Measurement of charge symmetry breaking by the comparison of π⁺d→ppη with π^-d→nnη

We report a measurement of charge symmetry breaking in the NNη system. We have measured the ratio of the differential cross sections of the charge-symmetric reactions π⁺d→ppη and π^-d→nnη in the energy region of the η threshold. Our result is R≡dσ(π⁺d→ppη)/dσ(π^-d→nnη)=0.938±0.009 after a phase-space correction is made for the difference in the threshold energies of the two reactions. The deviation of R from unity is an indication of charge symmetry breaking, which is mostly due to π⁰-η mixing. A theoretical model for η production which includes π⁰-η mixing was used to fit the data and yields a mixing angle of (1.5±0.4)°. Our result is consistent with the mixing angle determined in particle decay and isospin-forbidden processes as well as predictions by several theoretical analyses which yield ≈1° and another which yields ≈2°.