Search for Direct Processes inK±→π±π0γDecays

We have found 33 events of the type $K^{+}\to {\pi }^{+}{\pi }^0\gamma$ and 31 of the type $K^{-}\to {\pi }^{-}{\pi }^0\gamma$ in an experiment using spark chambers. The angles of the observed decay products were used to determine the angle between the charged pion and the $\gamma$ ray in the $K$ rest frame. The distribution of this angle showed no indication of a direct radiative process but instead was consistent with internal bremsstrahlung. In order to be quantitative concerning the amount of direct interaction, we selected 24 events with the charged-pion kinetic energy $T_{\pi }$ restricted to $58\le T_{\pi }\le 90$ MeV and carried out maximum-likelihood fits for various assumptions regarding the radiative amplitudes, final-state interaction, and amount of $\mathrm{CP}$ violation. Assuming no magnetic dipole, the largest value found by the maximum-likelihood fit for the direct (electric dipole) amplitude is $A_E=-{0.12}_{-0.25\mathrm{}}^{+0.21\mathrm{}}$ relative to the internal-bremsstrahlung amplitude. If the direct process occurs, and if $\mathrm{CP}$ is appreciably violated in the $K^{\pm }\to {\pi }^{\pm }{\pi }^0\gamma$ decay, there would be a difference between the rates for $K^{+}\to {\pi }^{+}{\pi }^0\gamma$ and $K^{-}\to {\pi }^{-}{\pi }^0\gamma$. Our result is $\frac{\Gamma (K^{-}\to {\pi }^{-}{\pi }^0\gamma )}{\Gamma (K^{+}\to {\pi }^{+}{\pi }^0\gamma )}-1=0\mathrm{}\pm 0.24$. Thus a large asymmetry suggested by some theories is ruled out by this observation.