Isospin - Motivation For Isospin

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 1⁄₂, 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 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 I₃ = +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.

These considerations would also prove useful in the analysis of meson-nucleon interactions after the discovery of the pions in 1947. The three pions (π+, π0, π−) could be assigned to an isospin triplet with I = 1 and I₃ = +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: a doublet I = 1⁄₂ of K mesons (K0, K0), a triplet I = 1 of Sigma baryons (Σ+, Σ0, Σ−) a singlet I = 0 Lambda baryon (Λ0), a quartet I = 3⁄₂ Delta baryons (Δ++, Δ+, Δ0, Δ−), 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.