title: Planet Nine

text: Other experts have varying degrees of skepticism. American astrophysicist Ethan Siegel, who previously speculated that planets may have been ejected from the Solar System during an early dynamical instability, is skeptical of the existence of an undiscovered planet in the Solar System. In a 2018 article discussing a survey that did not find evidence of clustering of the ETNOs' orbits he suggests the previously observed clustering could have been the result of observational bias and claims most scientists think Planet Nine does not exist. Planetary scientist Hal Levison thinks that the chance of an ejected object ending up in the inner Oort cloud is only about 2%, and speculates that many objects must have been thrown past the Oort cloud if one has entered a stable orbit.


title: Planet Nine

text: Further skepticism about the Planet Nine hypothesis arose in 2020, based on results from the Outer Solar System Origins Survey and the Dark Energy Survey, with the OSSOS documenting over 800 trans-Neptunian objects and the DES discovering 316 new ones. Both surveys adjusted for observational bias and concluded that of the objects observed there was no evidence for clustering. The authors go further to explain that practically all objects' orbits can be explained by physical phenomena rather than a ninth planet as proposed by Brown and Batygin. An author of one of the studies, Samantha Lawler, said the hypothesis of Planet Nine proposed by Brown and Batygin "does not hold up to detailed observations" pointing out the much larger sample size of 800 objects compared to the much smaller 14 and that conclusive studies based on said objects were "premature". She went further to explain the phenomenon of these extreme orbits could be due to gravitational occultation from Neptune when it migrated outwards earlier in the Solar System's history.


title: Planet Nine

text: The results of the Outer Solar System Survey (OSSOS) suggest that the observed clustering is the result of a combination of observational bias and small number statistics. OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight objects with semi-major axis > 150 AU with orbits oriented in a wide range of directions. After accounting for the observational biases of the survey, no evidence for the arguments of perihelion (ω) clustering identified by Trujillo and Sheppard was seen, and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with being random. Pedro Bernardinelli and his colleagues also found that the orbital elements of the ETNOs found by the Dark Energy Survey showed no evidence of clustering. However, they also noted that the sky coverage and number of objects found were insufficient to show that there was no Planet Nine. A similar result was found when these two surveys were combined with a survey by Trujillo and Sheppard. These result differed from an analysis of discovery biases in the previously observed ETNOs by Mike Brown. He found that after observation biases were accounted for, the clustering of longitudes of perihelion of 10 known ETNOs would be observed only 1.2% of the time if their actual distribution was uniform. When combined with the odds of the observed clustering of the arguments of perihelion, the probability was 0.025%. A later analysis of the discovery biases of 14 ETNOs by Brown and Batygin determined the probability of the observed clustering of the longitudes of perihelion and the orbital pole locations to be 0.2%.


title: Planet Nine

text: Simulations of 15 known objects evolving under the influence of Planet Nine also revealed differences from observations. Cory Shankman and his colleagues included Planet Nine in a simulation of many clones (objects with similar orbits) of 15 objects with semi-major axis > 150 AU and perihelion > 30 AU. While they observed alignment of the orbits opposite that of Planet Nine's for the objects with semi-major axis greater than 250 AU, clustering of the arguments of perihelion was not seen. Their simulations also showed that the perihelia of the ETNOs rose and fell smoothly, leaving many with perihelion distances between 50 AU and 70 AU where none had been observed, and predicted that there would be many other unobserved objects. These included a large reservoir of high-inclination objects that would have been missed due to most observations being at small inclinations, and a large population of objects with perihelia so distant that they would be too faint to observe. Many of the objects were also ejected from the Solar System after encountering the other giant planets. The large unobserved populations and the loss of many objects led Shankman et al. to estimate that the mass of the original population was tens of Earth masses, requiring that a much larger mass had been ejected during the early Solar System. Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing ETNOs is a temporary phenomenon that will disappear as more objects are detected.


title: Planet Nine

text: Ann-Marie Madigan and Michael McCourt postulate that an inclination instability in a distant massive belt is responsible for the alignment of the arguments of perihelion of the ETNOs. An inclination instability could occur in a disk of particles with high eccentricity orbits ("e" > 0.6) around a central body, such as the Sun. The self-gravity of this disk would cause its spontaneous organization, increasing the inclinations of the objects and aligning the arguments of perihelion, forming it into a cone above or below the original plane. This process would require an extended time and significant mass of the disk, on the order of a billion years for a 1–10 Earth-mass disk. Mike Brown considers Planet Nine a more probable explanation, noting that current surveys have not revealed a large enough scattered-disk to produce an "inclination instability". In Nice model simulations of the Solar System that included the self-gravity of the planetesimal disk an inclination instability did not occur. Instead, the simulation produced a rapid precession of the objects' orbits and most of the objects were ejected on too short of a timescale for an inclination instability to occur. In 2020 Madigan and colleagues showed that the inclination instability would require 20 Earth masses in a disk of objects with semi-major axes of a few hundred AU. An inclination instability in this disk could also reproduce the observed gap in the perihelion distances of the extreme TNOs, and the observed apsidal alignment following the inclination instability given sufficient time.


title: Planet Nine

text: Antranik Sefilian and Jihad Touma propose that a massive disk of moderately eccentric TNOs is responsible for the clustering of the longitudes of perihelion of the ETNOs. This disk would contain 10 Earth-mass of TNOs with aligned orbits and eccentricities that increased with their semi-major axes ranging from zero to 0.165. The gravitational effects of the disk would offset the forward precession driven by the giant planets so that the orbital orientations of its individual objects are maintained. The orbits of objects with high eccentricities, such as the observed ETNOs, would be stable and have roughly fixed orientations, or longitudes of perihelion, if their orbits were anti-aligned with this disk. Although Brown thinks the proposed disk could explain the observed clustering of the ETNOs, he finds it implausible that the disk could survive over the age of the Solar System. Batygin thinks that there is insufficient mass in the Kuiper belt to explain the formation of the disk and asks "why would the protoplanetary disk end near 30 AU and restart beyond 100 AU?"


title: Planet Nine

text: The Planet Nine hypothesis includes a set of predictions about the mass and orbit of the planet. An alternative hypothesis predicts a planet with different orbital parameters. Renu Malhotra, Kathryn Volk, and Xianyu Wang have proposed that the four detached objects with the longest orbital periods, those with perihelia beyond and semi-major axes greater than , are in "n":1 or "n":2 mean-motion resonances with a hypothetical planet. Two other objects with semi-major axes greater than are also potentially in resonance with this planet. Their proposed planet could be on a lower eccentricity, low inclination orbit, with eccentricity "e" < 0.18 and inclination "i" ≈ 11°. The eccentricity is limited in this case by the requirement that close approaches of to the planet be avoided. If the ETNOs are in periodic orbits of the third kind, with their stability enhanced by the libration of their arguments of perihelion, the planet could be in a higher inclination orbit, with "i" ≈ 48°. Unlike Batygin and Brown, Malhotra, Volk and Wang do not specify that most of the distant detached objects would have orbits anti-aligned with the massive planet.


title: Planet Nine

text: Trujillo and Sheppard argued in 2014 that a massive planet in a circular orbit with an average distance between and was responsible for the clustering of the arguments of perihelion of twelve TNOs with large semi-major axes. Trujillo and Sheppard identified a clustering near zero degrees of the arguments of perihelion of the orbits of twelve TNOs with perihelia greater than and semi-major axes greater than . After numerical simulations showed that the arguments of perihelion should circulate at varying rates, leaving them randomized after billions of years, they suggested that a massive planet in a circular orbit at a few hundred astronomical units was responsible for this clustering. This massive planet would cause the arguments of perihelion of the TNOs to librate about 0° or 180° via the Kozai mechanism so that their orbits crossed the plane of the planet's orbit near perihelion and aphelion, the closest and farthest points from the planet. In numerical simulations including a 2–15 Earth mass body in a circular low-inclination orbit between and the arguments of perihelia of Sedna and librated around 0° for billions of years (although the lower perihelion objects did not) and underwent periods of libration with a Neptune mass object in a high inclination orbit at 1,500 AU. Another process such as a passing star would be required to account for the absence of objects with arguments of perihelion near 180°.


