Neutral pion photoproduction on deuterium in baryon chiral perturbation theory to order q⁴

Threshold neutral pion photoproduction on the deuteron is studied in the framework of baryon chiral perturbation theory beyond next-to--leading order in the chiral expansion. To fourth order in small momenta, the amplitude is finite and a sum of two- and three-body interactions with no undetermined parameters. With accurate theoretical and experimental input from the single nucleon sector for the proton amplitude, we investigate the sensitivity of the threshold cross section to the elementary $\gamma n \to \pi^0 n$ amplitude. A precise measurement of the threshold cross section for $\gamma d \to \pi^0 d$ is called for.