Helicity dependence of the γ 3He → πX reactions in the Δ(1232) resonance region

The helicity dependences of the differential cross sections for the semi-inclusive γ 3 He → π 0 X and γ 3 He → π ± X reactions have been measured for the first time in the energy region 200<Eγ<450 MeV. The experiment was performed at the tagged photon beam facility of the MAMI accelerator in Mainz using a longitudinally polarised high-pressure 3He gas target. Hadronic products were measured with the large-acceptance Crystal Ball detector complemented with additional devices for charged-particle tracking and identification. Unpolarised differential cross sections and their helicity dependence are compared with theoretical calculations using the Fix-Arenh ̈ ovel model. The effect of the intermediate excitation of the Δ (1232) resonance can be clearly seen from this comparison, especially for the polarised case, where nuclear effects are relatively small. The model provides a better theoretical description of the unpolarised charged pion photoproduction data than the neutral pion channel. It does significantly better in describing the helicity-dependent data in both channels. These comparisons provide new information on the mechanisms involved in pion photoproduction on 3He and suggest that a polarised 3He target can providevaluable information on the corresponding polarised quasi-free neutron reactions.