Contribution of the Δ-isobar excitation mechanism to the pp → {pp}_s π⁰ reaction

The differential cross section of the reaction pp → {pp}_s π⁰, where {pp}_s is the proton pair in the ¹S₀ state at a small excitation energy E_pp < 3 MeV, is calculated using the box diagram with the excitation of the Δ-isobar. The model demonstrates a clear bump observed in the experimental data at the beam energy of ∼0.6 GeV and the zero diproton outgoing angle; however, its absolute value strongly depends on the off-shell extrapolation of the πNΔ vertex factor and underestimates the data.