clyde Lorrain Cowan Jr was an American physicist, the co-discoverer of the neutrino along with Frederick Reines. The discovery was made in 1956 in the neutrino experiment. Frederick Reines received the Nobel Prize in Physics in 1995 in both their names.
Frederick Reines was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist in history "so intimately associated with the discovery of an elementary particle and the subsequent thorough investigation of its fundamental properties".
In 1968, Raymond Davis, Jr. and John N. Bahcall successfully detected the first solar neutrinos in the Homestake experiment.Davis, along with Japanese physicist Masatoshi Koshiba were jointly awarded half of the 2002 Nobel Prize in Physics "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos (the other half went to Riccardo Giacconi for corresponding pioneering contributions which have led to the discovery of cosmic X-ray sources)."
John Norris Bahcall was an American astrophysicist, best known for his contributions to the solar neutrino problem, the development of the Hubble Space Telescope and for his leadership and development of the Institute for Advanced Study in Princeton.
The Homestake experiment was an experiment headed by astrophysicists Raymond Davis, Jr. and John N. Bahcall in the late 1960s. Its purpose was to collect and count neutrinos emitted by nuclear fusion taking place in the Sun. Bahcall did the theoretical calculations and Davis designed the experiment. After Bahcall calculated the rate at which the detector should capture neutrinos, Davis's experiment turned up only one third of this figure. The experiment was the first to successfully detect and count solar neutrinos, and the discrepancy in results created the solar neutrino problem. The experiment operated continuously from 1970 until 1994. The University of Pennsylvania took it over in 1984. The discrepancy between the predicted and measured rates of neutrino detection was later found to be due to neutrino "flavour" oscillations.
Masatoshi Koshiba is a Japanese physicist, known as one of the founders of Neutrino astronomy and jointly won the Nobel Prize in Physics in 2002.
This was followed by the first atmospheric neutrino detection in 1965 by two groups almost simultaneously. One was led by Frederick Reines who operated a liquid scintillator - the Case-Witwatersrand-Irvine or CWI detector - in the East Rand gold mine in South Africa at an 8.8 km water depth equivalent. The other was a Bombay-Osaka-Durham collaboration that operated in the Indian Kolar Gold Field mine at an equivalent water depth of 7.5 km. Although the KGF group detected neutrino candidates two months later than Reines CWI, they were given formal priority due to publishing their findings two weeks earlier.
The first generation of undersea neutrino telescope projects began with the proposal by Moisey Markov in 1960 "...to install detectors deep in a lake or a sea and to determine the location of charged particles with the help of Cherenkov radiation."
Moisey Alexandrovich Markov was a Soviet physicist-theorist who mostly worked in the area of quantum mechanics, nuclear physics and particle physics He is particularly known for having proposed the idea of underwater neutrino telescopes in 1960 that was originally developed in the master thesis of his student Igor Mikhailovich Zheleznykh.
Cherenkov radiation is an electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The characteristic blue glow of an underwater nuclear reactor is due to Cherenkov radiation.
The first underwater neutrino telescope began as the DUMAND project. DUMAND stands for Deep Underwater Muon and Neutrino Detector. The project began in 1976 and although it was eventually cancelled in 1995, it acted as a precursor to many of the following telescopes in the following decades.
The Baikal Neutrino Telescope is installed in the southern part of Lake Baikal in Russia. The detector is located at a depth of 1.1 km and began surveys in 1980. In 1993, it was the first to deploy three strings to reconstruct the muon trajectories as well as the first to record atmospheric neutrinos underwater.
AMANDA (Antarctic Muon And Neutrino Detector Array) used the 3 km thick ice layer at the South Pole and was located several hundred meters from the Amundsen-Scott station. Holes 60 cm in diameter were drilled with pressurized hot water in which strings with optical modules were deployed before the water refroze. The depth proved to be insufficient to be able to reconstruct the trajectory due to the scattering of light on air bubbles. A second group of 4 strings were added in 1995/96 to a depth of about 2000 m that was sufficient for track reconstruction. The AMANDA array was subsequently upgraded until January 2000 when it consisted of 19 strings with a total of 667 optical modules at a depth range between 1500 m and 2000 m. AMANDA would eventually be the predecessor to IceCube in 2005.
After the decline of DUMAND the participating groups split into three branches to explore deep sea options in the Mediterranean Sea. ANTARES was anchored to the sea floor in the region off Toulon at the French Mediterranean coast. It consists of 12 strings, each carrying 25 "storeys" equipped with three optical modules, an electronic container, and calibration devices down to a maximum depth of 2475 m.
