1. The d+d -> 4He+pi0 reaction tests both isospin conservation and charge symmetry.
2. The present framework for understanding charge symmetry breaking comes from chiral perturbation theory. In this structure there are two origins for CSB: the down-up quark mass difference and electromagnetic effects (parametrized by [delta]-mN and [bar-delta]-mN). This structure tells us to look in two places: the properties of the nucleon and in processes where the pion scatters from the nucleon. Charge conservation limits the consideration to neutral pions. For light systems, this leads to a search for the anisotropy in n+p --> d+pi0 and the total cross section in d+d --> 4He+pi0. For technical reasons, pi-eta mixing is treated as a separate cause, even though it, too, originates with quark mass differences and electromagnetic effects.
3. The most recent attempt to observe d+d --> 4He+pi0 near 1 GeV was also sensitive to two pion production and radiative capture. Subsequent to announcing a cross section at 1.1 GeV, it was pointed out that the rate was also consistent with double radiative capture.
4. We chose to place our target upstream of the 6-degree bend in the Cooler ring so that forward-going 4He just above threshold could be separated from the beam. Kinematic reconstruction required the building of a magnetic channel that could measure the 4He four-momentum.
5. The components of the experiment are drawn.
6. A Monte-Carlo calculation shows the separation of pion production from double radiative capture by calculating the pion missing mass based on 4He time of flight and wire chamber position information. The cross sections come from pre-experiment estimates.
7. The p+d --> 3He+pi0 reaction was used to commission the channel. Initial results show excellent performance. Pb-glass simulations with GEANT are accurate for the efficiency to with 3% of its value.
8. Particle identification can pick out 4He, but there is a very high 4He flux coming along with the beam.
9. The high energy photons are visible in the Pb-glass, but only a two-photon coincidence with the 4He is clean.
10. Other important items include a calibration of the Cooler beam energy, correct calculations of the energy loss of 3He and 4He in the magnetic channel, and corrections to the time offsets used in the calculation of the time of flight. Runs were made at two energies to check that the kinematic reconstruction was working correctly.
11. The luminosity was monitored using d+d elastic scattering at 90 degrees in the center of mass. The cross section for this process was calibrated against p+d elastic scattering using a molecular HD target. The reference p+d cross sections were recently measured to good precision at the energies that we needed at the KVI in Groningen.
12. The final results are two peaks at two energies sitting on top of a double radiative capture background. The cross sections are consistent with s-wave pion production.
13. Repeat of transparency 2.
14. Calculations are in progress. Some diagrams are based on chiral perturbation theory, others on pi-eta mixing. At the moment, the pi-eta mixing diagrams give the largest cross section. Many important effects remain to be included.
15. The summary was an illustration of how the neutron-proton mass difference and the n+p -> d+pi0 fore-aft asymmetry fit together with the d+d -> 4He+pi0 cross section to determine the quark mass difference and electromagnetic terms in chiral perturbation theory. On a plot of the two chiral perturbation theory strengths, the neutron-proton mass difference is a line with a slope of -1. The fore-aft asymmetry determines a line with a slope of 2 whose location depends on pi-eta mixing. The d+d -> 4He+pi0 cross section will potentially determine the strength of the mixing if present indications hold.
Another way to look at this is shown in an alternate transparency with a much more speculative premise. If the neutron-proton mass difference is assumed, then it is possible to plot the pi-eta mixing term against the quark mass difference term. Each experiment determines the intercept and width of a band. Theory is required to give the slope. The intercept and slope for the fore-aft asymmetry are shown. For the d+d -> 4He+pi0 experiment, a representative band is given that corresponds to dominance by pi-eta mixing at the current value. The width of the band represents the d+d -> 4He+pi0 cross section error.
16. A list of the experimental and theoretical collaborators.