title: Planet Nine

text: These simulations showed the basic idea of how a single large planet can shepherd the smaller TNOs into similar types of orbits. They were basic proof of concept simulations that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have. Thus they did not fully formulate a model that successfully incorporated all the clustering of the ETNOs with an orbit for the planet. But they were the first to notice there was a clustering in the orbits of TNOs and that the most likely reason was from an unknown massive distant planet. Their work is very similar to how Alexis Bouvard noticed Uranus' motion was peculiar and suggested that it was likely gravitational forces from an unknown 8th planet, which led to the discovery of Neptune.


title: Planet Nine

text: Raúl and Carlos de la Fuente Marcos proposed a similar model but with two distant planets in resonance. An analysis by Carlos and Raúl de la Fuente Marcos with Sverre J. Aarseth confirmed that the observed alignment of the arguments of perihelion could not be due to observational bias. They speculated that instead it was caused by an object with a mass between that of Mars and Saturn that orbited at some from the Sun. Like Trujillo and Sheppard they theorized that the TNOs are kept bunched together by a Kozai mechanism and compared their behavior to that of Comet 96P/Machholz under the influence of Jupiter. They also struggled to explain the orbital alignment using a model with only one unknown planet, and therefore suggested that this planet is itself in resonance with a more-massive world about from the Sun. In their article, Brown and Batygin noted that alignment of arguments of perihelion near 0° or 180° via the Kozai mechanism requires a ratio of the semi-major axes nearly equal to one, indicating that multiple planets with orbits tuned to the data set would be required, making this explanation too unwieldy.


title: Planet Nine

text: In 2019, Jakub Scholtz and James Unwin proposed that a primordial black hole was responsible for the clustering of the orbits of the ETNOs. Their analysis of OGLE gravitational lensing data revealed a population of planetary mass objects in the direction of the galactic bulge more numerous than the local population of stars. They propose that instead of being free floating planets, these objects are primordial black holes. Since their estimate of the size of this population is greater than the estimated population of free floating planets from planetary formation models they argue that the capture of a hypothetical primordial black hole would be more probable than the capture of a free floating planet. This could also explain why an object responsible for perturbing the orbits of the ETNOs, if it exists, has yet to be seen. A detection method was proposed in the paper, stating that the black hole is too cold to be detected over the CMB, but interaction with surrounding dark matter would produce gamma rays detectable by the FERMILAT. Konstantin Batygin commented on this, saying while it is possible for Planet Nine to be a primordial black hole, there is currently not enough evidence to make this idea more plausible than any other alternative. Edward Witten proposed a fleet of probes accelerated by radiation pressure that could discover a Planet Nine primordial black hole's location, however Thiem Hoang and Avi Loeb showed that any signal would be dominated by noise from the interstellar medium. Amir Siraj and Avi Loeb proposed a method for the Vera C. Rubin Observatory to detect flares from any low-mass black hole in the outer Solar System, including a possible Planet Nine primordial black hole.


title: Planet Nine

text: Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings. It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto. If Planet Nine exists and is close to perihelion, astronomers could identify it based on existing images. At aphelion, the largest telescopes would be required, but if the planet is currently located in between, many observatories could spot Planet Nine. Statistically, the planet is more likely to be close to its aphelion at a distance greater than 600 AU. This is because objects move more slowly when near their aphelion, in accordance with Kepler's second law. A 2019 study estimated that Planet Nine, if it exists, may be smaller and closer than originally thought. This would make the hypothetical planet brighter and easier to spot, with an apparent magnitude of 21–22. According to University of Michigan professor Fred Adams, by 2035, Planet Nine will either be observable or enough data will have been gathered to rule out its existence.


title: Planet Nine

text: The search of databases of stellar objects by Batygin and Brown has already excluded much of the sky along Planet Nine's predicted orbit. The remaining regions include the direction of its aphelion, where it would be too faint to be spotted by these surveys, and near the plane of the Milky Way, where it would be difficult to distinguish from the numerous stars. This search included the archival data from the Catalina Sky Survey to magnitude 21–22, Pan-STARRS to magnitude 21.5, and infrared data from the Wide-field Infrared Survey Explorer (WISE) satellite. In 2021, they also searched the first three years of data from the Zwicky Transient Facility (ZTF) without identifying Planet Nine. The search of the ZTF data alone has ruled out 56% of the parameter space for possible Planet Nine positions. As a result of ruling out mostly objects with small semi-major axes, the expected orbit of Planet Nine was pushed slightly further away.


title: Planet Nine

text: Other researchers have been conducting searches of existing data. David Gerdes, who helped develop the camera used in the Dark Energy Survey, claims that software designed to identify distant Solar System objects such as could find Planet Nine if it was imaged as part of that survey, which covered a quarter of the southern sky. Michael Medford and Danny Goldstein, graduate students at the University of California, Berkeley, are also examining archived data using a technique that combines images taken at different times. Using a supercomputer they will offset the images to account for the calculated motion of Planet Nine, allowing many faint images of a faint moving object to be combined to produce a brighter image. A search combining multiple images collected by WISE and NEOWISE data has also been conducted without detecting Planet Nine. This search covered regions of the sky away from the galactic plane at the "W1" wavelength (the 3.4 μm wavelength used by WISE) and is estimated to be able to detect a 10-Earth mass object out to 800–900 AU.


title: Planet Nine

text: Malena Rice and Gregory Laughlin applied a targeted shift-stacking search algorithm to analyze data from TESS sectors 18 and 19 looking for Planet Nine and candidate outer Solar System objects. Their search generated no serious evidence for the presence of a distant planet, but it produced 17 new outer Solar System body candidates located at geocentric distances in the range 80–200 AU, that need follow-up observations with ground-based telescope resources for confirmation. Early results from a survey with WHT aimed at recovering these distant TNO candidates have failed to confirm two of them.


title: Planet Nine

text: By 2022, a comparison between IRAS and AKARI data yielded no Planet Nine detection. It was noted that far-infrared data in the major portion of the sky are heavily contaminated by the emission from the galactic nebulae, making detection of Planet Nine thermal emission problematic close to the galactic plane or bulge.


title: Planet Nine

text: Because the planet is predicted to be visible in the Northern Hemisphere, the primary search is expected to be carried out using the Subaru Telescope, which has both an aperture large enough to see faint objects and a wide field of view to shorten the search. Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams expect the search to take up to five years. Brown and Batygin initially narrowed the search for Planet Nine down to roughly 2,000 square degrees of sky near Orion, a swath of space that Batygin thinks could be covered in about 20 nights by the Subaru Telescope. Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky. In December 2018, they spent four half–nights and three full nights observing with the Subaru Telescope. Due to the elusiveness of the hypothetical planet, it has been proposed that different detection methods be used when looking for a super-Earth mass planet ranging from using differing telescopes to using multiple spacecraft. In late April and early May 2020, Scott Lawrence and Zeeve Rogoszinski proposed the latter method for finding it as multiple spacecraft would have advantages that land-based telescopes do not have.


title: Planet Nine

text: Although a distant planet such as Planet Nine would reflect little light, due to its large mass it would still be radiating the heat from its formation as it cools. At its estimated temperature of the peak of its emissions would be at infrared wavelengths. This radiation signature could be detected by Earth-based submillimeter telescopes, such as ALMA, and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths. A search of part of the sky using archived data of the Atacama Cosmology Telescope has not detected Planet Nine. Jim Green of NASA's Science Mission Directorate is optimistic that it could be observed by the James Webb Space Telescope, the successor to the Hubble Space Telescope.


title: Planet Nine

text: The Zooniverse project, originally started in February 2017, is using archival data from the WISE spacecraft to search for Planet Nine. The project will also search for substellar objects like brown dwarfs in the neighborhood of the Solar System. 32,000 animations of four images each, which constitute 3% of the WISE spacecraft's data, have been uploaded to the Backyard Worlds website.


