Strong interaction

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In particle physics, the strong interaction, or strong force, or color force, holds quarks and gluons together to form protons and neutrons.

For an understanding of the interaction that holds protons and neutrons together to form atomic nuclei, see nuclear force.

The term 'strong interaction' is also used to describe food webs.

The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction. Of the four fundamental forces, the strong interaction is the most powerful.

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).

[edit] History

Before the 1970s, protons and neutrons were thought to be indivisible fundamental particles. It was known that:

1. Protons carried a positive electrical charge,

2. electric repulsion made same-charge particles repel each other, and

3. Multiple protons were bound together in the atomic nucleus.

So, what held protons together in the nucleus?

Another, stronger, attractive force was postulated to explain how protons were held together in the atomic nucleus, overcoming electromagnetic repulsion. For its high strength (at short distances), it was dubbed the "strong force". It was thought, at that time, this strong force was a fundamental force acting directly on the protons.

It was later discovered this phenomena was only a residual side-effect of another, truly fundamental, force acting directly on particles inside protons called quarks and gluons. This newly-discovered force was initially called the "color force." This has no relation to visible color.

Today, the term "strong force" is used for that strong nuclear force that acts directly on quarks and gluons. The original strong force that acts on protons is today called the nuclear force or residual strong nuclear force.

[edit] Details

The strong force is postulated in Quantum chromodynamics, QCD.

QCD is 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).

All particles 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. This refers to a group theoretical property explained in the article on color charge.

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 upon only elementary quark and gluon particles.

A residual effect of the strong force is called the nuclear force. The nuclear force acts between hadrons, such as nucleons in atomic nuclei.

The 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 strong force, unlike other forces, does not diminish in strength with increasing distance. As a result, quarks are always permanently bound together into hadrons. It is impossible to separate individual quarks as can be done with protons. In QCD, this phenomenon is called confinement. So only hadrons can be observed. This has been shown by many failed free quark searches. The elementary quark and gluon particles affected are unobservable directly.

[edit] See also

Weak interaction, electromagnetism and gravity
Standard model of particle physics and its field theoretical formulation.
Quantum field theory and gauge theory
Quantum chromodynamics and Quark matter
Internucleon force and nuclear physics
Coupling constant
Binding energy
Nuclear force
Color charge
CERN
Quark-gluon plasma

[edit] References

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

[edit] 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.