A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation. For this reason, much of what is known about quarks has been drawn from observations of hadrons. Quarks have various intrinsic properties, including electric charge, color charge, spin, they are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions known as fundamental forces, as well as the only known particles whose electric charges are not integer multiples of the elementary charge. There are six types, known as flavors, of quarks: up, strange, charm and top. Up and down quarks have the lowest masses of all quarks; the heavier quarks change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state.

Because of this, up and down quarks are stable and the most common in the universe, whereas strange, charm and top quarks can only be produced in high energy collisions. For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign; the quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for all six flavors; the top quark, first observed at Fermilab in 1995, was the last to be discovered. The Standard Model is the theoretical framework describing all the known elementary particles; this model contains six flavors of quarks, named up, strange, charm and top. Antiparticles of quarks are called antiquarks, are denoted by a bar over the symbol for the corresponding quark, such as u for an up antiquark.

As with antimatter in general, antiquarks have the same mass, mean lifetime, spin as their respective quarks, but the electric charge and other charges have the opposite sign. Quarks are spin-​1⁄2 particles, implying that they are fermions according to the spin–statistics theorem, they are subject to the Pauli exclusion principle, which states that no two identical fermions can occupy the same quantum state. This is in contrast to bosons. Unlike leptons, quarks possess color charge; the resulting attraction between different quarks causes the formation of composite particles known as hadrons. The quarks that determine the quantum numbers of hadrons are called valence quarks. There are two families of hadrons: baryons, with three valence quarks, mesons, with a valence quark and an antiquark; the most common baryons are the building blocks of the atomic nucleus. A great number of hadrons are known, most of them differentiated by their quark content and the properties these constituent quarks confer.

The existence of "exotic" hadrons with more valence quarks, such as tetraquarks and pentaquarks, was conjectured from the beginnings of the quark model but not discovered until the early 21st century. Elementary fermions are grouped into each comprising two leptons and two quarks; the first generation includes up and down quarks, the second strange and charm quarks, the third bottom and top quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, there is strong indirect evidence that no more than three generations exist. Particles in higher generations have greater mass and less stability, causing them to decay into lower-generation particles by means of weak interactions. Only first-generation quarks occur in nature. Heavier quarks can only be created in high-energy collisions, decay quickly. Studies of heavier quarks are conducted in artificially created conditions, such as in particle accelerators. Having electric charge, color charge, flavor, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: electromagnetism, strong interaction, weak interaction.

Gravitation is too weak to be relevant to individual particle interactions except at extremes of energy and distance scales. However, since no successful quantum theory of gravity exists, gravitation is not described by the Standard Model. See the table of properties below for a more complete overview of the six quark flavors' properties; the quark model was independently proposed by physicist

Edward Grant

Edward Grant is an American historian of medieval science. He was named a Distinguished Professor in 1983. Other honors include the 1992 George Sarton Medal, for "a lifetime scholarly achievement" as an historian of science. Grant is Distinguished Professor Emeritus, Department of History and Philosophy of Science, Indiana University, Bloomington. Before arriving at Indiana University in the fall of 1959, Professor Grant taught at the University of Maine and in the history of science program at Harvard University. Professor Grant was twice chair of his department where he taught courses on medieval science, natural philosophy and science and religion, he served as president of the History of Science Society from 1985–86. He has received many honors and awards, including the George Sarton Medal in 1992, the most prestigious award given by the History of Science Society that "recognizes those whose entire careers have been devoted to the field and whose scholarship is exceptional." In his book The Foundations of Modern Science in the Middle Ages: Their Religious and Intellectual Contexts, Grant discusses the developments and discoveries that culminated in the Scientific Revolution of the 17th century.

