Neutron-proton mass difference in nuclear matter
  Isospin-breaking effects in nuclear matter are 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. The approach predicts that the neutron-proton mass difference decreases in isospin-symmetric nuclear matter but by a very small amount only. 
Ulf-G. Mei{\ss}ner, A.M. Rakhimov, A. Wirzba, U.T. Yakhshiev