pp→ppπ0reaction near threshold: A chiral power counting approach

We use power-counting arguments as an organizing principle to apply chiral perturbation theory, including an explicit $\Delta$,to the $\mathrm{pp}\to \mathrm{pp}{\pi }^0$ reaction near threshold. There are two lowest-order leading mechanisms expected to contribute to the amplitude with similar magnitudes: an impulse term, and a $\Delta$-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 $\Delta$-excitation terms. In addition, we have modeled the short-ranged interaction using $\sigma$ and $\omega$ 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.