Precision measurement of charge symmetry breaking in $\mathrm{np}$ elastic scattering at 347 MeV

A nonzero difference of the analyzing powers due to charge symmetry breaking has been measured with high precision in $\mathrm{np}$ elastic scattering at a neutron beam energy of 347 MeV. The neutron beam and proton target were alternately polarized for the measurements of $A_n$ and $A_p.$ A mirror-symmetric detection system was used to cancel geometry-related systematic errors. From fits of the measured asymmetry angular distributions over the range of $53.4°<~{\theta }_{\mathrm{cm}}<~86.9°,$ the difference in the zero-crossing angles of the analyzing powers was determined to be $0.438°\pm 0.054°(\mathrm{stat}.\mathrm{})\pm 0.051°(\mathrm{syst}.\mathrm{})$ in the center-of-mass system. Using the experimentally determined slope of the analyzing power $dA/d\theta =(-1.35\pm 0.05)\times {10}^{-2}$ ${\mathrm{deg}}^{-1}$ (c.m.), this is equivalent to $\Delta A\equiv A_n-A_p=[59\pm 7(\mathrm{stat}.\mathrm{})\pm 7(\mathrm{syst}.\mathrm{})\pm 2(\mathrm{syst}.\mathrm{}\left)\right]\times {10}^{-4}.$ The shape of $\Delta A\left(\theta \right)$ in the vicinity of the zero-crossing angle has also been extracted. Predictions of nucleon-nucleon interaction models based on meson exchange agree well with the results.