Gluons carry thecolor charge of the strong interaction, thereby participating in the strong interaction as well as mediating it. Because gluons carry the color charge, QCD is more difficult to analyze compared toquantum electrodynamics (QED) where thephoton carries no electric charge.
The gluon is avector boson, which means it has aspin of 1 ħ. While massive spin-1 particles have three polarization states,massless gauge bosons like the gluon have only two polarization states becausegauge invariance requires the field polarization to be transverse to the direction that the gluon is traveling. Inquantum field theory, unbroken gauge invariance requires that gauge bosons have zero mass. Experiments limit the gluon's rest mass (if any) to less than a few MeV/c2. The gluon has negative intrinsicparity.
There are eight independent types of gluons in QCD. This is unlike the photon of QED or the threeW and Z bosons of theweak interaction.
Additionally, gluons are subject to thecolor charge phenomena.Quarks carry three types of color charge; antiquarks carry three types of anticolor. Gluons carry both color and anticolor. This gives ninepossible combinations of color and anticolor in gluons. The following is a list of those combinations (and their schematic names):
Thesepossible combinations are onlyeffective states, not theactual observed color states of gluons. To understand how they are combined, it is necessary to consider the mathematics of color charge in more detail.
The stable strongly interacting particles, including hadrons like the proton or the neutron, are observed to be "colorless". More precisely, they are in a "color singlet" state, and mathematically analogous to aspin singlet state.[11] The states allow interaction with other color singlets, but not other color states; because long-range gluon interactions do not exist, this illustrates that gluons in the singlet state do not exist either.[11]
There are eight remaining independent color states corresponding to the "eight types" or "eight colors" of gluons. Since the states can be mixed together, there are multiple ways of presenting these states. These are known as the "color octet", and a commonly used list for each is:[11]
These are equivalent to theGell-Mann matrices. The critical feature of these particular eight states is that they arelinearly 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 others. It is also impossible to add them to makerr,gg, orbb[12] the forbiddensinglet state. There are many other possible choices, but all are mathematically equivalent, at least equally complicated, and give the same physical results.
Formally, QCD is agauge theory withSU(3) gauge symmetry. Quarks are introduced asspinors inNfflavors, each in thefundamental representation (triplet, denoted3) of the color gauge group, SU(3). The gluons are vectors in theadjoint representation (octets, denoted8) of color SU(3). For a generalgauge 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 isn2 − 1.
In group theory, there are no color singlet gluons becausequantum chromodynamics has an SU(3) rather than aU(3) symmetry. There is no knowna priori reason for one group to be preferred over the other, but as discussed above, the experimental evidence supports SU(3).[11] If the group were U(3), the ninth (colorless singlet) gluon would behave like a "second photon" and not like the other eight gluons.[13]
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 areconfined withincomposite particles calledhadrons. This effectively limits the range of the strong interaction to10−15 m, roughly the size of anucleon. 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.
One consequence of the hadron-confinement property of gluons is that they are not directly involved in thenuclear forces between hadrons. The force mediators for these are other hadrons calledmesons.
Although in thenormal phase of QCD single gluons may not travel freely, it is predicted that there exist hadrons that are formed entirely of gluons — calledglueballs. There are also conjectures about otherexotic hadrons in which real gluons (as opposed tovirtual 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.
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 revealed in 1978 summer conferences,[2] thePLUTO 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 asthree-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[14][15] (see also the recollection[2] andPLUTO experiments).
In summer 1979, at higher energies at the electron-positron colliderPETRA (DESY), again three-jet topologies were observed, now clearly visible and interpreted as qq gluonbremsstrahlung, byTASSO,[16]MARK-J[17] and PLUTO experiments[18] (later in 1980 also byJADE[19]). The spin = 1 property of the gluon was confirmed in 1980 by TASSO[20] and PLUTO experiments[21] (see also the review[3]). In 1991 a subsequent experiment at theLEP storage ring atCERN again confirmed this result.[22]
The gluons play an important role in the elementary strong interactions betweenquarks and gluons, described by QCD and studied particularly at the electron-proton colliderHERA at DESY. The number and momentum distribution of the gluons in theproton (gluon density) have been measured by two experiments,H1 andZEUS,[23] in the years 1996–2007. The gluon contribution to the proton spin has been studied by theHERMES experiment at HERA.[24] The gluon density in the proton (when behaving hadronically) also has been measured.[25]
Color confinement is verified by the failure offree quark searches (searches of fractional charges). Quarks are normally produced in pairs (quark + antiquark) to compensate the quantum color and flavor numbers; however atFermilab single production oftop quarks has been shown.[b][26] Noglueball has been demonstrated.
