Investigation of Delta medium effects using the He-4(gamma,pi+ n) reaction

Measurements of the 4He(γ,π+n)3H and 4He(γ,π+p)nnn reactions in quasifree π production kinematic regimes have been performed using unpolarized and linearly polarized tagged photons in conjunction with large solid angle π and n detectors at the MAMI-B accelerator, Mainz. Differential cross sections are presented for photon energies spanning the Δ(1232) excitation region. These data combined with previously presented results for the 12C(γ,π+n)11B reaction provide an extensive data set for future investigations of Δ-medium effects in nuclei. The 4He(γ,π+n)3H cross sections are significantly smaller than theoretical predictions based on the free proton amplitude for Δ excitation with a crude correction for final state interactions, and exhibit different trends, which could possibly be a consequence of Δ-medium effects. From a comparison with published 12C(γ,π+p)11Be data, it is concluded that the 4He(γ,π+p)nnn reaction mechanisms are probably two-step processes, although the possibility that part or all of the cross section arises from interactions with preexisting Δ++ particles cannot be ruled out.