Measurement of the 2H(γ,π0) reaction near threshold

Total and differential cross sections for the reaction H-2(gamma, pi(0)) have been investigated within 20 MeV of threshold using the tagged photon facility at SAL in conjunction with the pi(0) spectrometer IGLOO. The differential measurements are the first to be reported in the threshold region, while the total cross section is a significant improvement over the only previous measurement. We give the first determination of the electric dipole amplitude for pi(0) photoproduction from the deuteron in the threshold region. The threshold amplitude (E-d) has recently been calculated within the framework of chiral perturbation theory (ChPT). The present result for E-d falls about 20% below the ChPT value. We confirm the predicted sign (negative). Above threshold we find no evidence for a unitarity cusp such as is observed in the dipole amplitude for H-1(gamma, pi(0)). Finally, the elementary P-wave amplitude P-1((+)) as deduced from the angular distributions is compared with ChPT predictions. A substantial discrepancy with theory is evident, perhaps a signal of two-body effects.