Editing Quantum chromodynamics (section)

{{short description|Theory of the strong nuclear interactions}}
{{Redirect|QCD}}
{{Standard model of particle physics|cTopic=quantum chromodynamics}}

In [[theoretical physics]], '''quantum chromodynamics''' ('''QCD''') is the study of the [[strong interaction]] between [[quark]]s mediated by [[gluon]]s. Quarks are fundamental particles that make up composite [[hadron]]s such as the [[proton]], [[neutron]] and [[pion]]. QCD is a type of [[quantum field theory]] called a [[non-abelian gauge theory]], with symmetry group [[special unitary group|SU(3)]]. The QCD analog of electric charge is a property called ''color''. Gluons are the [[force carrier]]s of the theory, just as photons are for the electromagnetic force in [[quantum electrodynamics]]. The theory is an important part of the [[Standard Model]] of [[particle physics]]. A large body of [[Quantum chromodynamics#Experimental tests|experimental evidence for QCD]] has been gathered over the years.

QCD exhibits three salient properties:
* [[Color confinement]]. Due to the force between two color charges remaining constant as they are separated, the energy grows until a quark–antiquark pair is [[mass–energy equivalence|spontaneously produced]], turning the initial hadron into a pair of hadrons instead of isolating a color charge. Although analytically unproven, color confinement is well established from [[lattice QCD]] calculations and decades of experiments.<ref>{{cite book
 |author=J. Greensite
 |year=2011
 |title=An introduction to the confinement problem
 |publisher=[[Springer Science+Business Media|Springer]]
 |isbn=978-3-642-14381-6
}}</ref>
* [[Asymptotic freedom]], a steady reduction in the strength of interactions between quarks and gluons as the energy scale of those interactions increases (and the corresponding length scale decreases).  The asymptotic freedom of QCD was discovered in 1973 by [[David Gross]] and [[Frank Wilczek]],<ref name=GrossWilczek>
{{cite journal
 |author1=D.J. Gross |author2=F. Wilczek |year=1973
 |title=Ultraviolet behavior of non-abelian gauge theories
 |journal=[[Physical Review Letters]]
 |volume=30 |issue= 26|pages= 1343–1346
 |bibcode=1973PhRvL..30.1343G
 |doi=10.1103/PhysRevLett.30.1343
|doi-access=free}}</ref> and independently by [[David Politzer]] in the same year.<ref name=Politzer>
{{cite journal
 |author=H.D. Politzer
 |year=1973
 |title=Reliable perturbative results for strong interactions
 |journal=[[Physical Review Letters]]
 |volume=30 |issue=26 |pages=1346–1349
 |bibcode=1973PhRvL..30.1346P
 |doi=10.1103/PhysRevLett.30.1346
|doi-access=free
 }}</ref>  For this work, all three shared the 2004 [[Nobel Prize in Physics]].<ref name=Nobel>{{cite web
 |url         = http://nobelprize.org/nobel_prizes/physics/laureates/2004/
 |title       = The Nobel Prize in Physics 2004
 |publisher   = Nobel Web
 |year        = 2004
 |access-date  = 2010-10-24
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20101106025744/http://nobelprize.org/nobel_prizes/physics/laureates/2004/
 |archive-date = 2010-11-06
}}</ref>
*[[Chiral symmetry breaking]], the [[spontaneous symmetry breaking]] of an important global symmetry of quarks, detailed below, with the result of generating masses for hadrons far above the masses of the quarks, and making pseudoscalar mesons exceptionally light.  [[Yoichiro Nambu]] was awarded the 2008 Nobel Prize in Physics for elucidating the phenomenon in 1960, a dozen years before the advent of QCD. Lattice simulations have confirmed all his generic predictions.