Charge symmetry breaking inn→-p→ scattering at 183 MeV

Abstract

We report the results of a precise test of charge symmetry in the scattering of polarized neutrons from polarized protons at a laboratory bombarding energy of 183 MeV. The observable sensitive to charge symmetry is the difference between the analyzing powers associated with the neutron spin [${\mathit{A}}_{\mathit{n}}$(θ)] and with the proton spin [${\mathit{A}}_{\mathit{p}}$(θ)]. Systematic errors and experimental ambiguities in the measurement of this difference are extensively discussed. Our result for ΔA≡${\mathit{A}}_{\mathit{n}}$-${\mathit{A}}_{\mathit{p}}$, averaged over the angular range 82.2°≤${\mathrm{\theta }}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$≤116.1°, is (33.1±5.9±4.3)×${10}^{\mathrm{-}4}$. With the statistical and systematic errors added in quadrature, this value is 3.4 standard deviations larger than the effect expected from pure photon exchange (the electromagnetic spin-orbit interaction) between the neutron and proton, and thus represents clear evidence of charge symmetry breaking in the strong interaction. We also extract information about the angular dependence of ΔA(θ), within limitations imposed by uncertainties in the measured beam and target polarizations. Both the angle-averaged value and the angular dependence measured for ΔA are in excellent agreement with predictions from meson-exchange theory, when these include both the effect of the n-p mass difference on one-pion exchange and the isospin mixing of ${\mathrm{\rho }}^0$ and ${\mathrm{\omega }}^0$ mesons. In particular, the Bonn nucleon-nucleon (NN) potential model accounts for the present results utilizing ρNN and ωNN coupling constant values deduced previously for this potential from fits to more conventional NN scattering data.