Positive Pion Production by Polarized X Rays between 227 and 373 MeV

Measurements have been made of the ratio of the ${\pi }^{+}$ photoproduction cross sections at right angles to and along the electric field vector. Data have been taken at 45°, 90°, and 135° at energies of 227, 240, 342, and 373 MeV. A comparison of the data with the predictions of a phenomenological analysis using only $S$ and $P$ waves shows less than 0.1% chance of obtaining such results without the inclusion of higher angular momenta, and hence, demonstrates even more convincingly the need for a meson current term which has been indicated by other measurements. A comparison is made with the relativistic dispersion relations of McKinley which include an approximation for the $\gamma$, $\rho$, $\pi$ coupling. At the resonance energy our polarization asymmetry is insensitive to this coupling and is in good agreement with the McKinley prediction. At lower energy the agreement is not as good but our data seem to substantiate the need for a negative $\gamma$, $\rho$, $\pi$ coupling constant.