KL→μ+μ−, the top-quark mass, and other related topics

The dispersive part of the $K_L\to {\mu }^{+}{\mu }^{-}$ amplitude due to the two-photon intermediate state is reconsidered, including vector-meson contributions in the hidden-symmetry scheme. We find that, contrary to the usual assumption $\mathrm{Re}{A}_{\gamma \gamma }\ll \mathrm{Re}{A}_{\mathrm{EW}}$, these two tend to cancel each other, and $B(K_L\to {\mu }^{+}{\mu }^{-})$ is rather insensitive to $m_t$. If $B(K_L\to {\mu }^{+}{\mu }^{-})>\mathrm{}7.0\mathrm{}\times {10}^{-9\mathrm{}}$, then a larger value of $f_B$ is preferred.