Polarized photon measurements of the ¹²C(γ⃗,pp) and ¹²C(γ⃗,pn) reactions for E_γ=160-350 MeV

The ¹²C(γ⃗,pp) and ¹²C(γ⃗,pn) reactions have been studied for E_γ=160-350 MeV using linearly polarized photons from the Glasgow tagged photon spectrometer at the Mainz microtron MAMI. Both reaction channels show a negative photon asymmetry Σ 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 Σ_(γ,pp), but not in Σ_(γ,pn). Further differences between the two reaction channels are observed in the E_γ dependence of Σ 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_γ>250 MeV, Σ has a substantial negative value which is similar for both reaction channels. This is attributed to two-step reactions following initial quasifree pion production.