ΔI=12Rule in Nonleptonic Weak Interactions

We investigate whether the $\Delta I=\frac{1}{2}$ rule for weak nonleptonic interactions can be embedded into unified theories of the weak and electromagnetic interactions. In such theories weak interactions are mediated not only by (gauge) vector-boson exchange, as usually envisaged, but also through exchange of Higgs scalar bosons. Although the former contributions cannot (it seems) be arranged to have a pure $\Delta I=\frac{1}{2}$ structure, we discuss a model in which the latter can be so arranged. Owing to strong-interaction complexities the effective strengths of these two sources of weak interactions cannot easily be assessed. The discussion nevertheless emphasizes the possibility that Higgs-exchange effects may have a significant role for the $\Delta I=\frac{1}{2}$ question.