Quarks have mass and exhibit spin, the type of intrinsic angularmomentum corresponding to rotation around an axis, equal to half the basicquantum mechanical unit of angular momentum, obeying Pauli’s exclusion principle. This principle that no two particles having half integral spin can exist in thesame quantum state(1985 Quarks). Quarks always occur in combination with other quarks, they never occuralone. Physicists have attempted to knock a single quark free from a groupusing a particle accelerator, but have failed.
Mesons contain a quark and anantiquark, up, down, and strange, while baryons contain three quarksdistinguished by flavours. Each has a charge that is a fraction of that of anelectron. Up and down quarks make up protons and neutrons, and can be observedin ordinary matter. Strange quarks can be observed in omega-minus and othershort lived subatomic particles which play on part in ordinary matter(1985Quarks). The interpretation of quarks as physical entities poses two problems. First, sometimes two or three identical quarks have to be in the same quantumstate which, because they have to have half integral spin, violates Pauli’sexclusion principal.
Second, quarks appear to not be able to be separated fromthe particles they make up. Although the force holding the quarks together isstrong it is improbable that it could withstand bombardment from high energy andneutrinions in a particle accelorator(1985 Quarks). Quantum chromodynamics(QCD) ascribes colours red, green, and blue toquarks and minus-red, minus-green, and minus-blue to antiquarks. Combinationsof quarks must contain equal mixtures of colours so that they cancel each otherout. Colour involves the exchange of massless particles, gluons.
Gluonstransfer the forces which bind quarks together. Quarks change colour as theyemit and absorb gluons. The exchange of gluons is what maintains the rightquark colour distribution. The forces carried by gluons weaken when they areclose together , at a distance of about 10-13 cm, about the diameter of a proton,quarks behave as if they were free.
This is called asymptomatic freedom(1985Quarks). When one draws the quarks apart the force gets stronger, this is indirect contrast with electromagnetic force which gets weaker with the square ofthe distance between the two bodies. Gluons can create other gluons when theymove between quarks. If a quark moves away from a group of others because ithas been hit by a speeding particle, gluons draw from the quarks motion in orderto create more gluons. The larger the number of gluons exchanged the strongerthe binding force.
Supplying additional energy to quarks results in conversionof energy to new quarks and antiquarks with which the first quark combines(1985Quarks). After the discovery of “bottom” and “charm” it was believed that allquarks occur in pairs. This led to the effort to find “top” quark. In 1984 thelaboratory of the European Council for Nuclear Research (CERN) in Genevaobtained experimental evidence of “top’s” existence.
The discovery of “top”completes the theory of natures basic components, quarks(1985 Quarks). Bibliography(1985) Quarks, Encyclopedia Britanica, Encyclopedia Britanica Inc. USA.