J. Ahrens

The ${}^{12}\mathrm{C}(\overrightarrow{\gamma },pp)$ and ${}^{12}\mathrm{C}(\overrightarrow{\gamma },pn)$ reactions have been studied for $E_{\gamma }=160\mathrm{}-350$ MeV using linearly polarized photons from the Glasgow tagged photon spectrometer at the Mainz microtron MAMI. Both reaction channels show a negative photon asymmetry $\Sigma$ for missing energies $E_m$ below 70 MeV, where direct emission nucleon pairs is expected. A strong peak at low $E_m$ is observed in ${\Sigma }_{(\gamma ,pp)},$ but not in ${\Sigma }_{(\gamma ,pn)}.$ Further differences between the two reaction channels are observed in the $E_{\gamma }$ dependence of $\Sigma$ and in the angular distributions of differential cross sections for photons polarized parallel, or perpendicular, to the reaction plane. Theoretical calculations using an unfactorized distorted wave treatment of direct two-nucleon emission do not agree with the magnitude of the photon asymmetry or the angular dependence of the differential cross sections for either channel. For $E_m>70\mathrm{}$ MeV and $E_{\gamma }>250\mathrm{}$ MeV, $\Sigma$ has a substantial negative value which is similar for both reaction channels. This is attributed to two-step reactions following initial quasifree pion production.