Editing Quantum chromodynamics (section)

==Experimental tests==<!-- This section is linked from [[Quantum chromodynamics]] -->
The notion of quark [[flavour (particle physics)|flavors]] was prompted by the necessity of explaining the properties of hadrons during the development of the [[quark model]]. The notion of color was necessitated by the puzzle of the {{SubatomicParticle|Delta++}}. This has been dealt with in the section on [[Quantum chromodynamics#History|the history of QCD]].

The first evidence for quarks as real constituent elements of hadrons was obtained in [[deep inelastic scattering]] experiments at [[SLAC]]. The first evidence for gluons came in [[three-jet event]]s at [[PETRA]].<ref>{{Cite journal |last=Bethke |first=S. |date=2007-04-01 |title=Experimental tests of asymptotic freedom |url=https://www.sciencedirect.com/science/article/pii/S0146641006000615 |journal=Progress in Particle and Nuclear Physics |language=en |volume=58 |issue=2 |pages=351–386 |doi=10.1016/j.ppnp.2006.06.001 |arxiv=hep-ex/0606035 |bibcode=2007PrPNP..58..351B |s2cid=14915298 |issn=0146-6410}}</ref>

Several good quantitative tests of perturbative QCD exist:
* The [[coupling constant#QCD and asymptotic freedom|running of the QCD coupling]] as deduced from many observations
* [[Bjorken scaling|Scaling violation]] in polarized and unpolarized [[deep inelastic scattering]]
* [[Vector boson]] production at [[collider]]s (this includes the [[Drell–Yan process]])
* [[Direct photons]] produced in hadronic collisions
* [[Jet (particle physics)|Jet cross sections]] in colliders
* [[Event shape observables]] at the [[LEP]]
* Heavy-quark production in colliders

Quantitative tests of non-perturbative QCD are fewer, because the predictions are harder to make. The best is probably the running of the QCD coupling as probed through [[lattice QCD|lattice]] computations of heavy-quarkonium spectra. There is a recent claim about the mass of the heavy meson B<sub>c</sub> . Other non-perturbative tests are currently at the level of 5% at best. Continuing work on masses and [[Form factor (QFT)|form factor]]s of hadrons and their weak matrix elements are promising candidates for future quantitative tests. The whole subject of [[quark matter]] and the [[quark–gluon plasma]] is a non-perturbative test bed for QCD that still remains to be properly exploited.{{Citation needed|date=September 2020}}

One qualitative prediction of QCD is that there exist composite particles made solely of [[gluon]]s called [[glueball]]s that have not yet been definitively observed experimentally.  A definitive observation of a glueball with the properties predicted by QCD would strongly confirm the theory.  In principle, if glueballs could be definitively ruled out, this would be a serious experimental blow to QCD.  But, as of 2013, scientists are unable to confirm or deny the existence of glueballs definitively, despite the fact that particle accelerators have sufficient energy to generate them.