Neutron-proton spin-correlation parameter Azz at 68 MeV.

Abstract

We report a first measurement of the spin-correlation parameter ${\mathit{A}}_{\mathit{z}\mathit{z}}$ in neutron-proton scattering at 67.5 MeV. The results, obtained in the angular range 105\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\mathrm{\ensuremath{\theta}}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}170\ifmmode^\circ\else\textdegree\fi{} with typical accuracies of 0.008, are highly sensitive to the $^{3}$${\mathit{S}}_{1}$${\mathrm{\ensuremath{-}}}^{3}$${\mathit{D}}_{1}$ mixing parameter ${\mathrm{\ensuremath{\epsilon}}}_{1}$. A phase-shift analysis based on the current world data yields a value of ${\mathrm{\ensuremath{\epsilon}}}_{1}$ significantly higher than predicted by modern potential models.