Proton Compton Effect by Polarized Photons at 90° in the c.m. System in the First Resonance Region

For the reaction $\gamma +p\to {\gamma }^{\prime }+p^{\prime }$ (proton Compton effect), we have measured the ratio $\frac{d{\sigma }_{\mathrm{II}}}{d{\sigma }_{\perp }}$ between the cross sections for linearly polarized photons, using the coherent bremsstrahlung beam of the Frascati electron synchrotron. At 90° in the c.m. system and in the photon energy region $300\le K\le 335$ MeV, we find $\frac{d{\sigma }_{\mathrm{II}}}{d{\sigma }_{\perp }}={{2.1}_{-0.4\mathrm{}}}^{+0.5\mathrm{}}$. In the absence of theoretical predictions based on the dispersive theory in this energy region, this result is compared with the values obtained using an isobaric model, taking into account various possible intermediate states.