NEMO (NEutrino Mediterranean Observatory) was pursued by Italian groups to investigate the feasibility of a cubic-kilometer scale deep-sea detector. A suitable site at a depth of 3.5 km about 100 km off Capo Passero at the South-Eastern coast of Sicily has been identified. From 2007-2011 the first prototyping phase tested a "mini-tower" with 4 bars deployed for several weeks near Catania at a depth of 2 km. The second phase as well as plans to deploy the full-size prototype tower will be pursued in the KM3NeT framework.
The NESTOR Project was installed in 2004 to a depth of 4 km and operated for one month until a failure of the cable to shore forced it to be terminated. The data taken still successfully demonstrated the detector's functionality and provided a measurement of the atmospheric muon flux. The proof of concept will be implemented in the KM3Net framework.
The second generation of deep-sea neutrino telescope projects reach or even exceed the size originally conceived by the DUMAND pioneers. IceCube, located at the South Pole and incorporating its predecessor AMANDA, was completed in December 2010. It currently consists of 5160 digital optical modules installed on 86 strings at depths of 1450 to 2550 m in the Antarctic ice. The KM3NeT in the Mediterranean Sea and the GVD are in their preparatory/prototyping phase. IceCube instruments 1 km3 of ice. GVD is also planned to cover 1 km3 but at a much higher energy threshold. KM3NeT is planned to cover several km3. Both KM3NeT and GVD could be completed by 2017 and it is expected that all three will form a global neutrino observatory.
In July 2018, the IceCube Neutrino Observatory announced that they have traced an extremely-high-energy neutrino that hit their Antarctica-based research station in September 2017 back to its point of origin in the blazar TXS 0506+056 located 3.7 billion light-years away in the direction of the constellation Orion. This is the first time that a neutrino detector has been used to locate an object in space and that a source of cosmic rays has been identified.
Since neutrinos interact only very rarely with matter, the enormous flux of solar neutrinos racing through the Earth is sufficient to produce only 1 interaction for 1036 target atoms, and each interaction produces only a few photons or one transmuted atom. The observation of neutrino interactions requires a large detector mass, along with a sensitive amplification system.
Given the very weak signal, sources of background noise must be reduced as much as possible. The detectors must be shielded by a large shield mass, and so are constructed deep underground, or underwater. They record upward going muons in charged current muon neutrino interactions. Upward because no other known particle can traverse the entire Earth. The detector must be at least 1 km deep to suppress downward traveling muons, and are subject to an irreducible background of extraterrestric neutrinos interacting in the Earth's atmosphere. This background also provides a standard calibration source. Sources of radioactive isotopes must also be controlled as they produce energetic particles when they decay. The detectors consist of an array of photomultiplier tubes (PMTs) housed in transparent pressure spheres which are suspended in a large volume of water or ice. The PMTs record the arrival time and amplitude of the Cherenkov light emitted by muons or particle cascades. The trajectory can then usually be reconstructed by triangulation if at least three "strings" are used to detect the events.
When astronomical bodies, such as the Sun, are studied using light, only the surface of the object can be directly observed. Any light produced in the core of a star will interact with gas particles in the outer layers of the star, taking hundreds of thousands of years to make it to the surface, making it impossible to observe the core directly. Since neutrinos are also created in the cores of stars (as a result of stellar fusion), the core can be observed using neutrino astronomy.Other sources of neutrinos- such as neutrinos released by supernovae- have been detected. There are currently goals to detect neutrinos from other sources, such as Active Galactic Nuclei (AGN), as well as Gamma-ray bursts and Starburst galaxies. Neutrino astronomy may also indirectly detect dark matter.
The muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 1/2, but with a much greater mass. It is classified as a lepton. As is the case with other leptons, the muon is not believed to have any sub-structure—that is, it is not thought to be composed of any simpler particles.
Cosmic rays are high-energy radiation, mainly originating outside the Solar System and even from distant galaxies. Upon impact with the Earth's atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are originated either from the sun or from outside of our solar system. Data from the Fermi Space Telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Active galactic nuclei also appear to produce cosmic rays, based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018.
The Cowan–Reines neutrino experiment was conducted by Clyde L. Cowan and Frederick Reines in 1956. The experiment confirmed the existence of neutrinos. Neutrinos, subatomic particles with no electric charge and very small mass, had been conjectured to be an essential particle in beta decay processes in the 1930s. With neither mass nor charge, such particles appeared to be impossible to detect. The experiment exploited a huge flux of (hypothetical) electron antineutrinos emanating from a nearby nuclear reactor and a detector consisting of large tanks of water. Neutrino interactions with the protons of the water were observed, verifying the existence and basic properties of this particle for the first time.
The Antarctic Muon And Neutrino Detector Array (AMANDA) is a neutrino telescope located beneath the Amundsen–Scott South Pole Station. In 2005, after nine years of operation, AMANDA officially became part of its successor project, the IceCube Neutrino Observatory.
