Measurement of the2H(γ,π0)reaction near threshold

Total and differential cross sections for the reaction ${}^2\mathrm{H}(\gamma ,{\pi }^0)$ 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 ${}^1\mathrm{H}(\gamma ,{\pi }^0).\mathrm{}$ Finally, the elementary $P$-wave amplitude $P_1^{\mathrm{}(+)\mathrm{}}$ 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.