Comment on Neutron-Proton Spin-Correlation Parameter A_{ZZ} at 68 Mev

Abstract

We present two arguments indicating that the large value for the $ε_1$ mixing parameter at 50 MeV, which the Basel group extracted from their recent $A_{zz}$ measurement, may be incorrect. First, there are nucleon-nucleon (NN) potentials which predict the $ε_1$ at 50 MeV substantially below the Basel value and reproduce the Basel $A_{zz}$ data accurately. Second, the large value for $ε_1$ at 50 MeV proposed by the Basel group can only be explained by a model for the NN interaction which is very unrealistic (no $ρ$-meson and essentially a point-like $πNN$ vertex) and overpredicts the $ε_1$ in the energy range where it is well determined (150--500 MeV) by a factor of two.