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On the Hadronic Neutron-Proton Mass Splitting in Chiral Soliton Models of Valence Quarks

In the framework of chiral soliton models of valence quarks with and without confinement, namely the chiral chromodielectric model and the linear sigma-model, the hadronic mass difference between the neutron and the proton is considered as a consequence of an up- and down-quark mass difference. In those models we described the nucleon as a chiral soliton which we endow with good quantum numbers and zero linear momentum by carrying out exact angular-momentum, isospin and linear-momentum projections. For a choice md − mu ∼ 3.2 MeV the experimental value for the hadronic neutron-proton mass difference is obtained in both models. This result contrasts with the application of the collective quantization methods to the same models for which the hadronic splitting resulting from a quark isospin symmetry-breaking term turns out to be too small.