Quarks basic quantum mechanical unit of angular momentum,

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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).

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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

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

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

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).

(1985) Quarks, Encyclopedia Britanica, Encyclopedia Britanica Inc. USA.

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