QUARK

any of a group of subatomic particles believed to be among the fundamental constituents of matter. In much the same way that protons and neutrons make up atomic nuclei, these particles themselves are thought to consist of quarks. Quarks constitute all hadrons (baryons and mesons)i.e., all particles that interact by means of the strong force, the force that binds the components of the nucleus. According to prevailing theory, quarks have mass and exhibit a spin (i.e., type of intrinsic angular momentum corresponding to a rotation around an axis through the particle) equal to one-half the basic quantum mechanical unit of angular momentum. The latter property implies that they obey the Pauli exclusion principle, which states that no two particles having half-integral spin can exist in exactly the same quantum state. Quarks appear to be truly fundamental. They have no apparent structure; that is, they cannot be resolved into something smaller. Quarks always seem to occur in combination with other quarks or antiquarks, never alone. For years physicists have attempted to 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 fanciful 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 shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It postulated the existence of three types of quarks, distinguished by distinctive flavours. These three quark types are now commonly designated as up (u), down (d), and strange (s). Each carries a fractional electric charge (i.e., a charge less than that of the electron). The up and down quarks are thought to make up protons and neutrons and are thus the ones observed in ordinary matter. Strange quarks occur as components of K mesons and various other extremely short-lived subatomic particles that were first observed in cosmic rays but that play no part in ordinary matter. The interpretation of quarks as actual physical entities posed two major problems. First, quarks had to have half-integral spin for the model to work, but at the same time they seemed to violate the Pauli exclusion principle. In many of the baryon configurations constructed of quarks, sometimes two or even three identical quarks had to be set in the same quantum statean arrangement prohibited by the exclusion principle. Second, quarks appeared to defy being freed from the particles they made up. Although the forces binding quarks were strong, it seemed improbable that they were powerful enough to withstand bombardment by high-energy electrons and neutrinos from particle accelerators. These problems were resolved by the introduction of the concept of colour, as formulated in quantum chromodynamics (QCD). In this theory of 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. Charge implies the exchange of photons between charged particles. Similarly, colour involves the exchange of massless particles called gluons among quarks. Just as photons carry electromagnetic force, gluons transmit the forces that bind quarks together. Quarks change their colour as they emit and absorb gluons, and the exchange of gluons maintains proper quark colour distribution. The binding forces carried by the gluons tend to be weak when quarks are close together. At a distance of approximately 10-13 cmabout the diameter of a protonquarks behave as though they were free. This condition is called asymptotic freedom. When one begins to draw the quarks apart, however, as if attempting to knock them out of a proton, the force grows stronger. This is in direct contrast to the electromagnetic force, which grows weaker with the square of the distance between the interacting bodies. As explained by QCD, gluons have the ability to 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 behaviour of quarks and provides a means of calculating their basic properties, it does not account for the flavours 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 creation of mesons, strongly suggests that quarks occur in pairs. This speculation led to efforts to find a sixth type of quark called top (t), after its proposed flavour. According to theory, the top quark carries a + 2/3 electric charge; its partner, the bottom quark, has a charge of - 1/3. In 1995 two independent groups of scientists at Fermi National Accelerator Laboratory, Batavia, Illinois, reported that they had found the top quark. A weighted average of their results gives the top quark a mass of 176 12 GeV (billion 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.