All quarks, of course, have A = 1/3. The u and d quarks form an isospin doublet, T = 1/2, while the other four quarks are isospin singlets, T = 0. Quote from Wikipedia: “In modern descriptions of hadron interaction, it has become more obvious to draw Feynman diagrams that trace through individual quarks composing the interacting baryons and mesons, rather than counting hypercharge (and other) quantum numbers.” The basic idea is that T commutes with strong interaction Hamiltonians, but not with electromagnetic Hamiltonians, which contain the charge Q which can be expressed in terms of T3. Thus if you take a strongly interacting system and slowly turn on the electromagnetic force, it removes a degeneracy, breaking the initial single state into a multiplet, with each multiplet member having a different value of T3 or Q.
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In nuclear physics, the neutron
is taken to have T3 = +1/2 and the proton to have T3
= -1/2. [This is the exact opposite of the convention in
particle physics.]
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The isobaric analog state of a bound single-neutron state in heavier nuclei is found as an unbound state, an isobaric analog resonance, in the corresponding nucleus where the neutron is replaced by a proton in the same level.
