The neutron-proton mass difference in (isospin asymmetric) nuclear matter and
finite nuclei is studied in the framework of a medium-modified Skyrme model.
The proposed effective Lagrangian incorporates both the medium influence of the
surrounding nuclear environment on the single nucleon properties and an
explicit isospin-breaking effect in the mesonic sector. Energy-dependent
charged and neutral pion optical potentials in the s- and p-wave channels are
included as well. The present approach predicts that the neutron-proton mass
difference is mainly dictated by its strong part and that it markedly decreases
in neutron matter. Furthermore, the possible interplay between the effective
nucleon mass in finite nuclei and the Nolen-Schiffer anomaly is discussed. In
particular, we find that a correct description of the properties of mirror
nuclei leads to a stringent restriction of possible modifications of the
nucleon's effective mass in nuclei.Comment: 10 pages, 8 figures, presentation at the 19th Int. IUPAP Conf. on
Few-Body Problems in Physics (Aug.31-Sep.5, 2009, Univ.of Bonn, Germany