title: Planet Nine

text: In April 2017, using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March 2017, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.


title: Planet Nine

text: The Zooniverse Catalina Outer Solar System Survey project, started in August 2020, is using archived data from the Catalina Sky Survey to search for TNOs. By looking for moving objects in animations, and depending on the size, distance and magnitude, citizen scientists might be able to find Planet Nine.


title: Planet Nine

text: Precise observations of Saturn's orbit using data from "Cassini" suggest that Planet Nine could not be in certain sections of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position. This data neither proves nor disproves that Planet Nine exists.


title: Planet Nine

text: An initial analysis by Fienga, Laskar, Manche, and Gastineau using Cassini data to search for Saturn's orbital residuals, small differences with its predicted orbit due to the Sun and the known planets, was inconsistent with Planet Nine being located with a true anomaly, the location along its orbit relative to perihelion, of −130° to −110° or −65° to 85°. The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of . At this location, Planet Nine would be approximately from the Sun, with right ascension close to 2 and declination close to −20°, in Cetus. In contrast, if the putative planet is near aphelion it would be located near right ascension 3.0 to 5.5 and declination −1° to 6°.


title: Planet Nine

text: A later analysis of "Cassini" data by astrophysicists Matthew Holman and Matthew Payne tightened the constraints on possible locations of Planet Nine. Holman and Payne developed a more efficient model that allowed them to explore a broader range of parameters than the previous analysis. The parameters identified using this technique to analyze the Cassini data was then intersected with Batygin and Brown's dynamical constraints on Planet Nine's orbit. Holman and Payne concluded that Planet Nine is most likely to be located within 20° of RA = 40°, Dec = −15°, in an area of the sky near the constellation Cetus.


title: Planet Nine

text: William Folkner, a planetary scientist at the Jet Propulsion Laboratory (JPL), has stated that the "Cassini" spacecraft was not experiencing unexplained deviations in its orbit around Saturn. An undiscovered planet would affect the orbit of Saturn, not "Cassini". This could produce a signature in the measurements of "Cassini", but JPL has seen no unexplained signatures in "Cassini" data.


title: Planet Nine

text: An analysis in 2016 of Pluto's orbit by Holman and Payne found perturbations much larger than predicted by Batygin and Brown's proposed orbit for Planet Nine. Holman and Payne suggested three possible explanations: systematic errors in the measurements of Pluto's orbit; an unmodeled mass in the Solar System, such as a small planet in the range of 60– (potentially explaining the Kuiper cliff); or a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.


title: Planet Nine

text: An analysis of the orbits of comets with nearly parabolic orbits identifies five new comets with hyperbolic orbits that approach the nominal orbit of Planet Nine described in Batygin and Brown's initial article. If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83–90° and a declination of 8–10°. Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.


title: Planet Nine

text: Malena Rice and Gregory Laughlin have proposed that a network of telescopes be built to detect occultations by Jupiter trojans. The timing of these occultations would provide precise astrometry of these objects enabling their orbits to be monitored for variations due to the tide from Planet Nine.


title: Planet Nine

text: In May 2022, it was suggested that the peculiar meteor CNEOS 2014-01-08 may have entered Earth-crossing orbit after a swing-by of Planet Nine. If that hypothesis is true, the trajectory back-tracing of CNEOS 2014-01-08 means Planet Nine may be currently located in the constellation of Aries, at right ascension 50°, and declination 11.8°.


title: Planet Nine

text: An analysis by Sarah Millholland and Gregory Laughlin identified a pattern of commensurabilities (ratios between orbital periods of pairs of objects consistent with both being in resonance with another object) of the ETNOs. They identify five objects that would be near resonances with Planet Nine if it had a semi-major axis of 654 AU: Sedna (3:2), 474640 Alicanto (3:1), (4:1), (5:1), and (5:1). They identify this planet as Planet Nine but propose a different orbit with an eccentricity "e" ≈ 0.5, inclination "i" ≈ 30°, argument of perihelion ω ≈ 150°, and longitude of ascending node Ω ≈ 50° (the last differs from Brown and Batygin's value of 90°).


title: Planet Nine

text: Carlos and Raúl de la Fuente Marcos also note commensurabilities among the known ETNOs similar to that of the Kuiper belt, where accidental commensurabilities occur due to objects in resonances with Neptune. They find that some of these objects would be in 5:3 and 3:1 resonances with a planet that had a semi-major axis of ≈700 AU.


title: Planet Nine

text: Three objects with smaller semi-major axes near 172 AU (, and ) have also been proposed to be in resonance with Planet Nine. These objects would be in resonance and anti-aligned with Planet Nine if it had a semi-major axis of 315 AU, below the range proposed by Batygin and Brown. Alternatively, they could be in resonance with Planet Nine, but have orbital orientations that circulate instead of being confined by Planet Nine if it had a semi-major axis of 505 AU.


title: Planet Nine

text: A later analysis by Elizabeth Bailey, Michael Brown and Konstantin Batygin found that if Planet Nine is in an eccentric and inclined orbit the capture of many of the ETNOs in higher-order resonances and their chaotic transfer between resonances prevent the identification of Planet Nine's semi-major axis using current observations. They also determined that the odds of the first six objects observed being in N/1 or N/2 period ratios with Planet Nine are less than 5% if it has an eccentric orbit.


title: Planet Nine

text: In late 2020 it was determined HD 106906 b, a candidate exoplanet, had an eccentric orbit that took it outside the debris disk of its binary host stars. Its orbit appears to be similar to the predictions made for Planet Nine's semi-major axis and it may serve as a proxy for Planet Nine that helps explain how such planetary orbits evolve, although this exoplanet is well over ten times as massive as Jupiter.


title: Planet Nine

text: Planet Nine does not have an official name and will not receive one unless its existence is confirmed via imaging. Only two planets, Uranus and Neptune, have been discovered in the Solar System during recorded history. However, many minor planets, including dwarf planets such as Pluto, asteroids, and comets have been discovered and named. Consequently, there is a well-established process for naming newly discovered Solar System objects. If Planet Nine is observed, the International Astronomical Union will certify a name, with priority usually given to a name proposed by its discoverers. It is likely to be a name chosen from Roman or Greek mythology.


title: Planet Nine

text: In their original article, Batygin and Brown simply referred to the object as "perturber", and only in later press releases did they use "Planet Nine". They have also used the names "Jehoshaphat" and "George" (a reference to William Herschel's proposed name for Uranus) for Planet Nine. Brown has stated: "We actually call it Phattie when we're just talking to each other." In 2018, Batygin has also informally suggested, based on a petition on Change.org, to name the planet after singer David Bowie, and to name any potential moons of the planet after characters from Bowie's song catalogue, such as Ziggy Stardust or Starman.


title: Planet Nine

text: Jokes have been made connecting "Planet Nine" to Ed Wood's 1959 science-fiction horror film "Plan 9 from Outer Space". In connection with the Planet Nine hypothesis, the film title recently found its way into academic discourse. In 2016, an article titled "Planet Nine from Outer Space" about the hypothesized planet in the outer region of the Solar System was published in "Scientific American". Several conference talks since then have used the same word play, as did a lecture by Mike Brown given in 2019.


title: Planet Nine

text: Persephone, the wife of the deity Pluto, had been a popular name commonly used in science fiction for a planet beyond Neptune (see Fictional planets of the Solar System). However, it is unlikely that Planet Nine or any other conjectured planet beyond Neptune will be given the name Persephone once its existence is confirmed, as it is already the name for asteroid 399 Persephone.


title: Planet Nine

text: In 2018, planetary scientist Alan Stern objected to the name "Planet Nine", saying, "It is an effort to erase Clyde Tombaugh's legacy and it's frankly insulting", suggesting the name Planet X until its discovery. He signed a statement with 34 other scientists saying, "We further believe the use of this term [Planet Nine] should be discontinued in favor of culturally and taxonomically neutral terms for such planets, such as Planet X, Planet Next, or Giant Planet Five." According to Brown, Planet X' is not a generic reference to some unknown planet, but a specific prediction of Lowell's which led to the (accidental) discovery of Pluto. Our prediction is not related to this prediction."


