gluon
the so-called messenger particle of the strong nuclear force, which
binds subatomic particles known as quarks within the protons and
neutrons of stable matter as well as within heavier, short-lived
particles created at high energies. Quarks interact by emitting
and absorbing gluons, just as electrically charged particles interact
through the emission and absorption of photons.
In quantum chromodynamics (QCD), the theory of the strong force,
the interactions of quarks are described in terms of eight types
of massless gluon, which, like the photon, all carry one unit of
intrinsic angular momentum, or spin. Like quarks, the gluons carry
a "strong charge" known as colour; this means that gluons can interact
between themselves through the strong force. In 1979 confirmation
of the conception came with the observation of the radiation of
gluons by quarks in studies of high-energy particle collisions at
the German national laboratory, Deutsches Elektronen-Synchrotron
(DESY; "German Electron-Synchrotron), in Hamburg.
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 state--an 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 cm--about the diameter of a proton--quarks 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.
Odgovorila:
Tatjana Maricic: Tatjana.Maricic@public.srce.hr
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