Neutral-Pion Photoproduction on a$^{7}$Li Nucleus in the Excitation Region of the First Pion—Nucleon Resonance

In order to explain elastic neutral-pion photoproduction on a 7Li nucleus in the excitation-energy region of the Δ(1232)3/2+ resonance, a new interpretation of coherent pion photoproduction on nuclei featuring two or more shells is proposed with allowance for a special feature of inelastic pion photoproduction on a nucleon. Coherence originating from nucleons of different nuclear shells is retained, while coherence from all of the nucleons is removed, since, after pion production on nucleons from different shells, they cannot cannot be in the same phases because of the difference in the conditions of their residence in the nucleus. Under this assumption, a noncontradictory description of all experimental data is obtained upon additionally including the interaction of Δ isobars in the nucleus that are produced along with neutral pions.