Measurement of the charge symmetry breaking d+d→4He+π0 reaction near threshold.

We describe the observation of the isospin violating and charge symmetry breaking reaction dd→απ0 just above threshold. Measurements using a magnetic channel (gated by two photons) of the 4He scattering angle and momentum (from time of flight) permitted reconstruction of the π0 "missing mass," the quantity used to separate distributions of 4He+π0 events from a continuum containing 4He+γ+γ and 4He+π0+γ events. A review has been completed of the apparatus, relevant geant simulations, various corrections, and the d+p scattering cross section used as a luminosity calibration. The new values of the total cross section for neutral pion production are 14.3±2.2(stat)±1.6(sys) pb at 228.5 MeV and 17.3±3.4(stat)±2.4(sys) pb at 231.8 MeV. The uncertainty remains dominated by statistical errors. These cross sections arise from the down-up quark mass difference and quark electromagnetic effects. [ABSTRACT FROM AUTHOR]