He emphasize how the roots of modern science were planted in the ancient and medieval worlds long before the modern period, that the Christian Latin civilization of Western Europe began the last stage of its intellectual development. One basic factor is how Christianity developed in the West with the establishment of the medieval universities around 1200. In God and Reason in the Middle Ages he argues that the Middle Ages acquired an undeserved reputation as an age of superstition and unreason. Edward Grant has published more than ninety articles and twelve books, including: Physical Science in the Middle Ages Much Ado About Nothing: Theories of Space and Vacuum from the Middle Ages to the Scientific Revolution Planets, Stars, & Orbs: The Medieval Cosmos, 1200–1687 The Foundations of Modern Science in the Middle Ages God and Reason in the Middle Ages Science and Religion, 400 B. C. to A. D. 1550: From Aristotle to Copernicus A History of Natural Philosophy from the Ancient World to the Nineteenth Century Harvard University Press

Zimbabwe Congress of Trade Unions

The Zimbabwe Congress of Trade Unions is the primary trade union federation in Zimbabwe. The general secretary of ZCTU is Wellington Chibebe and the president is Lovemore Matombo; the former General Secretary was Morgan Tsvangirai. Jeffrey Mutandare is a former President of the ZCTU; the ZCTU was formed on February 28, 1981 through the merger of six trade union centres: African Trade Union Congress, the National African Trade Union Congress, the Trade Union Congress of Zimbabwe, the United Trade Unions of Zimbabwe, the Zimbabwe Federation of Labour and the Zimbabwe Trade Union Congress. The ZCTU was established by the ruling party, ZANU-PF, with the aim of reducing industrial disputation, improving the influence of the government over the union movement. Prior to the creation of the state of Zimbabwe official trade unions in Rhodesia were controlled by the white minority and worked to preserve the economic privilege of white workers over the black majority. While trade unions representing the black majority workforce did exist, their activities were hampered by the racially discriminatory Rhodesian government, their close association with the black nationalist movement meant they were inactive during the Rhodesian Bush War.

Trade union membership increased following the establishment of majority-rule government in 1980, reached 200,000 in 1985. The number of strikes and industrial disputes increased against the wishes of the new government, it was to combat this trend that the government established the ZCTU, as well as promising the introduction of a minimum wage and limited industrial democracy. The initial leadership of the ZCTU was politically associated with the government, was dismissed in 1984 for corruption, their replacements were found to be corrupt, with the General Secretary removed for missappropriating funds in November 1986. The direction of the ZCTU changed after the appointment of Jeffrey Mutandare, of the Associated Mineworkers' Union. Mutandare was much more willing than previous leaders to criticise government policy, including the new Labour Relations Act of 1985, which he claimed centralised control over the trade union movement in the Ministry of Labour. In the 1990s the ZCTU grew opposed to the government of Robert Mugabe due to the government's pursuit of neoliberal economic policies, as well as perceived government corruption and authoritarianism.

At its 1995 Congress the ZCTU launched a major economic policy statement,'Beyond ESAP', criticising the Mugabe government's adoption of the Economic Structural Adjustment Programme. The ESAP involved the introduction of neoliberal macroeconomic policies, at the encouragement of the World Bank; the ZCTU was the main force behind the formation of the opposition party, the Movement for Democratic Change, established in 1999. The Secretary-General of the ZCTU, Morgan Tsvangirai became the President of the MDC; as of January 2014 the following unions were affiliated to the ZCTU. Zimbabwe Pulp & Paper Workers' Union Zimbabwe Domestic & Allied Workers Union Zimbabwe Chemicals & Plastics Allied Workers' Union Zimbabwe Textile Workers Union Zimbabwe Tobacco Industrial Workers’ Union Zimbabwe Metal, Energy & Allied Workers’ Union Zimbabwe Security Guards Union Zimbabwe Banks & Allied Workers’ Union Zimbabwe Furniture, Timber & Allied Trades Union Zimbabwe Educational Scientific, Social & Cultural Workers’ Union Zimbabwe Catering & Hotel Workers’ Union Zimbabwe Construction and Allied Trades Workers’ Union Zimbabwe Urban Councils Workers’ Union Zimbabwe Union of Journalists Zimbabwe Amalgamated Railway Workers’ Union Railway Association of Yard Operating Staff Zimbabwe Railways Artisans Union Railway Association of Enginemen National Union of the Clothing Industry National Engineering Workers’ Union Motor Vehicle Manufacturers Workers’ Union of Zimbabwe General Agriculture & Plantation Workers’ Union of Zimbabwe Federation of Food & Allied Workers’ Union of Zimbabwe Commercial Workers’ Union of Zimbabwe Cement, Lime & Allied Workers’ Union of Zimbabwe ZCTU website