Two-photon contribution to polarization inK+→π+μ+μ−

Short-distance physics involving virtual top and charm quarks contributes to ${\mu }^{+}$ (and ${\mu }^{-}$) polarization in the decay $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ Measurement of the parity-violating asymmetry $\frac{({\Gamma }_R-{\Gamma }_L)}{({\Gamma }_R+{\Gamma }_L)}$, where ${\Gamma }_R$ and ${\Gamma }_L$ are the rates to produce right- and left-handed ${\mu }^{+}$, may provide valuable information on the unitarity triangle. The parity-violating asymmetry also gets a contribution from Feynman diagrams with two-photon intermediate states. We estimate this two-photon contribution to the asymmetry and discuss briefly the two-photon contribution to time-reversal-odd asymmetries that involve both the ${\mu }^{+}$ and ${\mu }^{-}$ polarizations.