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Astroparticle physics, also called particle astrophysics, is a branch of
particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
that studies elementary particles of astronomical origin and their relation to astrophysics and
cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
. It is a relatively new field of research emerging at the intersection of particle physics,
astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, 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.


History

The field of astroparticle physics is evolved out of optical astronomy. With the growth of detector technology came the more mature astrophysics, which involved multiple physics subtopics, such as
mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to object ...
,
electrodynamics In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
,
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
,
plasma physics Plasma ()πλάσμα
, Henry George Liddell, R ...
,
nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies the ...
, relativity, and particle physics. Particle physicists found astrophysics necessary due to difficulty in producing particles with comparable energy to those found in space. For example, the cosmic ray spectrum contains particles with energies as high as 1020  eV, where a proton-proton collision at the Large Hadron Collider occurs at an energy of ~1012 eV. The field can be said to have begun in 1910, when a German physicist named
Theodor Wulf Theodor Wulf (28 July 1868 – 19 June 1946) was a German physicist and Jesuit priest who was one of the first experimenters to detect excess atmospheric radiation. Theodor Wulf became a Jesuit priest at the age of 20, before studying physic ...
measured the
ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
in the air, an indicator of gamma radiation, at the bottom and top of the
Eiffel Tower The Eiffel Tower ( ; french: links=yes, tour Eiffel ) is a wrought-iron lattice tower on the Champ de Mars in Paris, France. It is named after the engineer Gustave Eiffel, whose company designed and built the tower. Locally nicknamed "' ...
. He found that there was far more ionization at the top than what was expected if only terrestrial sources were attributed for this radiation. The Austrian physicist
Victor Francis Hess Victor Franz Hess (; 24 June 188317 December 1964) was an Austrian-American physicist, and Nobel laureate in physics, who discovered cosmic rays. Biography He was born to Vinzenz Hess and Serafine Edle von Grossbauer-Waldstätt, in Waldstein ...
hypothesized that some of the ionization was caused by radiation from the sky. In order to defend this hypothesis, Hess designed instruments capable of operating at high altitudes and performed observations on ionization up to an altitude of 5.3 km. From 1911 to 1913, Hess made ten flights to meticulously measure ionization levels. Through prior calculations, he did not expect there to be any ionization above an altitude of 500 m if terrestrial sources were the sole cause of radiation. His measurements however, revealed that although the ionization levels initially decreased with altitude, they began to sharply rise at some point. At the peaks of his flights, he found that the ionization levels were much greater than at the surface. Hess was then able to conclude that "a radiation of very high penetrating power enters our atmosphere from above." Furthermore, one of Hess's flights was during a near-total eclipse of the Sun. Since he did not observe a dip in ionization levels, Hess reasoned that the source had to be further away in space. For this discovery, Hess was one of the people awarded the
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
in 1936. In 1925,
Robert Millikan Robert Andrews Millikan (March 22, 1868 – December 19, 1953) was an American experimental physicist honored with the Nobel Prize for Physics in 1923 for the measurement of the elementary electric charge and for his work on the photoelectric ...
confirmed Hess's findings and subsequently coined the term '
cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ow ...
'. Many physicists knowledgeable about the origins of the field of astroparticle physics prefer to attribute this 'discovery' of cosmic rays by Hess as the starting point for the field.


Topics of research

While it may be difficult to decide on a standard 'textbook' description of the field of astroparticle physics, the field can be characterized by the topics of research that are actively being pursued. The journal '' Astroparticle Physics'' accepts papers that are focused on new developments in the following areas: * High-energy cosmic-ray physics and astrophysics; * Particle cosmology; * Particle astrophysics; * Related astrophysics: Supernova,
Active Galactic Nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...
, Cosmic Abundances,
Dark Matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
etc.; * High-energy, VHE and UHE gamma-ray astronomy; * High- and low-energy neutrino astronomy; * Instrumentation and detector developments related to the above-mentioned fields.


Open questions

One main task for the future of the field is simply to thoroughly define itself beyond working definitions and clearly differentiate itself from astrophysics and other related topics. Current unsolved problems for the field of astroparticle physics include characterization of
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
and
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...
. Observations of the orbital velocities of stars in the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
and other galaxies starting with
Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959. Biography The son of a teacher, Baade finished school in 1912. He then studied maths, physics and astr ...
and
Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical an ...
in the 1930s, along with observed velocities of galaxies in galactic clusters, found motion far exceeding the energy density of the visible matter needed to account for their dynamics. Since the early nineties some candidates have been found to partially explain some of the missing dark matter, but they are nowhere near sufficient to offer a full explanation. The finding of an accelerating universe suggests that a large part of the missing dark matter is stored as dark energy in a dynamical vacuum. Another question for astroparticle physicists is why is there so much more matter than antimatter in the universe today.
Baryogenesis In physical cosmology, baryogenesis (also known as baryosynthesis) is the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (a ...
is the term for the hypothetical processes that produced the unequal numbers of baryons and antibaryons in the early universe, which is why the universe is made of matter today, and not antimatter.


