Charge-symmetry breaking in<i>np</i>elastic scattering at 477 MeV

Abstract

The effect of isospin-violating, charge-symmetry-breaking (CSB) terms in the np interaction has been observed at TRIUMF by measuring the difference in the zero-crossing angles of the neutron and proton analyzing powers, ${A}_{n}$ and ${A}_{p}$, at a neutron energy of 477 MeV. The scattering asymmetries were measured with a neutron beam incident on a polarizable proton target. To reduce systematic errors, interleaved measurements of ${A}_{n}$ and ${A}_{p}$ were made using the same beam and target (apart from their respective polarization states). Neutrons and protons were detected in coincidence in the center-of-mass angle range from 59\ifmmode^\circ\else\textdegree\fi{}--80\ifmmode^\circ\else\textdegree\fi{}. The difference in zero-crossing angles was 0.340\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.162\ifmmode^\circ\else\textdegree\fi{} (\ifmmode\pm\else\textpm\fi{}0.058\ifmmode^\circ\else\textdegree\fi{}), which yields \ensuremath{\Delta}A\ensuremath{\equiv}${A}_{n}$-${A}_{p}$=0.0047\ifmmode\pm\else\textpm\fi{}0.0022 (\ifmmode\pm\else\textpm\fi{}0.0008) using dA/d${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.=\mathrm{\ensuremath{-}}0.013\phantom{\rule{0ex}{0ex}}82}$ ${\mathrm{deg}}^{\mathrm{\ensuremath{-}}1}$. The second errors represent systematic effects. This result is in good agreement with recent theoretical calculations which include CSB effects due to the np mass difference in \ensuremath{\pi}, \ensuremath{\rho}, and 2\ensuremath{\pi} exchange, electromagnetic coupling of the neutron anomalous magnetic moment to the proton current, \ensuremath{\rho}-\ensuremath{\omega}-meson mixing, and short- and medium-range effects of the up- and down-quark mass difference.