title: Cabaret (1972 film)

text: Cabaret is a 1972 American musical drama film directed by Bob Fosse, and starring Liza Minnelli, Michael York and Joel Grey.


title: Cabaret (1972 film)

text: Set in Berlin during the Weimar Republic in 1931, under the presence of the growing Nazi Party, the film is loosely based on the 1966 Broadway musical "Cabaret" by Kander and Ebb, which was adapted from Christopher Isherwood's semi-autobiographical novel "The Berlin Stories" (1945) as well as John Van Druten's 1951 play "I Am a Camera", which was itself adapted from Isherwood's novel. Multiple numbers from the stage score were used for the film, which also featured three other songs by Kander and Ebb, including two written for the adaptation. In the traditional manner of musical theater, most major characters in the stage version sing to express their emotions and advance the plot; in the film, however, the musical numbers are entirely diegetic. All of them take place inside the club, with one exception: "Tomorrow Belongs to Me", the only song sung by neither the Master of Ceremonies nor Sally Bowles.


title: Cabaret (1972 film)

text: After the box-office failure of his 1969 film version of "Sweet Charity", Fosse bounced back with "Cabaret" in 1972, a year that made him one of the most honored working directors in Hollywood. The film also brought Minnelli, daughter of Judy Garland and Vincente Minnelli, her own first chance to sing on screen, and she won the Academy Award for Best Actress. With Academy Awards for Best Supporting Actor (Grey), Best Director (Fosse), Best Cinematography, Best Art Direction, Best Sound, Best Original Song Score and Adaptation, and Best Film Editing, "Cabaret" holds the record for most Oscars earned by a film not honored for Best Picture. It is listed as number 367 on "Empire"s 500 greatest films of all time. "Cabaret" opened to glowing reviews and strong box office, eventually taking in more than $40 million. In addition to its eight Oscars, it won Best Picture citations from the National Board of Review and the Hollywood Foreign Press Association, and took Best Supporting Actor honors for Grey from the National Board of Review, the Hollywood Foreign Press, and the National Society of Film Critics. In 1995, "Cabaret" was the twelfth live-action musical film selected by the Library of Congress for preservation in the United States National Film Registry as being deemed "culturally, historically, or aesthetically significant".


title: Cabaret (1972 film)

text: In 1931 Berlin, a young, openly promiscuous American Sally Bowles performs at the Kit Kat Klub. A new British arrival in the city, Brian Roberts, moves into the boarding house where Sally lives. A reserved academic and writer, Brian must give English lessons to earn a living while completing his doctorate. Sally tries to seduce Brian, but he tells her that on three previous occasions he has tried to have sexual relationships with women, all of which failed. They become friends, and Brian witnesses Sally's bohemian life in the last days of the Weimar Republic. When Brian consoles Sally after her father cancels his meeting with her, they become lovers, concluding that his previous failures with women were because they were "the wrong three girls".


title: Cabaret (1972 film)

text: Maximilian von Heune, a rich, married playboy and baron, befriends Sally and takes her and Brian to his country estate where they are both spoiled and courted. After a somewhat enigmatic experience with Brian, Max drops his pursuit of the pair in haste. During an argument, Sally tells Brian that she has been having sex with Max, and Brian reveals that he has as well. Brian and Sally later reconcile, and Sally reveals that Max left them 300 marks and mockingly compares the sum with what a professional prostitute earns.


title: Cabaret (1972 film)

text: Sally learns that she is pregnant but is unsure of the father. Brian offers to marry her and take her back to his university life in Cambridge. At first, they celebrate their resolution to start this new life together, but after a picnic between Sally and Brian, in which Brian acts distant and uninterested, Sally becomes disheartened by the vision of herself as a bored faculty wife washing dirty diapers. Ultimately, she has an abortion, without informing Brian in advance. When he confronts her, she shares her fears, and the two reach an understanding. Brian departs for England, and Sally continues her life in Berlin, embedding herself in the Kit Kat Klub.


title: Cabaret (1972 film)

text: A subplot concerns Fritz Wendel, a German Jew passing as a Protestant, who is in love with Natalia Landauer, a wealthy German Jewish heiress who holds him in contempt and suspects his motives. Through Brian, Sally advises him to be more aggressive, which eventually enables Fritz to win her love. However, to gain her parents' consent for their marriage, Fritz must reveal his background, which he does and the two are married by a rabbi.


title: Cabaret (1972 film)

text: The rise of fascism is an ever-present undercurrent throughout the film. Their progress can be tracked through the characters' changing actions and attitudes. While in the beginning of the film, a Nazi is expelled from the Kit Kat Klub, the final shot of the film shows the cabaret's audience is dominated by uniformed Nazis. The rise of the Nazis is also demonstrated in a rural beer garden scene. A blonde boy sings to an audience of all ages ("Tomorrow Belongs to Me") about the beauties of nature and youth. It is revealed that the boy is wearing a Hitler Youth uniform. The ballad transforms into a militant Nazi anthem, and one by one nearly all the adults and young people watching rise and join in the singing. "Do you still think you can control them?" Brian asks Max. Later, Brian's confrontation with a Nazi on a Berlin street leads to his being beaten.


title: Cabaret (1972 film)

text: In addition, in a joint uncredited role, Oliver Collignon plays the Nazi youth, with his singing voice for "Tomorrow Belongs to Me" dubbed by Mark Lambert.


title: Cabaret (1972 film)

text: The 1972 film was based upon Christopher Isherwood's semi-autobiographical stories about Weimar-era Berlin during the Jazz Age. In 1929, Isherwood moved to Berlin in order to pursue life as an openly gay man and to enjoy the city's libertine nightlife. His expatriate social circle included W.H. Auden, Stephen Spender, Paul Bowles, and Jean Ross. While in Berlin, Isherwood shared lodgings with Ross, a British cabaret singer and aspiring film actress from a wealthy Anglo-Scottish family.


title: Cabaret (1972 film)

text: While rooming together at Nollendorfstrasse 17 in Schöneberg, Isherwood and Ross met John Blomshield, a wealthy playboy who inspired the film character of Baron Maximilian von Heune. Blomshield sexually pursued both Isherwood and Ross for a short while, and he invited them to accompany him on a trip abroad. He then abruptly disappeared without saying goodbye. Following Blomshield's disappearance, Ross became pregnant with the child of jazz pianist and later actor Peter van Eyck. After Eyck abandoned Ross, she underwent a near-fatal abortion facilitated by Isherwood who pretended to be her heterosexual impregnator.


title: Cabaret (1972 film)

text: While Ross recovered from the botched abortion procedure, the political situation rapidly deteriorated in Germany. As Berlin's daily scenes featured "poverty, unemployment, political demonstrations and street fighting between the forces of the extreme left and the extreme right," Isherwood, Spender, and other British nationals realized that they must flee the country. "There was a sensation of doom to be felt in the Berlin streets," Spender recalled.


title: Cabaret (1972 film)

text: By the time Adolf Hitler implemented the Enabling Act of 1933 which cemented his dictatorship, Isherwood, Ross, Spender, and others had fled Germany and returned to England. Many of the Berlin cabaret denizens befriended by Isherwood would later flee abroad or perish in concentration camps. These factual events served as the genesis for Isherwood's 1937 novella "Sally Bowles" which was later adapted into the 1955 film "I Am a Camera" and the 1966 "Cabaret" musical.


title: Cabaret (1972 film)

text: In July 1968, Cinerama made a verbal agreement to make a film version of the 1966 Broadway musical but pulled out in February 1969. In May 1969, Allied Artists paid a company record $1.5 million for the film rights and planned a company record budget. The cost of $4,570,000 was split evenly with ABC Pictures.


