= Highest-weight weak-isospin selection in kaon decay
Suppose a transition operator obeys $[I_3,H]=H/2$ and $[I_+,H]=0$, and acts on the lower component of an isospin-half <kaon>. Its output has third component zero, and $I_+^2H|K\rangle=HI_+^2|K\rangle=0$. This excludes an isospin-two output. A spinless parent decaying to two spinless <pions> has zero relative <orbital angular momentum>, so <Bose-Einstein statistics> excludes the antisymmetric isospin-one sector. The output is therefore the <two-isovector singlet>. In the equal-mass isospin limit, charged and neutral two-pion decay widths have ratio two, including the identical-particle counting.
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