Gluons (
glue and the
suffix
on) are
elementary expressions of
quark interaction, and are indirectly involved with
the binding of
protons and
neutrons together in
atomic
nuclei. The
antiparticle of a gluon
is another gluon (see
Eight gluon
colors below).
In technical terms, they are
vector
gauge bosons
that mediate
strong color charge interactions of
quarks in
quantum
chromodynamics (QCD). Unlike the
electrically neutral photon of
quantum
electrodynamics (QED), gluons themselves carry color charge and
therefore participate in the strong interaction in addition to
mediating it, making QCD significantly harder to analyze than
QED.
Properties
The gluon is a vector boson; like the
photon,
it has a
spin of 1. While massive
spin1 particles have three polarization
states, massless gauge bosons like the gluon have only two
polarization states because
gauge
invariance requires the polarization to be transverse. In
quantum field theory, unbroken
gauge invariance requires that gauge bosons have zero mass
(experiment limits the gluon's mass to less than a few
MeV/c
^{2}). The gluon has negative intrinsic
parity.
Numerology of gluons
Unlike the single
photon of QED or the three
W and Z bosons of the
weak interaction, there are eight
independent types of gluon in QCD.
This may be difficult to understand intuitively.
Quarks carry three types of
color charge; antiquarks carry three types of
anticolor. Gluons may be thought of as carrying both color and
anticolor, but to correctly understand how they are combined, it is
necessary to consider the mathematics of color charge in more
detail.
Color charge and superposition
In
quantum mechanics, the states
of particles may be added according to the
principle of superposition; that is,
they may be in a "combined state" with a
probability, if
some particular quantity is measured, of giving several different
outcomes. A relevant illustration in the case at hand would be a
gluon with a color state described by:
 (r\bar{b}+b\bar{r})/\sqrt{2}
This is read as "red–antiblue plus blue–antired." (The factor of
the square root of two is required for
normalization, a detail which is
not crucial to understand in this discussion.) If one were somehow
able to make a direct measurement of the color of a gluon in this
state, there would be a 50% chance of it having red–antiblue color
charge and a 50% chance of blue–antired color charge.
Color singlet states
It is often said that the
stable
stronglyinteracting particles observed in nature are
"colorless," but more precisely they are in a "color singlet"
state, which is mathematically analogous to a
spin singlet state. Such states allow
interaction with other color singlets, but not with other color
states; because longrange gluon interactions do not exist, this
illustrates that gluons in the singlet state do not exist
either.
The color singlet state is:
 (r\bar{r}+b\bar{b}+g\bar{g})/\sqrt{3}
In words, if one could measure the color of the state, there would
be equal probabilities of it being redantired, blueantiblue, or
greenantigreen.
Eight gluon colors
There are eight remaining independent color states, which
correspond to the "eight types" or "eight colors" of gluons.
Because states can be mixed together as discussed above, there are
many ways of presenting these states, which are known as the "color
octet." One commonly used list is:
(r\bar{b}+b\bar{r})/\sqrt{2} 

i(r\bar{b}b\bar{r})/\sqrt{2} 
(r\bar{g}+g\bar{r})/\sqrt{2} 

i(r\bar{g}g\bar{r})/\sqrt{2} 
(b\bar{g}+g\bar{b})/\sqrt{2} 

i(b\bar{g}g\bar{b})/\sqrt{2} 
(r\bar{r}b\bar{b})/\sqrt{2} 

(r\bar{r}+b\bar{b}2g\bar{g})/\sqrt{6} 
These are equivalent to the
GellMann
matrices; the translation between the two is that redantired
is the upperleft matrix entry, redantiblue is the left middle
entry, blueantigreen is the bottom middle entry, and so on. The
critical feature of these particular eight states is that they are
linearly independent, and also
independent of the singlet state; there is no way to add any
combination of states to produce any other. (It is also impossible
to add them to make r\bar{r}, g\bar{g}, or b\bar{b}; otherwise the
forbidden
singlet state could also be
made.) There are many other possible choices, but all are
mathematically equivalent, at least equally complex, and give the
same physical results.
Group theory details
Technically, QCD is a
gauge theory with
SU gauge symmetry. Quarks are introduced as
spinor fields in
N_{f}
flavour, each in the
fundamental
representation (triplet, denoted
3) of the
color gauge group, SU(3). The gluons are vector fields in the
adjoint representation
(octets, denoted
8) of color SU(3). For a general
gauge group, the number of forcecarriers
(like photons or gluons) is always equal to the dimension of the
adjoint representation. For the simple case of SU(
N), the
dimension of this representation is .
In terms of group theory, the assertion that there are no color
singlet gluons is simply the statement that
quantum chromodynamics has an
SU rather than a
U
symmetry. There is no known
a priori
reason for one group to be preferred over the other, but as
discussed above, the experimental evidence supports SU(3).
Confinement
Since gluons themselves carry color charge, they participate in
strong interactions. These gluongluon interactions constrain color
fields to stringlike objects called "flux tubes", which exert
constant force when stretched. Due to this force,
quarks are
confined
within
composite particles called
hadrons. This effectively limits the range of
the strong interaction to 10
^{−15} meters, roughly the size
of an
atomic nucleus. (Beyond a
certain distance, the energy of the flux tube binding two quarks
increases linearly. At a large enough distance, it becomes
energetically more favorable to pull a quarkantiquark pair out of
the vacuum rather than increase the length of the flux tube.)
Gluons also share this property of being confined within hadrons.
One consequence is that gluons are not directly involved in the
nuclear forces between hadrons. The
force mediators for these are other hadrons called
mesons.
Although in the
normal phase of
QCD single gluons may not travel freely, it is predicted that
there exist
hadrons which are formed entirely
of gluons — called
glueballs. There are also conjectures
about other
exotic
hadrons in which real gluons (as opposed to
virtual ones found in ordinary hadrons)
would be primary constituents. Beyond the normal phase of QCD (at
extreme temperatures and pressures),
quark gluon plasma forms. In such a
plasma there are no hadrons; quarks and gluons become free
particles.
Experimental observations
The first direct experimental evidence of gluons was found in 1979
when
threejet events were observed
at the electronpositron collider
PETRA.
However, just before PETRA appeared on the scene, the PLUTO
experiment at
DORIS showed event topologies
suggestive of a threegluon decay.
Experimentally, confinement is verified by the failure of
free quark searches. Free gluons have
never been observed, however at Fermilab single production of top
quarks has been statistically shown. Although there have been hints
of exotic hadrons, no glueball has been observed either.
Quarkgluon plasma has been found recently at
the Relativistic Heavy Ion Collider (RHIC) at Brookhaven
National Laboratories (BNL).
See also
References and external links
 D.J. Griffiths (1987), pp. 280–281
 D.J. Griffiths (1987), p. 281
 D.J. Griffiths (1987), p. 280
External links