  We generalize the linear sigma model in order to develop a chiral-invariant
model of nuclear structure. The model is natural, and contains not only the
usual sigma meson which is the chiral partner of the pion but also a new
chiral-singlet that is responsible for the medium-range nucleon-nucleon
attraction. This approach provides significant advantages in terms of its
description of nuclear matter and finite nuclei in comparison with conventional
models based on the linear sigma model.
