Polarized photon measurements of the12C(γ→,pp)and12C(γ→,pn)reactions forEγ=160−350MeV

Abstract

The ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},pp)$ and ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},pn)$ reactions have been studied for ${E}_{\ensuremath{\gamma}}=160\ensuremath{-}350$ MeV using linearly polarized photons from the Glasgow tagged photon spectrometer at the Mainz microtron MAMI. Both reaction channels show a negative photon asymmetry $\ensuremath{\Sigma}$ 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 ${\ensuremath{\Sigma}}_{(\ensuremath{\gamma},pp)},$ but not in ${\ensuremath{\Sigma}}_{(\ensuremath{\gamma},pn)}.$ Further differences between the two reaction channels are observed in the ${E}_{\ensuremath{\gamma}}$ dependence of $\ensuremath{\Sigma}$ 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 ${E}_{m}>70$ MeV and ${E}_{\ensuremath{\gamma}}>250$ MeV, $\ensuremath{\Sigma}$ has a substantial negative value which is similar for both reaction channels. This is attributed to two-step reactions following initial quasifree pion production.