A comparative analysis of FSI effects in the γ d → pn π 0 reaction at low and high neutron momenta

We report measurements of the tensor analyzing power T 20 for incoherent neutral pion photoproduction on the deuteron, γd → pnπ 0 , at photon energies 300–600 MeV. The data reveal a clear kinematic transition in the reaction mechanism. At low neutron momenta (p n < 200 MeV/c), T 20 is governed by the deuteron D-wave component, providing a direct probe of the deuteron structure. At high neutron momenta (p n > 200 MeV/c), the observable becomes sensitive to the reaction dynamics, with final state interactions dominating the production mechanism. This transition demonstrates that T 20 can discriminate between quasi-free and rescattering-dominated processes. The results are compared with theoretical calculations based on the MAID2007 model with explicit inclusion of NN and πN rescattering, showing good agreement across both kinematic regimes.