title: Cabaret (1972 film)

text: In 1971, Bob Fosse learned through Harold Prince, director of the original Broadway production, that Cy Feuer was producing a film adaptation of "Cabaret" through ABC Pictures and Allied Artists. This was the first film produced in the revival of Allied Artists. Determined to direct the film, Fosse begged Feuer to hire him. However, Fosse had previously directed the unsuccessful film adaptation of "Sweet Charity", a box office failure which made chief executives Manny Wolf and Marty Baum reluctant to hire him. Wolf and Baum preferred a more renowned or established director such as Billy Wilder, Joseph L. Mankiewicz or Gene Kelly.


title: Cabaret (1972 film)

text: Eager to hire Fosse, Feuer appealed to the studio heads, citing Fosse's talent for staging and shooting musical numbers, adding that if inordinate attention was given to filming the book scenes at the expense of the musical numbers, the whole film could fail. Fosse ultimately was hired. Over the next months, Fosse met with previously hired screenwriter Jay Presson Allen to discuss the screenplay.


title: Cabaret (1972 film)

text: As production neared, Fosse became increasingly dissatisfied with Allen's script which was based on Joe Masteroff's original book of the stage version. Fosse hired Hugh Wheeler to rewrite and revise Allen's work. Wheeler was referred to as a "research consultant," and Allen retained screenwriting credit. Wheeler, a friend of Christopher Isherwood, knew that Isherwood had been critical of the stage musical due to its bowdlerizations of his material. Wheeler went back to Isherwood's original stories in order to ensure a more faithful adaptation of the source material. In particular, Wheeler restored the subplot about the gigolo and the Jewish heiress. Wheeler also drew on gay author Christopher Isherwood's openness about his homosexuality to make the leading male character a bisexual man "rather than the heterosexual as he had been in the stage musical."


title: Cabaret (1972 film)

text: Fosse decided to increase the focus on the Kit Kat Klub, where Sally performs, as a metaphor for the decadence of Germany in the 1930s by eliminating all but one of the musical numbers performed outside the club. The only remaining outside number is "Tomorrow Belongs to Me", a folk song rendered spontaneously by patrons at an open-air café in one of the film's most effective scenes. In addition, the show's original songwriters Kander and Ebb wrote two new songs, "Mein Herr" and "Money", and incorporated "Maybe This Time", a song they had composed in 1964 and first sung by Kaye Ballard.


title: Cabaret (1972 film)

text: Feuer had cast Liza Minnelli as Sally Bowles and Joel Grey (reprising his stage role) long before Fosse was attached to the project. Fosse was given the option of using Grey as Master of Ceremonies or walking away from the production. Fosse hired Michael York as Sally Bowles's bisexual love interest, a casting choice which Minnelli initially believed was incorrect until she performed with him. Several smaller roles, as well as the remaining four dancers in the film, eventually were cast in West Germany.


title: Cabaret (1972 film)

text: Minnelli had auditioned to play Sally in the original Broadway production but was deemed too inexperienced at the time, even though she had won Broadway's Tony Award for Best Actress in a Musical. By the time "Cabaret" reached the screen, however, Minnelli was a film star having earned an Oscar nomination as the emotionally damaged college student in "The Sterile Cuckoo" (1969).


title: Cabaret (1972 film)

text: For her performance as Sally in the film, Minnelli reinterpreted the character and—at the explicit suggestion of her father, film and stage director Vincente Minnelli—she deliberately imitated film actress Louise Brooks, a flapper icon and sex symbol of the Jazz Age. Brooks, much like the character of Sally Bowles in the film, was an aspiring actress and American expat who temporarily moved to Weimar Berlin in search of international stardom. Minnelli later recalled:


title: Cabaret (1972 film)

text: In particular, Minnelli drew upon Brooks' "Lulu makeup and helmet-like coiffure." For the meeting between Sally Bowles and Brian Roberts, Minnelli modeled her movements and demeanor upon Brooks; in particular, the scene in "Pandora's Box" (1929) where Brooks' carefree character of Lulu is first introduced. Ultimately, Minnelli would win the Academy Award for Best Actress for her portrayal of Sally Bowles.


title: Cabaret (1972 film)

text: Fosse and Feuer traveled to West Germany, where producers chose to shoot the film, in order to finish assembling the film crew. During this time, Fosse highly recommended Robert L. Surtees for cinematographer, but Feuer and the top executives saw Surtees's work on "Sweet Charity" as one of the film's many artistic problems. Producers eventually chose British cinematographer Geoffrey Unsworth. Designers Rolf Zehetbauer, Hans Jürgen Kiebach and Herbert Strabel served as production designers. Charlotte Flemming designed costumes. Dancers Kathryn Doby, Louise Quick and John Sharpe were brought on as Fosse's dance aides.


title: Cabaret (1972 film)

text: Rehearsals and filming took place entirely in West Germany. For reasons of economy, indoor scenes were shot at the Bavaria Film Studios in Grünwald, outside Munich. Prior to filming, Fosse would complain every afternoon on the set of "Willy Wonka & the Chocolate Factory" because that film was overrunning and keeping him from starting work on the same stage.


title: Cabaret (1972 film)

text: Although the songs throughout the film allude to and advance the narrative, every song except "Tomorrow Belongs to Me" is executed in the context of a Kit Kat Klub performance. The voice heard on the radio reading the news throughout the film in German was that of associate producer Harold Nebenzal, whose father Seymour Nebenzahl produced such notable Weimar films as "M" (1931), "Testament of Dr. Mabuse" (1933), and "Threepenny Opera" (1931).


title: Cabaret (1972 film)

text: The film significantly differs from the Broadway musical. In the stage version, Sally is English (as she was in Isherwood's "Goodbye to Berlin"). In the film adaptation, she is American. Cliff Bradshaw was renamed Brian Roberts and made British (as was Isherwood, upon whom the character was based), rather than American as in the stage version. The characters and plotlines involving Fritz, Natalia and Max were pulled from "I Am a Camera" and did not appear in the stage production of "Cabaret" (or in "Goodbye to Berlin").


title: Cabaret (1972 film)

text: The most significant change involves the excision of the two main characters: Fraulein Schneider, who runs a boarding house, and her love interest, Herr Schultz, a German grocer. Their doomed romance plot, and the consequences of a Gentile falling in love with a Jew during the rise of antisemitism was cut. With the removals were "So What?" and "What Would You Do", sung by Schneider, the song "Meeskite", sung by Schultz, and their two duets "It Couldn't Please Me More (The Pineapple Song)" (cut) and "Married" (reset as a piano instrumental, and a phonograph record), as well as a short reprise of "Married", sung alone by Schultz.


title: Cabaret (1972 film)

text: Kander and Ebb wrote several new songs and removed others. "Don't Tell Mama" was replaced by "Mein Herr", and "The Money Song" (retained in an instrumental version as "Sitting Pretty") was replaced by "Money, Money." "Mein Herr" and "Money, Money", which were composed for the film, were integrated into the stage musical alongside the original numbers. The song "Maybe This Time", which Sally performs at the cabaret, was not written for the film, but was intended for actress Kaye Ballard. Although "Don't Tell Mama" and "Married" were removed as performed musical numbers, both still appear in the film: "Mama"'s bridge section appears as an instrumental played on Sally's gramophone; "Married" initially plays on the piano in Fraulein Schneider's parlor, and later heard on Sally's gramophone in a German translation ("Heiraten") sung by cabaret singer Greta Keller. Additionally, "If You Could See Her", performed by the MC, originally concluded with the line "She wouldn't be meeskite at all" onstage. The film changes this to "She wouldn't look Jewish at all," a return to Ebb's original lyrics.


