Photoproduction ofπ0on Protons by PolarizedγRays

The asymmetry $\Sigma (k,{\theta }^{*})=\frac{(d{\sigma }_{\perp }-d{\sigma }_{\parallel })}{(d{\sigma }_{\perp }+{\sigma }_{\parallel })}$ of the polarized cross sections for ${\pi }^0$ photoproduction has been measured at ${\theta }^{*}=90\mathrm{}°$ for energies $k$ of the incident photon in the range 230-380 MeV. The experiment has been performed with the polarized $\gamma$-ray beam of the Frascati 1-GeV electron synchrotron. The experimental results are compared with the present theoretical predictions in order to investigate the importance of $\omega$ exchange in the $t$ channel and the contribution of the $E_{1+}^{}{}_{}{}^{\mathrm{}\left(3\right)\mathrm{}}$ multipole at the 33 resonance. The theory with $\omega$ exchange is in the best agreement with the experiment.