What is isospin?

Question

What is isospin?

Answer 1

In physics, and specifically, particle physics, isospin (isotopic spin, isobaric spin) is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term isotopic spin is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number of nucleons. 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.

Isospin was introduced by Werner Heisenberg in 1932 (although it was named by Eugene Wigner in 1937) 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 nucleons, baryons of spin 1/?2, were grouped together 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 1/2 particle has two states, the two were said to be of isospin 1/2. The proton and neutron were then associated with different isospin projections Iz = +1/2 and -1/2 respectively. When constructing a physical theory of nuclear forces, one could then simply assume that it does not depend on isospin.

http://en.wikipedia.org/wiki/Isospin  (too long to quote)

Answer 2

property that is characteristic of families of related subatomic particles differing principally in the values of their electric charge. The families of similar particles are known as isospin multiplets: two-particle families are called doublets, three-particle families are called triplets, and so on.