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In nuclear physics and particle physics, isospin (''isotopic spin'', ''isobaric spin'') is a quantum number related to the strong interaction. Particles that are affected equally by the strong force but have different charges (e.g. protons and neutrons) can be treated as being different states of the same particle with isospin values related to the number of charge states.〔http://www.thefreedictionary.com/isospin〕 Although it does not have the units of angular momentum and is not a type of spin, the formalism that describes it is mathematically similar to that of angular momentum in quantum mechanics, which means it can be coupled in the same manner. For example, a proton-neutron pair can be coupled in a state of total isospin 1 or 0.〔 〕 It is a dimensionless quantity and the name derives from the fact that the mathematical structures used to describe it are very similar to those used to describe the intrinsic angular momentum (spin). This term was derived from ''isotopic spin'', a confusing term to which nuclear physicists prefer ''isobaric spin'', which is more precise in meaning. Isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions of baryons and mesons. Isospin symmetry remains an important concept in particle physics, and a close examination of this symmetry historically led directly to the discovery and understanding of quarks and of the development of Yang–Mills theory. ==Motivation for isospin== Isospin was introduced by Werner Heisenberg in 1932〔 〕 to explain symmetries of the then newly discovered neutron: * The mass of the neutron and the proton are almost identical: they are nearly degenerate, and both are thus often called nucleons. Although the proton has a positive charge, and the neutron is neutral, they are almost identical in all other respects. * The strength of the strong interaction between any pair of nucleons is the same, independent of whether they are interacting as protons or as neutrons. Thus, isospin was introduced as a concept well before the development in the 1960s of the quark model which provides our modern understanding. The name ''isospin'' however, was introduced by Eugene Wigner in 1937.〔 〕 Protons and neutrons, baryons of spin , were grouped together as nucleons because they both have nearly the same mass and interact in nearly the same way. Thus, it was convenient to treat them as being different states of the same particle. Since a spin particle has two states, the two were said to be of isospin . The proton and neutron were then associated with different isospin projections ''I''3 = + and − respectively. When constructing a physical theory of nuclear forces, one could then simply assume that it does not depend on isospin. These considerations would also prove useful in the analysis of meson-nucleon interactions after the discovery of the pions in 1947. The three pions (, , ) could be assigned to an isospin triplet with ''I'' = 1 and ''I''3 = +1, 0 or −1. By assuming that isospin was conserved by nuclear interactions, the new mesons were more easily accommodated by nuclear theory. As further particles were discovered, they were assigned into isospin multiplets according to the number of different charge states seen: 2 doublets, ''I'' = − and ''I'' = of K mesons (, ),(, ), a triplet ''I'' = 1 of Sigma baryons (, , ) a singlet ''I'' = 0 Lambda baryon (), a quartet ''I'' = Delta baryons (, , , ), and so on. This multiplet structure was combined with strangeness in Murray Gell-Mann's eightfold way, ultimately leading to the quark model and quantum chromodynamics. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isospin」の詳細全文を読む スポンサード リンク
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