strong interaction

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In particle physics, the strong interaction (also called the strong force, strong nuclear force, or color force) holds quarks and gluons together to form protons, neutrons and other particles. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction. The word strong is used since the strong interaction is the "strongest" of the four fundamental forces; its typical field strength is 100 times the strength of the electromagnetic force, some 10⁵ times as great as that of the weak force, and about 10³⁹ times that of gravitation.

The strong interaction is also the force that binds protons and neutrons together. In this context it is called the nuclear force (or residual strong force), and it is the residue strong interaction between the quarks that make up the protons and neutrons.

The strong force is thought to be mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. This is detailed in the theory of quantum chromodynamics (QCD).

History

Before the 1970s, protons and neutrons were thought to be indivisible elementary particles. It was known that protons carried a positive electrical charge. Despite the fact that electromagnetic repulsion made same-charge particles repel each other, multiple protons were bound tightly together in the atomic nucleus along with uncharged neutrons, but the binding mechanism was unknown.

A stronger attractive force was postulated to explain how the atomic nucleus was bound together despite the protons' mutual electromagnetic repulsion. This hypothesized force was called the strong force, which was believed to be a fundamental force that acted on the nucleons (the protons and neutrons that make up the nucleus). Experiments suggested that this force bound protons and neutrons together with equal strength.

It was later discovered protons and neutrons were not fundamental particles, but were made up of constituent particles called quarks. The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together in the protons and neutrons. The theory of quantum chromodynamics explains that quarks carry what is called a color charge, although it has no relation to visible color.^[1] Quarks with unlike color charge attract one another as a result of the strong interaction, which is mediated by particles called gluons.

Details

The behavior of the strong force

The contemporary 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).

Quarks and gluons are the only fundamental particles which carry non-vanishing color charge, and hence participate in strong interactions. The strong force itself acts directly only upon elementary quark and gluon particles.

All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parametrized by the strong coupling constant. This strength is modified by the gauge color charge of the particle, a group theoretical property which has nothing to do with ordinary visual color.

The strong force acting between quarks, unlike other forces, does not diminish in strength with increasing distance, after a limit (about the size of a hadron) has been reached. It remains at a strength of about 100,000 newtons,^{[citation needed]} no matter what distance separates the quarks after this limiting distance is reached. In QCD, this phenomenon is called color confinement; it implies that only hadrons, not individual free quarks, can be observed. The explanation is that the amount of work done against a force of 100,000 newtons is enough to create particle-antiparticle pairs within a very short distance of an interaction. In simple terms, the very energy applied to pull two quarks apart will turn into new quarks that pair up again with the original ones. The failure of all experiments that have searched for free quarks is considered to be evidence for this phenomenon.

The elementary quark and gluon particles affected are unobservable directly, but instead emerge as jets of newly created hadrons, whenever energy is deposited into a quark-quark bond, as when a quark in a proton is struck by a very fast quark (in an impacting proton) during an accelerator experiment. However, quark-gluon plasmas have been observed.

The behavior of the residual strong force (nuclear force)

A residual effect of the strong force is called the nuclear force. The nuclear force acts between hadrons, such as nucleons in atomic nuclei. This "residual strong force," acting indirectly, transmits gluons that form part of the virtual pi and rho mesons, which, in turn, transmit the nuclear force between nucleons.

The residual strong force is thus a minor residuum of the strong force which 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 forces) are much weaker than the electromagnetic forces that hold the atoms internally together.

Unlike the strong force itself, the nuclear force, or residual strong force, does diminish in strength, strongly with distance. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see Yukawa potential. This fact, together with the less-rapid decrease of the disruptive electromagnetic force between protons with distance, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82.

See also

• Binding energy
• Color charge
• Coupling constant
• Nuclear physics
• QCD matter
• Quantum field theory and Gauge theory
• Standard model of particle physics and Standard Model (mathematical formulation)
• Weak interaction, electromagnetism and gravity

References

Further reading

• Griffiths, David J. (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4. 
• Halzen, Francis; Martin, Alan D. (1984). Quarks and Leptons: An Introductory Course in Modern Particle Physics. John Wiley & Sons. ISBN 0-471-88741-2. 
• Kane, Gordon L. (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5. 
• Morris, Richard (2003). The Last Sorcerers: The Path from Alchemy to the Periodic Table. Washington, D.C.: Joseph Henry Press. ISBN 0-309-50593-3. 

External links

• MISN-0-280: The Strong Interaction (PDF file) by J.R. Christman for Project PHYSNET.
• The Alice Experiment at CERN is the heavy ion collaboration that will investigate aspects of the Strong Nuclear Force upon the completion of the building of the Large Hadron Collider at CERN.
• The Star Experiment at the Relativistic Heavy Ion Collider at Brookhaven Nation Laboratory in New York, USA. Investigates many aspects of the Strong Nuclear Force including the theoretical Quark Gluon Plasma.