Isospin Invariance

Isospin invariance follows from the fact that the strong interactions are independent of quark type, and so do not distinguish up quarks from down quarks. Furthermore, the masses of the up and down quarks are small compared to their energy in a proton or neutron, and thus protons and neutrons have close to equal masses.

As far as strong interactions are concerned, protons and neutrons behave identically. Isospin is the invariance that relates strong interaction  processes or states that differ only by replacing some number of protons by an equal number of neutrons.

Isospin is a compound word which suggests two ideas-- isotopes and spin.

Isospin is only very loosely related to the concept of an isotope and has nothing whatever to do with spin!  However the mathematics of isospin is similar to the mathematics of spin for spin 1/2 particles, and this is where the spin part of the name arose.

Isotopes are different nuclei of the same chemical element -- differing in the number of neutrons but containing the same number of protons.

Isobars (in particle physics, not weather maps) are different nuclei with the same number of baryons (i.e., nucleons -- neutrons plus protons). Each isobar is a particular isotope of some element, but each member of a set of isobars is a different element. All members of a set of isobars have approximately the same mass.

Isospin is a symmetry that relates members of a set of isobars.

The simplest such set has only one baryon. There are two such states, one proton and one neutron.

Clearly, since electromagnetic interactions depend on electric charge, isospin invariance does not apply for electromagnetic interactions. However, since strong interaction processes occur more rapidly than electromagnetic or weak processes, isospin invariance is a useful tool to analyze rapid decays.

As further strongly-interacting particles such as mesons and more massive baryons were discovered it was recognized that isospin invariance could also be applied to these particles. Particles of similar mass have similar strong-interaction behavior independent of their electric charges. For example, the three pi mesons (charges +1, 0, and -1) all have similar strong interaction rates. These particles differ only by replacing up quarks by down quark (or anti-u quarks by anti-d quarks).

Isospin at the Quark Level

At the quark level the difference between a proton and a neutron is due to the difference between up quarks (u) and down quarks (d).

A proton has a basic quark structure uud, whereas a neutron has the structure udd.

Fundamental strong interactions (gluon exchanges) are independent of quark flavor. Isospin invariance is a manifestation of this fact for particles that differ only by replacing up quarks with down quarks (or vice versa). Thus isospin invariance is a special case of flavor invariance.

Electromagnetic and weak interactions do depend on quark flavor.