Charge symmetry breaking in the reaction np → dπ0

Charge Symmetry Breaking (CSB) in the strong interaction is due ultimately to the mass difference between the u and d quarks, but is described conveniently by meson-exchange models. TRIUMF experiment E704 measures the foreward-backward asymmetry (A(fb)) in np --> d pi(0), which must be zero in the centre-of-mass if charge symmetry is conserved. The angle-integrated value of A(fb) is calculated to be -35 x 10(-4) near 280 MeV with the dominant contribution coming from (pi(0)-eta) and (pi(0)-eta') mixing. The experiment uses a neutron beam of 279.5 MeV, a liquid hydrogen target, and the SASP spectrometer to detect deuterons from the np --> d pi(0) reaction. A measurement of the pp --> d pi(+) distribution, for which A(fb) must be zero, provides a strong test of systematic errors.