Editing Strong interaction (section)

== Behavior of the strong interaction ==

The strong interaction is observable at two ranges, and mediated by different force carriers in each one. On a scale less than about 0.8&nbsp;[[femtometre|fm]] (roughly the radius of a nucleon), the force is carried by [[gluon|gluons]] and holds [[quark|quarks]] together to form protons, neutrons, and other hadrons. On a larger scale, up to about 3&nbsp;fm, the force is carried by [[meson|mesons]] and binds nucleons ([[proton|protons]] and [[neutron|neutrons]]) together to form the [[atomic nucleus|nucleus]] of an [[atom]].<ref name="auto">{{cite web| url = http://webhome.phy.duke.edu/~kolena/modern/forces.html#005| title = The four forces: the strong interaction Duke University Astrophysics Dept website}}</ref> In the former context, it is often known as the '''color force''', and is so strong that if hadrons are struck by high-energy particles, they produce [[Jet (particle physics)|jets]] of massive particles instead of emitting their constituents (quarks and gluons) as freely moving particles. This property of the strong force is called [[color confinement]].

{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
|+ Two layers of strong interaction
|-
! Interaction  !! range !! held !! carrier !! result
|-
| Strong || style=text-align:right |< {{val|0.8|u=fm}} || quark || gluon || hadron
|-
| Residual Strong || style=text-align:right | {{val|1|–|3|u=fm}}  || hadron || meson || nucleus
|}

=== Within hadrons ===
[[File:Gluon coupling.svg|thumb|right|400px|The fundamental [[coupling (physics)|couplings]] of the strong interaction, from left to right: (a) gluon radiation, (b) gluon splitting and (c,d) gluon self-coupling.]]
The word ''strong'' is used since the strong interaction is the "strongest" of the four fundamental forces. At a distance of 10<sup>−15</sup>&nbsp;m, its strength is around 100&nbsp;times that of the [[electromagnetic force]], some 10<sup>6</sup>&nbsp;times as great as that of the weak force, and about 10<sup>38</sup>&nbsp;times that of [[gravitation]].

The strong force is described by [[quantum chromodynamics]] (QCD), a part of the [[Standard Model]] of particle physics. Mathematically, QCD is a non-abelian [[gauge theory]] based on a local (gauge) [[symmetry group]] called [[SU(3)]].

The force carrier particle of the strong interaction is the gluon, a massless [[gauge boson]]. Gluons are thought to interact with quarks and other gluons by way of a type of charge called [[color charge]]. Color charge is analogous to electromagnetic charge, but it comes in three types (±red, ±green, and ±blue) rather than one, which results in different rules of behavior. These rules are described by [[quantum chromodynamics]] (QCD), the theory of quark–gluon interactions.
Unlike the [[photon]] in electromagnetism, which is neutral, the gluon carries a color charge. Quarks and gluons are the only fundamental particles that carry non-vanishing color charge, and hence they participate in strong interactions only with each other. The strong force is the expression of the gluon interaction with other quark and gluon particles.

All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parameterized by the strong [[coupling constant]]. This strength is modified by the gauge color charge of the particle, a [[Group theory|group-theoretical]] property.

The strong force acts between quarks. Unlike all other forces (electromagnetic, weak, and gravitational), the strong force does not diminish in strength with increasing distance between pairs of quarks. After a limiting distance (about the size of a [[hadron]]) has been reached, it remains at a strength of about {{val|10000|ul=N}}, no matter how much farther the distance between the quarks.<ref name=Fritzsch1983/>{{rp|164}}  As the separation between the quarks grows, the energy added to the pair creates new pairs of matching quarks between the original two; hence it is impossible to isolate quarks. The explanation is that the amount of work done against a force of {{val|10000|u=N}} is enough to create particle–antiparticle pairs within a very short distance. The energy added to the system by pulling two quarks apart would create a pair of new quarks that will pair up with the original ones. In QCD, this phenomenon is called [[color confinement]]; as a result, only hadrons, not individual free quarks, can be observed. The failure of all experiments that have searched for [[free quark]]s is considered to be evidence of this phenomenon.

The elementary quark and gluon particles involved in a high energy collision are not directly observable. The interaction produces jets of newly created hadrons that are observable. Those hadrons are created, as a manifestation of mass–energy equivalence, when sufficient energy is deposited into a quark–quark bond, as when a quark in one proton is struck by a very fast quark of another impacting proton during a [[particle accelerator]] experiment. However, [[quark–gluon plasma]]s have been observed.<ref>{{cite news |url=http://physics.about.com/od/physicsqtot/fl/Quark-Gluon-Plasma.htm |title=Quark–gluon plasma is the most primordial state of matter |newspaper=About.com Education |access-date=2017-01-16 |archive-url=https://web.archive.org/web/20170118030724/http://physics.about.com/od/physicsqtot/fl/Quark-Gluon-Plasma.htm |archive-date=2017-01-18 |url-status=dead }}</ref>

=== Between hadrons ===
{{main|Nuclear force}}
[[File:Pn_Scatter_Quarks.svg|thumb|300x300px|A [[Feynman diagram]] (shown by the animation in the lead) with the individual [[quark]] constituents shown, to illustrate how the fundamental strong interaction gives rise to the [[nuclear force]]. Straight lines are quarks, while multi-colored loops are [[Gluon|gluons]] (the carriers of the fundamental force).]]
While color confinement implies that the strong force acts without distance-diminishment between pairs of quarks in compact collections of bound quarks (hadrons), at distances approaching or greater than the radius of a proton, a residual force (described below) remains. It manifests as a force between the "colorless" hadrons, and is known as the ''[[nuclear force]]'' or ''residual strong force'' (and historically as the ''strong nuclear force'').

The nuclear force acts between hadrons, known as [[meson]]s and [[baryon]]s. This "residual strong force", acting indirectly, transmits gluons that form part of the virtual [[pion|π]] and [[rho meson|ρ]]&nbsp;[[meson]]s, which, in turn, transmit the force between nucleons that holds the nucleus (beyond [[hydrogen-1]] nucleus) together.<ref>{{cite web |title=3. The Strong Force |url=http://www.damtp.cam.ac.uk/user/tong/pp/pp3.pdf |publisher=Department of Applied Mathematics and Theoretical Physics, University of Cambridge |access-date=10 January 2023 |archive-url=https://web.archive.org/web/20211022113145/http://www.damtp.cam.ac.uk/user/tong/pp/pp3.pdf |archive-date=22 October 2021}}</ref>

The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker ''between'' neutrons and protons, because it is mostly neutralized ''within'' them, in the same way that electromagnetic forces between neutral atoms ([[van der Waals force]]s) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms.<ref name=Fritzsch1983>
{{cite book
 |last=Fritzsch |first=H.
 |year=1983
 |title=Quarks: The Stuff of Matter
 |url=https://archive.org/details/quarksstuffofmat00frit |url-access=registration |publisher=Basic Books
 |isbn=978-0-465-06781-7
 |pages=[https://archive.org/details/quarksstuffofmat00frit/page/167 167–168]
}}</ref>

Unlike the strong force, the residual strong force diminishes with distance, and does so rapidly. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see ''[[Yukawa potential]]''. The rapid decrease with distance of the attractive residual force and the less rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with [[atomic number]]s larger than 82 (the element lead).

Although the nuclear force is weaker than the strong interaction itself, it is still highly energetic: transitions produce [[gamma ray]]s. The mass of a nucleus is significantly different from the summed masses of the individual nucleons. This [[mass defect]] is due to the potential energy associated with the nuclear force. Differences between mass defects power [[nuclear fusion]] and [[nuclear fission]].