  In this Letter, I work out spin entanglement properties of neutron-proton
scattering using the exact S-matrix, generalizing previous works based on S
wave. The dependence of spin entanglement on momentum, scattering angle, and
initial spin configuration is investigated for realistic nuclear forces, while
low-energy properties of spin entanglement are analyzed within the framework of
pionless effective field theory at leading order. New connections are found
between spin entanglement and symmetry enhancement of strong interactions.
These results lead to a more complete understanding of how spin entanglement is
generated via neutron-proton interaction. They also lay the theoretical
foundation for controllable production of entangled nucleon-nucleon pairs in
future experiments.