title: Cabaret (1972 film)

text: The film opened at the Ziegfeld Theatre in New York City on February 13, 1972 with a single performance benefit grossing $2,538. It started regular showings at the Ziegfeld from February 14, grossing $8,684 in its opening day, and a house record $80,278 for the week. It grossed another $165,038 from 6 other theatres in 6 key cities reported by "Variety", placing it tenth at the US box office. After seven months of release, it had grossed $5.3 million in the New York metropolitan area. "Variety" estimated that this represented 30% of the film's total compared to the normal 15% for the market, one of the few big-budget films to perform much better in New York. Based on this estimate, the film had grossed around $17 million. By year end, "Variety" reported that it had earned theatrical rentals of $10,885,000, making it the eighth most successful film of the year. Following the film's success at the Academy Awards in March 1973, it reached number one at the US box office with a gross of $1,880,000 for the week, a record for Allied Artists. It remained number one for a second week. By May 1973, the film had earned rentals of $16 million in the United States and Canada and $7 million in other countries and reported a profit of $4,904,000. By the end of 1973, "Variety" had updated the film's rentals in the United States and Canada to $18,175,000.


title: Cabaret (1972 film)

text: Roger Ebert gave a positive review in January 1972, saying: "This is no ordinary musical. Part of its success comes because it doesn't fall for the old cliché that musicals have to make you happy. Instead of cheapening the movie version by lightening its load of despair, director Bob Fosse has gone right to the bleak heart of the material and stayed there well enough to win an Academy Award for Best Director."


title: Cabaret (1972 film)

text: A.D. Murphy of "Variety" wrote "The film version of the 1966 John Kander-Fred Ebb Broadway musical "Cabaret" is most unusual: it is literate, bawdy, sophisticated, sensual, cynical, heart-warming, and disturbingly thought-provoking. Liza Minnelli heads a strong cast. Bob Fosse's generally excellent direction recreates the milieu of Germany some 40 years ago."


title: Cabaret (1972 film)

text: Roger Greenspun of "The New York Times" wrote in February 1972 that ""Cabaret" is one of those immensely gratifying imperfect works in which from beginning to end you can literally feel a movie coming to life." Likewise, Pauline Kael of "The New Yorker" wrote a review that same month in which she applauded the film:


title: Cabaret (1972 film)

text: Although "Cabaret" (1972) was well received by film critics upon its release, author Christopher Isherwood and other persons upon whom the film's characters were based were less receptive towards the cinematic adaptation. Isherwood himself was critical of the 1972 film due to what he perceived as its negative portrayal of homosexuality:


title: Cabaret (1972 film)

text: Similarly, Isherwood's friend Jean Ross—upon whom the character of Sally Bowles was based—was ambivalent about the film. She felt the depiction of 1930s Berlin "was quite, quite different" from reality. Nevertheless, she conceded that the depiction of their social circle of British expatriates as pleasure-seeking libertines was accurate: "We were all utterly against the bourgeois standards of our parents' generation. That's what took us to [Weimar-era] Berlin. The climate was freer there." Such ambivalence towards "Cabaret" (1972) was not unique among Isherwood's circle.


title: Cabaret (1972 film)

text: The poet Stephen Spender lamented how "Cabaret" (1972) glossed over Weimar Berlin's crushing poverty:


title: Cabaret (1972 film)

text: In 2002, Jamie Russell of the BBC wrote that the film was "the first musical ever to be given an X certificate, Bob Fosse's "Cabaret" launched Liza Minnelli into Hollywood superstardom and re-invented the musical for the Age of Aquarius." In 2013, film critic Peter Bradshaw listed "Cabaret" at number one on his list of "Top 10 musicals", describing it as "satanically catchy, terrifyingly seductive...directed and choreographed with electric style by Bob Fosse..."Cabaret" is drenched in the sexiest kind of cynicism and decadent despair."


title: Cabaret (1972 film)

text: Although less explicit compared with other films made in the 1970s, "Cabaret" dealt explicitly with topics like corruption, sexual ambiguity, false dreams, and Nazism. Tim Dirks at Filmsite.org notes: "The sexually-charged, semi-controversial, kinky musical was the first one ever to be given an X rating (although later re-rated) with its numerous sexual flings and hedonistic club life. There was considerable sexual innuendo, profanity, casual sex talk (homosexual and heterosexual), some evidence of anti-Semitism, and even an abortion in the film." It was also rated X in the UK and later re-rated as 15.


title: Cabaret (1972 film)

text: On the topic of Nazism, there was little consensus among critics about the possibly fascist implications of the film and play. However, critic Steven Belletto wrote a critique of "Cabaret" in the "Criticism" journal, published by Wayne State University Press, in which he highlighted the anti-fascist themes in the film present both within and outside of the musical acts. According to Belletto, "despite the ways that the film has been understood by a variety of critics, ["Cabaret"] rejects the logic of fascist certainty by staging various numbers committed to irony and ambiguity."


title: Cabaret (1972 film)

text: The "Tomorrow Belongs to Me" scene was controversial, with Kander and Ebb, both of whom were Jewish, sometimes being wrongly accused of using a historical Nazi song. According to an article in "Variety" in November 1976, the film was censored in West Berlin when it was first released there theatrically, with the sequence featuring the Hitler Youth singing "Tomorrow Belongs to Me" having been deleted. This elimination was made "because of the feeling that it might stir up resentments in the audience by showing the sympathizers for the Nazi movement during the '30s." The sequence was restored, however, when the film was shown on West German television on November 7, 1976.


title: Cabaret (1972 film)

text: Another topic of discussion was the song "If You Could See Her", which closed with the line: "If you could see her through my eyes, she wouldn't look Jewish at all." The point of the song was showing anti-Semitism as it begins to run rampant in Berlin, but there were a number of Jewish groups who interpreted the lyrics differently.


title: Cabaret (1972 film)

text: "Cabaret" earned a total of 10 Academy Award nominations (winning 8 of them) and holds the record for most Academy Awards won by a film which did not win the Academy Award for Best Picture.


title: Cabaret (1972 film)

text: Shortly before the Academy Awards, Bob Fosse won 2 Tony Awards for directing and choreographing "Pippin", his biggest stage hit to date. Months later, he won the Primetime Emmy Award for choreographing and directing Liza Minnelli's television special "Liza with a Z", he became the first director to win all three awards in one year.


title: Cabaret (1972 film)

text: "Cabaret" has been cited by "TV Guide" as among the greatest films made and in "Movieline" magazine as one of the "100 Best Movies Ever". It was included in Film4's "100 Greatest Films of All Time" at #78 and in "The San Francisco Chronicle"s "Hot 100 Films of the Past", being hailed as "the last great musical. Liza Minnelli plays Sally Bowles, an American adrift in pre-Nazi Berlin, in Bob Fosse's stylish, near-perfect film."


title: Cabaret (1972 film)

text: David Benedict has written in "The Guardian" about "Cabaret" influence in musical films: "Back then, musicals were already low on film-goers' lists, so how come it was such a success? Simple: "Cabaret" is the musical for people who hate them. Given the vibrancy of its now iconic numbers – Liza Minnelli in bowler and black suspenders astride a bentwood chair belting out 'Mein Herr' or shimmying and shivering with pleasure over 'Money' with Joel Grey – it sounds strange to say it, but one of the chief reasons why "Cabaret" is so popular is that it's not shot like a musical."


title: Cabaret (1972 film)

text: The film has been listed as one of the most important for queer cinema for its depictions of bisexuality, arguably transgressive at the time of its 1972 post-Code release and has been credited with turning Liza Minnelli into a gay icon. Film blogs have selected it as "the gayest winner in the history of the Academy."


title: Cabaret (1972 film)

text: The film was first released to DVD in 1998. There have been releases in 2003, 2008, and 2012. The film's international ancillary distribution rights are owned by ABC (now part of The Walt Disney Company), Fremantle (UK), Warner Bros. (which acquired the film as part of its purchase of Lorimar Productions, which had acquired the film library of Allied Artists) has US domestic distribution rights.


title: Cabaret (1972 film)

text: In April 2012, Warner unveiled a new restoration of the film at the TCM Classic Film Festival. A DigiBook edition was later released on Blu-ray on February 5, 2013. Before this restoration, "Cabaret" had been sold on a standard-definition DVD from Warner Bros., but the film was unavailable in high-definition or for digital projections in cinemas. The original camera negative is lost, and a surviving interpositive had a vertical scratch that ran through 1,000 feet, or 10 minutes, of one of its reels, as confirmed by Ned Price, vice president of mastering and restoration for Warner Bros. The damage ostensibly was inflicted by a grain of dirt that had rolled through the length of the reel, beginning with a scene in which Michael York's character confronts a pro-Nazi boarding house resident, and had cut into the emulsion. The marred frames were digitally restored, but "the difficult part was matching the grain structure so the fix was invisible." After automated digital repair attempts failed, the 1,000 feet of damaged film was hand painted using a computer stylus.


