Strong nuclear interactions


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Strong nuclear interactions

One of the fundamental physical interactions, which acts between a pair of hadrons. Hadrons include the nucleons, that is, neutrons and protons; the strange baryons, such as lambda (&Lgr;) and sigma (&Sgr;); the mesons, such as pion (&pgr;) and rho (&rgr;); and the strange meson, kaon (K). The nature of the interaction is determined principally through observations of the collision of a hadron pair. From this it is found that the interaction has a short range of about 10-15 m (10-13 in.) and is by far the dominant force within this range, being much larger than the electromagnetic interaction, which is next in magnitude. The strong interaction conserves parity and is time-reversal-invariant. See Baryon, Hadron, Meson, Nucleon, Parity (quantum mechanics), Strange particles, Symmetry laws (physics)

The interaction between baryons for distances greater than 10-15 m arises from the exchange of mesons. At relatively large distances, single-pion exchange dominates (illus. a). At shorter separation distances, the two-pion systems such as the &rgr; become important (illus. b). The interaction between the strange baryons, and between the strange baryons and the nucleons, is moderated by kaon exchange. To summarize this description, the interaction between baryons in the SU(3) multiplet is the consequence of the exchange of SU(3) spin 0 and spin 1 bosons. In a second approximation, the exchange of two pions (illus.c, d), or more generally, the exchange of two members of the SU(3) spin 0 and spin 1 multiplet, is responsible for a component of the strong nuclear interaction. See Unitary symmetry

Interaction between nucleons ( a ) from exchange of single pion, ( b ) from exchange of &rgr;-meson, a two-pion system, ( c ) from exchange of two separate pions with formation of excited state of nucleon, and ( d ) without formation of excited stateenlarge picture
Interaction between nucleons (a) from exchange of single pion, (b) from exchange of &rgr;-meson, a two-pion system, (c) from exchange of two separate pions with formation of excited state of nucleon, and (d) without formation of excited state

The range of the interaction generated by these exchanges can be calculated by using the formula below,

where m is the mass of the exchanged particles, ℏ is Planck's constant divided by 2&pgr;, and c is the speed of light. According to the above equation, the range of the interaction developed when a single pion is exchanged (illus. a) is equal to 1.4 × 10-15 m (5.5 × 10-14 in.), while that due to two-pion exchange (illus. d) is 0.7 × 10-15 m (2.8 × 10-14 in.).

At short separation distances the quark-gluon structure of the baryons must be taken into account. The interaction must be considered as a property of the six-quark-plus-gluon system. The decisive elements are the Pauli principle obeyed by the quarks, and the mismatch between the six-quark wave function and the two-baryon wave function. Thus, at short distances the interaction is effectively repulsive or more generally independent of the kinetic energy of the baryons at infinite separations. See Elementary particle, Exclusion principle, Fundamental interactions, Gluons, Quarks