The DUMAND Project was a proposed underwater neutrino telescope to be built in the Pacific Ocean, off the shore of the island of Hawaii, five kilometers beneath the surface. It would have included thousands of strings of instruments occupying a cubic kilometer of the ocean.
An air shower is an extensive cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray enters the atmosphere. When a particle, which could be a proton, a nucleus, an electron, a photon, or (rarely) a positron, strikes an atom's nucleus in the air it produces many energetic hadrons. The unstable hadrons decay in the air speedily into other particles and electromagnetic radiation, which are part of the shower components. The secondary radiation rains down, including x-rays, muons, protons, antiprotons, alpha particles, pions, electrons, positrons, and neutrons.
The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 1018 eV. In Earth's atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an "air shower") can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km2 per century, the Auger Observatory has created a detection area of 3,000 km2 (1,200 sq mi)—the size of Rhode Island, or Luxembourg—in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes.
The IceCube Neutrino Observatory is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. Its thousands of sensors are located under the Antarctic ice, distributed over a cubic kilometre.
The NESTOR Project is an international scientific collaboration whose target is the deployment of a neutrino telescope on the sea floor off Pylos, Greece.
ANTARES is the name of a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon, France. It is designed to be used as a directional neutrino telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the Earth, a complement to the South Pole neutrino detector IceCube that detects neutrinos from both hemispheres. The name comes from Astronomy with a Neutrino Telescope and Abyss environmental RESearch project; the acronym is also the name of the prominent star Antares. Other neutrino telescopes designed for use in the nearby area include the Greek NESTOR telescope and the Italian NEMO telescope, which are both in early design stages.
Astroparticle physics, also called particle astrophysics, is a branch of particle physics that studies elementary particles of astronomical origin and their relation to astrophysics and cosmology. It is a relatively new field of research emerging at the intersection of particle physics, astronomy, astrophysics, detector physics, relativity, solid state physics, and cosmology. Partly motivated by the discovery of neutrino oscillation, the field has undergone rapid development, both theoretically and experimentally, since the early 2000s.
The Cubic Kilometre Neutrino Telescope, or KM3NeT, is a future European research infrastructure that will be located at the bottom of the Mediterranean Sea. It will host the next-generation neutrino telescope in the form of a water Cherenkov detector with an instrumented volume of about five cubic kilometres distributed over three locations in the Mediterranean: KM3NeT-Fr, KM3NeT-It and KM3NeT-Gr. The KM3NeT project continues work done under the ANTARES, NEMO and NESTOR neutrino telescope projects.
The Baikal Deep Underwater Neutrino Telescope (BDUNT) is a neutrino detector conducting research below the surface of Lake Baikal (Russia) since 2003. The first detector was started in 1990 and completed in 1998. It was upgraded in 2005 and again starting in 2015 to build the Baikal Gigaton Volume Detector (Baikal-GVD.) BDUNT has studied neutrinos coming through the earth with results on atmospheric muon flux. BDUNT picks up a lot of atmospheric neutrinos created by cosmic rays interacting with the atmosphere — as opposed to cosmic neutrinos which can give clues to cosmic events and are therefore of greater interest to physicists.
The solar neutrino problem concerned a large discrepancy between the flux of solar neutrinos as predicted from the Sun's luminosity and measured directly. The discrepancy was first observed in the mid-1960s and finally resolved around 2002.
Multi-messenger astronomy is astronomy based on the coordinated observation and interpretation of disparate "messenger" signals. Interplanetary probes can visit objects within the Solar System, but beyond that, information must rely on "extrasolar messengers". The four extrasolar messengers are electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays. They are created by different astrophysical processes, and thus reveal different information about their sources.
Herbert Hwa-sen Chen was a theoretical and experimental physicist at the University of California at Irvine known for his contributions in the field of neutrino detection. Chen's work on observations of elastic neutrino-electron scattering provided important experimental support for the electroweak theory of the standard model of particle physics. In 1984 Chen realized that the deuterium of heavy water could be used as a detector that would distinguish the flavors of solar neutrinos. This idea led Chen to develop plans for the Sudbury Neutrino Observatory that would eventually make fundamental measurements demonstrating that neutrinos were particles with mass.
TXS 0506+056 is a very high energy blazar – a quasar with a relativistic jet pointing directly towards Earth – of BL Lac-type. With a redshift of 0.3365 ± 0.0010, it is about 1.75 gigaparsecs from Earth. Its approximate location on the sky is off the left shoulder of the constellation Orion. Discovered as a radio source in 1983, the blazar has since been observed across the entire electromagnetic spectrum.