Charge symmetry breaking in<i>n</i>→-<i>p</i>→ 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 [${\mathit{A}}_{\mathit{n}}$(\ensuremath{\theta})] and with the proton spin [${\mathit{A}}_{\mathit{p}}$(\ensuremath{\theta})]. Systematic errors and experimental ambiguities in the measurement of this difference are extensively discussed. Our result for \ensuremath{\Delta}A\ensuremath{\equiv}${\mathit{A}}_{\mathit{n}}$-${\mathit{A}}_{\mathit{p}}$, averaged over the angular range 82.2\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\mathrm{\ensuremath{\theta}}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}116.1\ifmmode^\circ\else\textdegree\fi{}, is (33.1\ifmmode\pm\else\textpm\fi{}5.9\ifmmode\pm\else\textpm\fi{}4.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. 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 \ensuremath{\Delta}A(\ensuremath{\theta}), within limitations imposed by uncertainties in the measured beam and target polarizations. Both the angle-averaged value and the angular dependence measured for \ensuremath{\Delta}A 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 ${\mathrm{\ensuremath{\rho}}}^{0}$ and ${\mathrm{\ensuremath{\omega}}}^{0}$ mesons. In particular, the Bonn nucleon-nucleon (NN) potential model accounts for the present results utilizing \ensuremath{\rho}NN and \ensuremath{\omega}NN coupling constant values deduced previously for this potential from fits to more conventional NN scattering data.