Charge symmetry violation in $np→dπ0$

While charge symmetry breaking (CSB) in the strong interaction is ultimately due to the mass difference between u and d quarks, meson-exchange models describe the data most consistently. A high precision measurement of CSB has been undertaken at TRIUMF to complement those data and lead to a richer understanding of the dynamical quark effects in the NN system. The observable of interest is the forward–backward asymmetry $(Afb)$ in $np→dπ0,$ which must be zero in the center-of-mass if charge symmetry is conserved and is predicted to be approximately $−35×10−4$ with the dominant contributions being an order of magnitude larger than those of the elastic scattering CSB measurements carried out at TRIUMF and IUCF. The measurement used a 279.5 MeV neutron beam, a liquid hydrogen target, and the SASP spectrometer positioned at 0°. With these kinematics and the large acceptance of SASP the full deuteron distribution was detected in one setting of the spectrometer thereby eliminating many systematic uncertainties. A measurement of the $pp→dπ+$ distribution accompanied the primary measurement as a robust test of the analysis and simulation codes. The experimental challenges and status are presented.