pp→ppπ⁰ reaction near threshold: A chiral power counting approach

We use power-counting arguments as an organizing principle to apply chiral perturbation theory, including an explicit Δ, to the pp → pp π⁰ reaction near threshold. There are two lowest-order leading mechanisms expected to contribute to the amplitude with similar magnitudes: an impulse term, and a Δ-excitation mechanism. We examine formally subleading but potentially large mechanisms, including pion-rescattering and short-ranged contributions. We show that the pion-rescattering contribution is enhanced by off-shell effects and has a sign opposite to that of a recent estimate based on a PCAC pion interpolating field. Our result is that the impulse term interferes destructively with the pion-rescattering and Δ-excitation terms. In addition, we have modeled the short-ranged interaction using σ and ω exchange mechanisms. A recoil correction to the impulse approximation is small. The total amplitude obtained including all of these processes is found to yield cross sections substantially smaller than the measured ones.