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→ppπ0 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. copyright 1996 The American Physical Society