MEASUREMENT OF THE H-2(GAMMA,PI(0)) 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 IGLO O. The differential measurements are the first to be reported in the t hreshold region, while the total cross section is a significant improv ement over the only previous measurement. We give the first determinat ion of the electric dipole amplitude for pi(0) photoproduction from th e deuteron in the threshold region. The threshold amplitude (E-d) has recently been calculated within the framework of chiral perturbation t heory (ChPT). The present result for E-d falls about 20% below the ChP T value. We confirm the predicted sign (negative). Above threshold we find no evidence for a unitarity cusp such as is observed in the dipol e amplitude for H-1(gamma, pi(0)). Finally, the elementary P-wave ampl itude P-1((+)) as deduced from the angular distributions is compared w ith ChPT predictions. A substantial discrepancy with theory is evident , perhaps a signal of two-body effects.