A chiral bag model with a soft surface : structure and solutions of the fuzzy and the modified fuzzy bag model

Abstract

In the present work we propose a new bag model for hadrons, called the modi ed fuzzy bag model (MFBM). The distinguishing feature of this model is the suppression of the pion eld, as it enters the bag, by means of a scalar potential for the pions, while still preserving chiral symmetry. The mechanism of pion suppression in the MFBM is similar to the mechanism of quark suppression in the fuzzy bag model (FBM). The standard chiral transformation for the pion eld su ers a natural alteration in the MFBM, and as a result the model is chiral invariant. We present also a discussion of the FBM and study, in the quark sector, the implications of the soft surface of the bag on the expectation value of the mass operator. In the pion-quark sector, we study the e ects of the suppression of the pion eld on the form factor for the pion-nucleon interaction, on the pion-nucleon coupling constant g NN and on the nucleon axial charge gA. Calculations of the pion-nucleon form factor exhibit, in particular, an improvement over previous results. The pionic axial current induces, in the MFBM, a nonvanishing and orientation dependent contribution to axial charge. An analysis of the asymptotic behaviour of the axial charge shows that the role of the surface is to increase the di erence of the contributions associated to di erent orientations.