title: Neutrino

text: A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of) 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 excluding massless 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 strong interaction.


title: Neutrino

text: Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos muon neutrinos (), or tau neutrinos (), in association with the corresponding charged lepton. Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values, 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 2022, but laboratory experiments and cosmological observations have determined the differences of their squares, an upper limit on their sum (< 2.14×10kg),


title: Neutrino

text: 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.


title: Neutrino

text: Neutrinos are created by various radioactive decays; the following list is not exhaustive, but includes some of those processes:


title: Neutrino

text: 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 65 billion () solar neutrinos, per second per square centimeter. Neutrinos can be used for tomography of the interior of the earth.


title: Neutrino

text: Research is intense in the hunt to elucidate the essential nature of neutrinos, with aspirations of finding:


title: Neutrino

text: 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 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 electron or beta particle in the process of beta decay and had a mass similar to the electron.


title: Neutrino

text: 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.


title: Neutrino

text: 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"):


title: Neutrino

text: Fermi's paper, written in 1934, unified Pauli's neutrino with Paul Dirac's positron and Werner Heisenberg's neutron–proton model and gave a solid theoretical basis for future experimental work. The journal "Nature" rejected Fermi's paper, saying that the theory was "too remote from reality". He submitted the paper to an Italian journal, which accepted it, but the general lack of interest in his theory at that early date caused him to switch to experimental physics.


title: Neutrino

text: 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. The first evidence of the reality of neutrinos came in 1938 via simultaneous cloud-chamber measurements of the electron and the recoil of the nucleus.


title: Neutrino

text: 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,


title: Neutrino

text: 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:


title: Neutrino

text: 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.


title: Neutrino

text: In February 1965, the first neutrino found in nature was identified by a group which included Jacques Pierre Friederich (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.


title: Neutrino

text: In 1962, Lederman, Schwartz, and Steinberger showed that more than one type of neutrino exists by first detecting interactions of the muon neutrino (already hypothesised with the name "neutretto"),


title: Neutrino

text: 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.


title: Neutrino

text: 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, to R. Davis, who conceived and led the Homestake experiment, and to A.B. McDonald, who led the SNO experiment, which could detect all of the neutrino flavors and found no deficit.


title: Neutrino

text: A practical method for investigating neutrino oscillations was first suggested by Bruno Pontecorvo in 1957 using an analogy with kaon oscillations; over the subsequent 10 years, he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 Stanislav Mikheyev and Alexei Smirnov (expanding on 1978 work by Lincoln Wolfenstein) noted that flavor oscillations can be modified when neutrinos propagate through matter. This so-called Mikheyev–Smirnov–Wolfenstein effect (MSW effect) is important to understand because many neutrinos emitted by fusion in the Sun pass through the dense matter in the solar core (where essentially all solar fusion takes place) on their way to detectors on Earth.


title: Neutrino

text: Starting in 1998, experiments began to show that solar and atmospheric neutrinos change flavors (see Super-Kamiokande and Sudbury Neutrino Observatory). This resolved the solar neutrino problem: the electron neutrinos produced in the Sun had partly changed into other flavors which the experiments could not detect.


title: Neutrino

text: Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations. Especially relevant in this context are the reactor experiment KamLAND and the accelerator experiments such as MINOS. The KamLAND experiment has indeed identified oscillations as the neutrino flavor conversion mechanism involved in the solar electron neutrinos. Similarly MINOS confirms the oscillation of atmospheric neutrinos and gives a better determination of the mass squared splitting. 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.


title: Neutrino

text: 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.


title: Neutrino

text: Around 1 second after the Big Bang, neutrinos decoupled, giving rise to a background level of neutrinos known as the cosmic neutrino background (CNB).


title: Neutrino

text: R. Davis and M. Koshiba were jointly awarded the 2002 Nobel Prize in Physics. Both conducted pioneering work on solar neutrino detection, and Koshiba's work also resulted in the first real-time observation of neutrinos from the SN 1987A supernova in the nearby Large Magellanic Cloud. These efforts marked the beginning of neutrino astronomy.


title: Neutrino

text: SN 1987A represents the only verified detection of neutrinos from a supernova. However, many stars have gone supernova in the universe, leaving a theorized diffuse supernova neutrino background.


title: Neutrino

text: Neutrinos have half-integer spin (); therefore they are fermions. Neutrinos are leptons. They have only been observed to interact through the weak force, although it is assumed that they also interact gravitationally.


title: Neutrino

text: 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.


title: Neutrino

text: 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 discrete mass eigenstates. Although only differences of squares of the three mass values are known as of 2016, experiments have shown that these masses are tiny in magnitude. From cosmological measurements, it has been calculated that the sum of the three neutrino masses must be less than one-millionth that of the electron.


title: Neutrino

text: More formally, neutrino flavor eigenstates (creation and annihilation combinations) are not the same as the neutrino mass eigenstates (simply labeled "1", "2", and "3"). As of 2016, it is not known which of these three is the heaviest. In analogy with the mass hierarchy of the charged leptons, the configuration with mass 2 being lighter than mass 3 is conventionally called the "normal hierarchy", while in the "inverted hierarchy", the opposite would hold. Several major experimental efforts are underway to help establish which is correct.


title: Neutrino

text: 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.


title: Neutrino

text: A non-zero mass allows neutrinos to possibly have a tiny magnetic moment; if so, neutrinos would interact electromagnetically, although no such interaction has ever been observed.


title: Neutrino

text: 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.


title: Neutrino

text: There are other possibilities in which neutrinos could oscillate even if they were massless: If Lorentz symmetry were not an exact symmetry, neutrinos could experience Lorentz-violating oscillations.


title: Neutrino

text: 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 MSW effect. Only a small fraction of the neutrino's energy is transferred to the material.


title: Neutrino

text: 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.


title: Neutrino

text: 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.


title: Neutrino

text: Antineutrinos are produced in nuclear beta decay together with a beta particle (in beta decay a neutron decays into a proton, electron, and antineutrino). All antineutrinos observed thus far had right-handed helicity (i.e., only one of the two possible spin states has ever been seen), while neutrinos were all left-handed.


title: Neutrino

text: Antineutrinos were first detected as a result of their interaction with protons in a large tank of water. This was installed next to a nuclear reactor as a controllable source of the antineutrinos (see Cowan–Reines neutrino experiment).


title: Neutrino

text: Researchers around the world have begun to investigate the possibility of using antineutrinos for reactor monitoring in the context of preventing the proliferation of nuclear weapons.


title: Neutrino

text: Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Particles that have this property are known as 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.


title: Neutrino

text: , 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, EXO, SNO+,


title: Neutrino

text: 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.


title: Neutrino

text: Neutrinos can interact with a nucleus, changing it to another nucleus. This process is used in radiochemical neutrino detectors. In this case, the energy levels and spin states within the target nucleus have to be taken into account to estimate the probability for an interaction. In general the interaction probability increases with the number of neutrons and protons within a nucleus.


title: Neutrino

text: It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. For this reason, in early experiments a special reaction channel was chosen to facilitate the identification: the interaction of an antineutrino with one of the hydrogen nuclei in the water molecules. A hydrogen nucleus is a single proton, so simultaneous nuclear interactions, which would occur within a heavier nucleus, don't need to be considered for the detection experiment. Within a cubic metre of water placed right outside a nuclear reactor, only relatively few such interactions can be recorded, but the setup is now used for measuring the reactor's plutonium production rate.


