Gluons are elementary particles that act as
the exchange particles for the strong force
between quarks, analogous to the exchange
of photons in the electromagnetic force between
two charged particles.
In technical terms, gluons are vector gauge
bosons that mediate strong interactions of
quarks in quantum chromodynamics.
Gluons themselves carry the color charge of
the strong interaction.
This is unlike the photon, which mediates
the electromagnetic interaction but lacks
an electric charge.
Gluons 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 spin-1 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.
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:
This is read as "red–antiblue plus blue–antired".
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 strongly
interacting 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 long-range gluon interactions do not
exist, this illustrates that gluons in the
singlet state do not exist either.
The color singlet state is:
In words, if one could measure the color of
the state, there would be equal probabilities
of it being red-antired, blue-antiblue, or
green-antigreen.
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:
These are equivalent to the Gell-Mann matrices;
the translation between the two is that red-antired
is the upper-left matrix entry, red-antiblue
is the upper middle entry, blue-antigreen
is the middle right 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.
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(3)
gauge symmetry.
Quarks are introduced as spinor fields in
Nf flavors, each in the fundamental representation
of the color gauge group, SU(3).
The gluons are vector fields in the adjoint
representation of color SU(3).
For a general gauge group, the number of force-carriers
is always equal to the dimension of the adjoint
representation.
For the simple case of SU(N), the dimension
of this representation is N2 − 1.
In terms of group theory, the assertion that
there are no color singlet gluons is simply
the statement that quantum chromodynamics
has an SU(3) rather than a U(3) 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 gluon-gluon interactions constrain color
fields to string-like 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 quark-antiquark 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 that are formed entirely
of gluons — called glueballs.
There are also conjectures about other exotic
hadrons in which real gluons would be primary
constituents.
Beyond the normal phase of QCD, quark gluon
plasma forms.
In such a plasma there are no hadrons; quarks
and gluons become free particles.
Experimental observations
Quarks and gluons manifest themselves by fragmenting
into more quarks and gluons, which in turn
hadronize into normal particles, correlated
in jets.
As shown in 1978 summer conferences the PLUTO
experiments at the electron-positron collider
DORIS reported the first evidence that the
hadronic decays of the very narrow resonance
Y(9.46) could be interpreted as three-jet
event topologies produced by three gluons.
Later published analyses by the same experiment
confirmed this interpretation and also the
spin 1 nature of the gluon.
In summer 1979 at higher energies at the electron-positron
collider PETRA again three-jet topologies
were observed, now interpreted as qq gluon
bremsstrahlung, now clearly visible, by TASSO,
MARK-J and PLUTO experiments.
The spin 1 of the gluon was confirmed in 1980
by TASSO and PLUTO experiments.
In 1991 a subsequent experiment at the LEP
storage ring at CERN again confirmed this
result.
The gluons play an important role in the elementary
strong interactions between quarks and gluons,
described by QCD and studied particularly
at the electron-proton collider HERA at DESY.
The number and momentum distribution of the
gluons in the proton have been measured by
two experiments, H1 and ZEUS, in the years
1996 till today.
The gluon contribution to the proton spin
has been studied by the HERMES experiment
at HERA.
The gluon density in the photon has also been
measured.
Color confinement is verified by the failure
of free quark searches.
Quarks are normally produced in pairs to compensate
the quantum color and flavor numbers; however
at Fermilab single production of top quarks
has been shown.
No glueball has been demonstrated.
Deconfinement was claimed in 2000 at CERN
SPS in heavy-ion collisions, and it implies
a new state of matter: quark–gluon plasma,
less interacting than in the nucleus, almost
as in a liquid.
It was found at the Relativistic Heavy Ion
Collider at Brookhaven in the years 2004–2010
by four contemporaneous experiments.
A quark–gluon plasma state has been confirmed
at the CERN Large Hadron Collider by the three
experiments ALICE, ATLAS and CMS in 2010.
See also
Quark
Hadron
Meson
Gauge boson
Quark model
Quantum chromodynamics
Quark–gluon plasma
Color confinement
Glueball
Gluon field
Gluon field strength tensor
Exotic hadrons
Standard Model
Three-jet events
Deep inelastic scattering
References
Further reading
A. Ali and G. Kramer.
"JETS and QCD: A historical review of the
discovery of the quark and gluon jets and
its impact on QCD".
European Physical Journal H 36: 245–326.
arXiv:1012.2288.
Bibcode:2011EPJH...36..245A. doi:10.1140e2011-10047-1. 
