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In physics, a quark (pronounced /kwɔrk/ or /kwɑrk/ ) is a type of subatomic particle. In technical terms, quarks are elementary fermions which engage in the strong interaction due to their color charge. Because of the phenomenon of color confinement, quarks are never found on their own in nature: they are always bound together in composite particles named hadrons. The most common hadrons are the proton and the neutron, which compose atomic nuclei.

There are six different types of quarks, known as flavors : up (symbol: u ), down ( d ), charm ( c ), strange ( s ), top ( t ), and bottom ( b ). The lightest flavors, the up quark and the down quark, are generally stable and are very common in the universe, as they are found in protons and neutrons. The more massive charm, strange, top and bottom quarks are unstable and rapidly decay; these can only be produced under high energy conditions, such as in particle accelerators and in cosmic rays. For every quark flavor there is a corresponding antiparticle, called antiquark , that differs from the quark only in that some of its properties have the opposite sign.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. There was little evidence for the theory until 1968, when electron–proton scattering experiments indicated the existence of substructure within the proton resembling three "sphere-like" regions within the proton. By 1995, when the top quark was observed at Fermilab, all the six flavors had been observed.

Since quarks are not found in isolation, their properties can only be deduced from experiments on hadrons. An exception to this is the top quark, which decays so rapidly that it does not produce hadrons at all, and instead is observed through the identification of the particles it has decayed into. Furthermore, it has been theorized in some of the Big Bang theories, that in the very beginning, our extremely hot universe may have contained single quarks, including "free" top quarks, in a quark-gluon plasma.

Classification

The Standard Model is the theoretical framework describing all the currently known elementary particles, plus the as-yet-unobserved Higgs boson. This model comprises six flavors of quarks, named up , down , charm , strange , top and bottom ; the top and bottom flavors are also known as truth and beauty , respectively. Any two quarks of the same flavor are identical particles, meaning that all of their properties are the same.

In the Standard Model, particles of matter, including quarks, are classified as fermions, meaning that their spin quantum number (a property related to their intrinsic angular momentum) is half-integer; as a consequence, they are subject to the Pauli exclusion principle, stating that no two fermions of the same flavor can ever simultaneously occupy the same state. This contrasts with particles mediating forces, which are bosons: that is, they have integer spin, and hence the Pauli exclusion principle does not apply to them. Among elementary fermions, quarks differ from leptons (the best-known flavor of which is the electron), in that they, unlike leptons, have a color charge, a property causing them to engage in strong interaction, the force keeping quarks bound together in hadrons.

Elementary fermions are grouped into three generations, each one comprising two leptons and two quarks. The first generation includes up and down quarks, the second includes charm and strange quarks, and the third includes top and bottom quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there is strong indirect evidence that there cannot exist more than three generations. Particles in higher generations generally have greater mass and are less stable, tending to decay into lower-generation, less massive particles by means of weak interaction. Typically, only the first-generation up and down quarks are in common natural occurrence; heavier quarks can only be created in high-energy conditions, such as in cosmic rays, and quickly decay. Most studies conducted on heavier quarks have been performed in artificially created conditions, such as in particle accelerators.

Antiparticles of quarks are called antiquark s, and denoted by a bar over the letter for the quark, such as u for an up antiquark. Like antimatter in general, antiquarks have the same mass and spin of their respective quarks, but the electric charge and other charges have the opposite sign.

Having electric charge, flavor, color charge and mass, quarks are the only known elementary particles engaging in all the four fundamental interactions of contemporary physics: respectively, electromagnetism, weak interaction, strong interaction and gravitation. The last is usually irrelevant at subatomic scales, and is not described by the Standard Model.

See the table of properties below for a more complete analysis of the six quark flavors' properties.

History

The quark theory was first postulated by physicists Murray Gell-Mann and George Zweig in 1964. At the time of the theory's initial proposal, the "particle zoo" consisted of several leptons and many different hadrons. Gell-Mann and Zweig developed the quark theory to explain the hadrons; they proposed that various combinations of quarks and antiquarks were the components of the hadrons, which were at the time considered to be indivisible.

The Gell-Mann–Zweig model predicted three quarks, which they named up , down and strange . At the time, the pair of physicists ascribed various properties and values to the three new proposed particles, such as electric charge and spin. The initial reaction of the physics community to the proposal was mixed, many having reservations regarding the actual physicality of the quark concept. They believed the quark was merely an abstract concept that could be temporarily used to help explain certain concepts that were not well understood, rather than an actual entity that existed in the way that Gell-Mann and Zweig had envisioned.

In less than a year, extensions to the Gell-Mann–Zweig model were proposed when another duo of physicists, Sheldon Lee Glashow and James Bjorken, predicted the existence of a fourth flavor of quark, which they referred to as charm . The addition was proposed because it expanded the power and self-consistency of the theory: it allowed a better description of the weak interaction (the mechanism that allows quarks to decay); equalized the number of quarks with the number of known leptons; and implied a mass formula that correctly reproduced the masses of the known mesons.

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton had substructure. However, whilst the concept of hadron substructure had been proven, there was still apprehension towards the quark model: the substructures became known at the time as partons (a term proposed by Richard Feynman, and supported by some experimental project reports), but it "was unfashionable to identify them explicitly with quarks". These partons were later identified as up and down quarks. Their discovery also validated the existence of the strange quark, because it was necessary to the model Gell-Mann and Zweig had proposed.

In a 1970 paper, Glashow, John Iliopoulos, and Luciano Maiani gave more compelling theoretical arguments for the as-yet undiscovered charm quark. The number of proposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation could be explained if there were another pair of quarks. They named the two additional quarks top and bottom .

It was the observation of the charm quark that finally convinced the physics community of the quark model's correctness. Following a decade without empirical evidence supporting the flavor's existence, it was created and observed almost simultaneously by two teams in November 1974: one at the Stanford Linear Accelerator Center under Samuel Ting and one at Brookhaven National Laboratory under Burton Richter. The two parties had assigned the discovered particle two different names, J and ψ. The particle hence became formally known as the J/ψ meson and it was considered a quark–antiquark pair of the charm flavor that Glashow and Bjorken had predicted, or the charmonium.

In 1977, the bottom quark was observed by Leon Lederman and a team at Fermilab. This indicated that a top quark probably existed, because the bottom quark was without a partner. However, it was not until eighteen years later, in 1995, that the top quark was finally observed. The top quark's discovery was quite significant, because it proved to be significantly more massive than expected, almos