Two Photon Contribution to Polarization in $K^+ \rightarrow \pi^+ \mu^+   \mu^-$
  Short distance physics involving virtual top and charm quarks contributes to $\mu^+$ (and $\mu^-$) polarization in the decay $K^+ \rightarrow \pi^+ \mu^+ \mu^-$. Measurement of the parity violating asymmetry $(\Gamma_R - \Gamma_L)/(\Gamma_R + \Gamma_L)$, 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. 
Ming Lu, Mark B. Wise, and Martin J. Savage