Quarks- any group of subatomic particles believed to be among the basic components if
matter
Quarks are believed to be the fundamental constituents of matter, and have no apparent
structure. They are the particles that make up protons and neutrons, which make up the
nucleus of atoms. Also, particles that interact by means of the strong force, the force
that holds parts of the nucleus together, are explained in terms of quarks. Other
baryons are explained in terms of quarks(1985 Quarks).
Quarks have mass and exhibit spin, the type of intrinsic angular momentum corresponding
to rotation around an axis, equal to half the basic quantum mechanical unit of angular
momentum, obeying Pauli's exclusion principle. This principle that no two particles
having half integral spin can exist in the same quantum state(1985 Quarks).
Quarks always occur in combination with other quarks, they never occur alone.
Physicists have attempted to knock a single quark free from a group using a particle
accelerator, but have failed. Mesons contain a quark and an antiquark, up, down, and
strange, while baryons contain three quarks distinguished by flavours. Each has a charge
that is a fraction of that of an electron. Up and down quarks make up protons and
neutrons, and can be observed in ordinary matter. Strange quarks can be observed in
omega-minus and other short lived subatomic particles which play on part in ordinary
matter(1985 Quarks).
The interpretation of quarks as physical entities poses two problems. First, sometimes
two or three identical quarks have to be in the same quantum state which, because they
have to have half integral spin, violates Pauli's exclusion principal. Second, quarks
appear to not be able to be separated from the particles they make up. Although the
force holding the quarks together is strong it is improbable that it could withstand
bombardment from high energy and neutrinions in a particle accelorator(1985 Quarks).
Quantum chromodynamics(QCD) ascribes colours red, green, and blue to quarks and
minus-red, minus-green, and minus-blue to antiquarks. Combinations of quarks must
contain equal mixtures of colours so that they cancel each other out. Colour involves
the exchange of massless particles, gluons. Gluons transfer the forces which bind quarks
together. Quarks
change colour as they emit and absorb gluons. The exchange of gluons is what maintains
the right quark colour distribution. The forces carried by gluons weaken when they are
close 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(1985 Quarks).
When one draws the quarks apart the force gets stronger, this is in direct contrast with
electromagnetic force which gets weaker with the square of the distance between the two
bodies. Gluons can create other gluons when they move between quarks. If a quark moves
away from a group of others because it has been hit by a speeding particle, gluons draw
from the quarks motion in order to create more gluons. The larger the number of gluons
exchanged the stronger the binding force. Supplying additional energy to quarks results
in conversion of energy to new quarks and antiquarks with which the first quark
combines(1985 Quarks).
After the discovery of "bottom" and "charm" it was believed that all quarks occur in
pairs. This led to the effort to find "top" quark. In 1984 the laboratory of the
European Council for Nuclear Research (CERN) in Geneva obtained 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.
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