Charge-symmetry breaking innpelastic scattering at 477 MeV

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°–80°. The difference in zero-crossing angles was 0.340°±0.162° (±0.058°), which yields ΔA≡$A_n$-$A_p$=0.0047±0.0022 (±0.0008) using dA/d${\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}=\mathrm{-}0.01382}$ ${\mathrm{deg}}^{\mathrm{-}1}$. 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 π, ρ, and 2π exchange, electromagnetic coupling of the neutron anomalous magnetic moment to the proton current, ρ-ω-meson mixing, and short- and medium-range effects of the up- and down-quark mass difference.