Threshold $pp\to ppπ^0$ up to one-loop accuracy

The $pp\to pp\pi^0$ cross section near threshold is computed up to one-loop order including the initial and final state interactions using the hybrid heavy baryon chiral perturbation theory. With the counter terms whose coefficients are fixed by the resonance-saturation assumption, we find that the one-loop contributions are quite important and bring our theoretical value of the cross section closer to the experimental data. The short-ranged contributions are treated by means of a cutoff, and only a mild cutoff dependence is observed when all diagrams of the given chiral order are summed. To the order treated, heavy baryon chiral perturbation theory, however, converges rather slowly calling for further studies of the process including going to higher orders.