Observation of the Repulsive Nature of theP-Wave Pion-Nucleus Interaction at LargeZ

We have measured the zero-energy $P$-wave pion-nucleus interaction for Pd ($Z=46\mathrm{}$) using the pionic-atom nuclear-resonance effect. The observed ratios of selected pionic x-ray intensities for two isotopes of Pd imply that the interaction is repulsive for this value $Z$, in contrast to its observed attractive nature for $Z<~30$. This change in the $P$-wave pion-nucleus interaction is in agreement with a previously untested theoretical prediction.