Any of the subatomic particles that are built from quarks, and thus react through strong nuclear force, are hadrons. The hadrons include mesons and baryons. All known subatomic particles except bosons and leptons, are hadrons. Except for protons and for neutrons that are bound in nuclei, all hadrons have short lives and are produced in the high-energy collisions of subatomic particles (Carrigan 35). The other three basic forces of nature also affect hadron behavior: all are subject to gravitation; charged hadrons obey electromagnetic laws; and some hadrons break up by way of the weak nuclear

force, while others decay via the strong electromagnetic forces.

Mesons are any member of a family of subatomic parti

of muon neutrinos. In the mid-1970s, particle physicists discovered yet another variety of charged lepton, the tau. A tau-neutrino and tau-antineutrino are associated with this third charged lepton (Schwarz 78). All types of neutrino have masses much smaller than those of their charged partners, if they have any mass at all (Martin 76). For example, experiments show that the mass of the electron-neutrino must be less than 0.0004 that of the electron. There is, however, no compelling theoretical reason for the mass of the neutrino to be exactly zero. Indeed, the shortfall in the number of neutrinos detected on

electromagnetic, weak, and gravitational forces and do not take part in strong

create other gluons as they move between quarks. Thus, if a quark starts to speed away from its companions after being struck by an accelerated particle, the gluons utilize energy that they draw from the quark's motion to produce more gluons. The larger the number of gluons exchanged among quarks, the stronger the binding forces become. Supplying additional energy to extract the quark only results in the conversion of that energy into new quarks and antiquarks with which the first quark combines . Although QCD cogently explains the behavior of quarks and provides a means of calculating their basic properties, it does not account for the flavors of "charm" and "bottom" associated with two types of heavy quarks that were found in the late 1970s. The discovery of the charmed (c) and bottom (b) quarks and their associated antiquarks, achieved through the

knock a quark out of a baryon in experiments with particle accelerators to observe it in a free state but have not yet succeeded in doing so. Throughout the 1960s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists, Murray Gell-Mann of the United States and Yuval Ne'eman of Israel, proposed a particle classification scheme called the Eightfold Way, based on the mathematical symmetry group SU(3), that described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, adopting the term from a passage in James Joyce's novel Finnegans Wake. (The American physicist George Zweig developed a similar theory independently that same year and called his fundamental particles "aces.") Gell-Mann's model provided a simple picture in which all mesons are

electron volts). (The next heaviest quark, the bottom, has a mass of 4.8 GeV.) It has yet to be explained why the top quark is so much more massive than the other elementary particles, but its existence completes the prevailing theoretical scheme of nature's fundamental building blocks.

strong interactions, developed in 1977, the term colour has nothing to do with the colours of the everyday world but rather represents a special quantum property of quarks. The colours red, green, and blue are ascribed to quarks, and their opposites, minus-red, minus-green, and minus-blue, to antiquarks. According to QCD, all combinations of quarks must contain equal mixtures of these imaginary colours so that they will cancel out one another, with the resulting particle having no net colour. A baryon, for example, always consists of a combination of one red, one green, and one blue quark. The property of colour in strong interactions plays a role analogous to an electric charge in electromagnetic interactions (Martin 190). Charge implies the exchange of photons between charged particles. Similarly, colour involves the exchange

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