Experimental facilities

The rapid development of this field has led to the design of new types of infrastructure. In underground laboratories or with specially designed telescopes, antennas and satellite experiments, astroparticle physicists employ new detection methods to observe a wide range of cosmic particles including neutrinos, gamma rays and cosmic rays at the highest energies. They are also searching for
dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
and
gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
. Experimental particle physicists are limited by the technology of their terrestrial accelerators, which are only able to produce a small fraction of the energies found in nature. Facilities, experiments and laboratories involved in astroparticle physics include: *
IceCube The IceCube Neutrino Observatory (or simply IceCube) is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10). Its thousands of sensors are located under ...
(
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
). The longest particle detector in the world, was completed in December 2010. The purpose of the detector is to investigate high energy neutrinos, search for dark matter, observe supernovae explosions, and search for exotic particles such as magnetic monopoles. *
ANTARES (telescope) 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 ...
. (
Toulon Toulon (, , ; oc, label= Provençal, Tolon , , ) is a city on the French Riviera and a large port on the Mediterranean coast, with a major naval base. Located in the Provence-Alpes-Côte d'Azur region, and the Provence province, Toulon is th ...
,
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan area ...
). A Neutrino detector 2.5 km under the Mediterranean Sea off the coast of Toulon, France. Designed to locate and observe neutrino flux in the direction of the southern hemisphere. * XENONnT, the upgrade of
XENON1T Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
, is a dark matter direct search experiment located at the Gran Sasso National Laboratories and will be sensitive to WIMPs with SI cross section of 10−48 cm2. *
BOREXINO Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier t ...
, a real-time detector, installed at Laboratori Nazionali del Gran Sasso, designed to detect neutrinos from the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
with an organic liquid scintillator target. *
Pierre Auger Observatory 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 Ear ...
(
Malargüe Malargüe () is a city in the southwest part of province of Mendoza, Argentina, about 370 km south of the provincial capital Mendoza. It is the head town of the Malargüe Department, and it has about 27,000 inhabitants as per the . Overvie ...
,
Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...
). Detects and investigates high energy cosmic rays using two techniques. One is to study the particles interactions with water placed in surface detector tanks. The other technique is to track the development of air showers through observation of ultraviolet light emitted high in the Earth's atmosphere. *
CERN Axion Solar Telescope The CERN Axion Solar Telescope (CAST) is an experiment in astroparticle physics to search for axions originating from the Sun. The experiment, sited at CERN in Switzerland, was commissioned in 1999 and came online in 2002 with the first da ...
(CERN, Switzerland). Searches for axions originating from the Sun. * NESTOR Project ( Pylos,
Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders ...
). The target of the international collaboration is the deployment of a neutrino telescope on the sea floor off of Pylos, Greece. *
Kamioka Observatory The is a neutrino and gravitational waves laboratory located underground in the Mozumi mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experim ...
is a neutrino and gravitational waves laboratory located underground in the Mozumi Mine near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. * Laboratori Nazionali del Gran Sasso is a laboratory that hosts experiments that require a low noise background environment. Located within the
Gran Sasso Gran Sasso d'Italia (; ) is a massif in the Apennine Mountains of Italy. Its highest peak, Corno Grande (2,912 metres), is the highest mountain in the Apennines, and the second-highest mountain in Italy outside the Alps. The mountain lies wi ...
mountain, near L'Aquila (Italy). Its experimental halls are covered by 1400m of rock, which protects experiments from cosmic rays. *
SNOLAB SNOLAB is a Canadian underground science laboratory specializing in neutrino and dark matter physics. Located 2 km below the surface in Vale's Creighton nickel mine near Sudbury, Ontario, SNOLAB is an expansion of the existing facilities con ...
*
Aspera European Astroparticle network ASPERA (or AStroParticle European Research Area) is a network of national government agencies responsible for coordinating and funding national research efforts in astroparticle physics. Members ASPERA comprises the following agencies: NFWO, ...
Started in July 2006 and is responsible for coordinating and funding national research efforts in Astroparticle Physics. *
Telescope Array Project The Telescope Array project is an international collaboration involving research and educational institutions in Japan, The United States, Russia, South Korea, and Belgium. The experiment is designed to observe air showers induced by ultra-high- ...
( Delta, Utah) An experiment for the detection of ultra high energy cosmic rays (UHECRs) using a ground array and fluorescence techniques in the desert of west Utah.


See also

* '' Astroparticle Physics'' (journal) *
Urca process In astroparticle physics, an Urca process is a reaction which emits a neutrino and which is assumed to take part in cooling processes in neutron stars and white dwarfs. The process was first discussed by George Gamow and Mário Schenberg while th ...
*
Unsolved problems in physics The following is a list of notable unsolved problems grouped into broad areas of physics. Some of the major unsolved problems in physics are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon ...


References

*


External links


Aspera European network portalwww.astroparticle.org: all about astroparticle physics...Aspera newsAstroparticle physics news on TwitterVirtual Institute of Astroparticle PhysicsHelmholtz Alliance for Astroparticle Physics
at
UCLA The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California. UCLA's academic roots were established in 1881 as a teachers college then known as the southern branch of the California ...

Journal of Cosmology and Astroparticle Physics Astroparticle Physics in the Netherlands Astroparticle and High Energy PhysicsASD: Astroparticle Physics Laboratory
at
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

Teaching Astroparticle Physics
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