A gluon () is an elementary particle that
acts as the exchange particle (or gauge boson)
for the strong force between quarks. It is
analogous to the exchange of photons in the
electromagnetic force between two charged
particles. In layman's terms, they "glue"
quarks together, forming hadrons such as protons
and neutrons.
In technical terms, gluons are vector gauge
bosons that mediate strong interactions of
quarks in quantum chromodynamics (QCD). 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 (quantum electrodynamics).
== 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 (experiments
limit the gluon's rest mass to less than a
few meV/c2). The gluon has negative intrinsic
parity.
== Counting 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. This gives nine possible
combinations of color and anticolor in gluons.
The following is a list of those combinations
(and their schematic names):
red-antired (
r
r
¯
{\displaystyle r{\bar {r}}}
), red-antigreen (
r
g
¯
{\displaystyle r{\bar {g}}}
), red-antiblue (
r
b
¯
{\displaystyle r{\bar {b}}}
)
green-antired (
g
r
¯
{\displaystyle g{\bar {r}}}
), green-antigreen (
g
g
¯
{\displaystyle g{\bar {g}}}
), green-antiblue (
g
b
¯
{\displaystyle g{\bar {b}}}
)
blue-antired, (
b
r
¯
{\displaystyle b{\bar {r}}}
), blue-antigreen (
b
g
¯
{\displaystyle b{\bar {g}}}
), blue-antiblue (
b
b
¯
{\displaystyle b{\bar {b}}}
)
These are not the actual color states of observed
gluons, but rather effective states. 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
b
¯
+
b
r
¯
)
/
2
.
{\displaystyle (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 that 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 strongly
interacting particles (such as the proton
and the neutron, i.e. hadrons) 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:
(
r
r
¯
+
b
b
¯
+
g
g
¯
)
/
3
.
{\displaystyle (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 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 critical feature of these particular eight
states is that they are linearly independent,
and also independent of the singlet state,
hence 32 − 1 or 23. There is no way to add
any combination of these states to produce
any other, and it is also impossible to add
them to make rr, gg, or bb the forbidden singlet
state. There are many other possible choices,
but all are mathematically equivalent, at
least equally complicated, and give the same
physical results.
=== Group theory details ===
Technically, QCD is a gauge theory with SU(3)
gauge symmetry. Quarks are introduced as spinors
in Nf flavors, each in the fundamental representation
(triplet, denoted 3) of the color gauge group,
SU(3). The gluons are vectors in the adjoint
representation (octets, denoted 8) of color
SU(3). For a general gauge group, the number
of force-carriers (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 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). The U(1) group for electromagnetic
field combines with a slightly more complicated
group known as SU(2) – S stands for "special"
– which means the corresponding matrices
have determinant 1 in addition to being unitary.
== 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 1×10−15 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 (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 ==
Quarks and gluons (colored) manifest themselves
by fragmenting into more quarks and gluons,
which in turn hadronize into normal (colorless)
particles, correlated in jets. As shown in
1978 summer conferences, the PLUTO detector
at the electron-positron collider DORIS (DESY)
produced the first evidence that the hadronic
decays of the very narrow resonance Υ(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 (see also the recollection and
PLUTO experiments).
In summer 1979, at higher energies at the
electron-positron collider PETRA (DESY), again
three-jet topologies were observed, now interpreted
as qq gluon bremsstrahlung, now clearly visible,
by TASSO,MARK-J
and PLUTO experiments (later in 1980 also
by JADE). The spin 1 of the gluon was confirmed
in 1980 by TASSO and PLUTO experiments (see
also the review). 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 (gluon density)
have been measured by two experiments, H1
and ZEUS, in the years 1996-2007. The gluon
contribution to the proton spin has been studied
by the HERMES experiment at HERA. The gluon
density in the proton (when behaving hadronically)
also has been measured.Color confinement is
verified by the failure of free quark searches
(searches of fractional charges). Quarks are
normally produced in pairs (quark + antiquark)
to compensate the quantum color and flavor
numbers; however at Fermilab single production
of top quarks has been shown (technically
this still involves a pair production, but
quark and antiquark are of different flavor).
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 (RHIC) 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 (LHC) by the three experiments ALICE,
ATLAS and CMS in 2010.The Continuous Electron
Beam Accelerator Facility at Jefferson Lab,
also called the Thomas Jefferson National
Accelerator Facility, in Newport News, Virginia,
is one of 10 Department of Energy facilities
doing research on gluons. The Virginia lab
is competing with another facility on Long
Island, New York, Brookhaven National Laboratory,
for funds to build a new electron-ion collider.
== See also