title: Neutrino

text: Very much like neutrons do in nuclear reactors, neutrinos can induce fission reactions within heavy nuclei. So far, this reaction has not been measured in a laboratory, but is predicted to happen within stars and supernovae. The process affects the abundance of isotopes seen in the universe. Neutrino-induced disintegration of deuterium nuclei has been observed in the Sudbury Neutrino Observatory, which uses a heavy water detector.


title: Neutrino

text: 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 boson. This particle can decay into any light neutrino and its antineutrino, and the more available types of light neutrinos,


title: Neutrino

text: the shorter the lifetime of the  boson. Measurements of the lifetime have shown that three light neutrino flavors couple to the . The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicists' intuition that there should be exactly three types of neutrino.


title: Neutrino

text: There are several active research areas involving the neutrino. Some are concerned with testing predictions of neutrino behavior. Other research is focused on measurement of unknown properties of neutrinos; there is special interest in experiments that determine their masses and rates of CP violation, which cannot be predicted from current theory.


title: Neutrino

text: International scientific collaborations install large neutrino detectors near nuclear reactors or in neutrino beams from particle accelerators to better constrain the neutrino masses and the values for the magnitude and rates of oscillations between neutrino flavors. These experiments are thereby searching for the existence of CP violation in the neutrino sector; that is, whether or not the laws of physics treat neutrinos and antineutrinos differently.


title: Neutrino

text: The KATRIN experiment in Germany began to acquire data in June 2018 to determine the value of the mass of the electron neutrino, with other approaches to this problem in the planning stages.


title: Neutrino

text: Despite their tiny masses, neutrinos are so numerous that their gravitational force can influence other matter in the universe.


title: Neutrino

text: The three known neutrino flavors are the only candidates for dark matter that are experimentally established elementary particles – specifically, they would be hot dark matter. However, the currently known neutrino types seem to be essentially ruled out as a substantial proportion of dark matter, based on observations of the cosmic microwave background. It still seems plausible that heavier, sterile neutrinos might compose warm dark matter, if they exist.


title: Neutrino

text: 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, heavy sterile neutrinos would have a mass of at least 45.6 GeV.


title: Neutrino

text: 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, results are overall ambiguous.


title: Neutrino

text: 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.


title: Neutrino

text: 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.


title: Neutrino

text: Cosmic ray neutrino experiments detect neutrinos from space to study both the nature of neutrinos and the cosmic sources producing them.


title: Neutrino

text: 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.


title: Neutrino

text: 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).


title: Neutrino

text: 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 −0.0006% (±0.0012%).


title: Neutrino

text: 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.


title: Neutrino

text: 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.


title: Neutrino

text: In June 2012, CERN announced that new measurements conducted by all four Gran Sasso experiments (OPERA, ICARUS, Borexino and LVD) found agreement between the speed of light and the speed of neutrinos, finally refuting the initial OPERA claim.


title: Neutrino

text: The Standard Model of particle physics assumed that neutrinos are massless. The experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavour 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.


title: Neutrino

text: The strongest upper limit on the masses of neutrinos comes from cosmology: the Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total energy of all three types of neutrinos exceeded an average of 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.


title: Neutrino

text: 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.


title: Neutrino

text: 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. While this shows that neutrinos have mass, the absolute neutrino mass scale is still not known. This is because neutrino oscillations are sensitive only to the difference in the squares of the masses.


title: Neutrino

text: As of 2020, the best-fit value of the difference of the squares of the masses of mass eigenstates 1 and 2 is , while for eigenstates 2 and 3 it is . Since is the difference of two squared masses, at least one of them must have a value which is at least the square root of this value. Thus, there exists at least one neutrino mass eigenstate with a mass of at least .


title: Neutrino

text: A number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments, especially using nuclear beta decay. Upper limits on the effective electron neutrino masses come from beta decays of tritium. The Mainz Neutrino Mass Experiment set an upper limit of "m" < 2.2 eV/"c" at 95% Confidence Level. Since June 2018 the KATRIN experiment searches for a mass between and in tritium decays. The February 2022 upper limit is m < 0.8 eV c at 90% CL in combination with a previous campaign by KATRIN from 2019.


title: Neutrino

text: 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.


title: Neutrino

text: If the neutrino is a Majorana particle, the mass may be calculated by finding the half-life of neutrinoless double-beta decay of certain nuclei. The current lowest upper limit on the Majorana mass of the neutrino has been set by KamLAND-Zen: 0.060–0.161 eV.


title: Neutrino

text: Since the neutrino is an elementary particle it does not have a size in the same sense as everyday objects: Like all other Standard Model fundamental particles, neutrinos are point-like, with neither width nor volume. Consequences of having a conventional "size" are absent: There is no minimum distance between them, and neutrinos cannot be condensed into a separate uniform substance that occupies a finite volume.


title: Neutrino

text: In one sense, particles with mass have a wavelength (the Compton wavelength) which is useful for estimating their cross-sections for collisions. The smaller a particle's mass, the larger its Compton wavelength. Based on the upper limit of given above, the "matter wave" of a neutrino would be on the order of at least () or longer, comparable to the wavelengths of ultraviolet light at the shortest UV wavelength(s) (UVC). This extremely long wavelength (for a particle with mass) leads physicists to suspect that even though neutrinos follow Fermi statistics, that their behavior may be much like a wave, making them seem Bosonic, and thus placing them near the border between particles (fermions) and waves (bosons).


title: Neutrino

text: 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.


title: Neutrino

text: 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.


title: Neutrino

text: 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 ~; consequently, the fraction of neutrinos with "wrong" helicity among them cannot exceed .


title: Neutrino

text: An unexpected series of experimental results for the rate of decay of heavy highly charged radioactive ions circulating in a storage ring has provoked theoretical activity in an effort to find a convincing explanation.


title: Neutrino

text: The observed phenomenon is known as the GSI anomaly, as the storage ring is a facility at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt Germany.


title: Neutrino

text: The rates of weak decay of two radioactive species with half lives of about 40 seconds and 200 seconds were found to have a significant oscillatory modulation, with a period of about 7 seconds.


title: Neutrino

text: As the decay process produces an electron neutrino, some of the suggested explanations for the observed oscillation rate propose new or altered neutrino properties. Ideas related to flavour oscillation met with skepticism.


title: Neutrino

text: Nuclear reactors are the major source of human-generated neutrinos. The majority of energy in a nuclear reactor is generated by fission (the four main fissile isotopes in nuclear reactors are , , and ), the resultant neutron-rich daughter nuclides rapidly undergo additional beta decays, each converting one neutron to a proton and an electron and releasing an electron antineutrino 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. For a typical nuclear reactor with a thermal power of ,


title: Neutrino

text: the total power production from fissioning atoms is actually , of which is radiated away as antineutrino radiation and never appears in the engineering. This is to say, of fission energy is "lost" from this reactor and does not appear as heat available to run turbines, since antineutrinos penetrate all building materials practically without interaction.


title: Neutrino

text: 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. Only antineutrinos with an energy above threshold of can trigger inverse beta decay and thus be unambiguously identified (see below).


title: Neutrino

text: 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 ( this number) below the energy threshold; these lower-energy antineutrinos are invisible to present detector technology.


title: Neutrino

text: 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. Such a setup is generally known as a "neutrino factory".


title: Neutrino

text: Nuclear weapons also produce very large quantities of neutrinos. Fred Reines and Clyde Cowan considered the detection of neutrinos from a bomb prior to their search for reactor neutrinos; a fission reactor 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).


title: Neutrino

text: 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, and Borexino. The main background in the geoneutrino measurements are the antineutrinos coming from reactors.


title: Neutrino

text: Atmospheric neutrinos result from the interaction of cosmic rays with atomic nuclei in the Earth's atmosphere, creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from Tata Institute of Fundamental Research (India), Osaka City University (Japan) and Durham University (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in Kolar Gold Fields in India in 1965.


title: Neutrino

text: The details of the operation of the Sun are explained by the Standard Solar Model. In short: when four protons fuse to become one helium nucleus, two of them have to convert into neutrons, and each such conversion releases one electron neutrino.


title: Neutrino

text: 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.