Editing Nucleon (section)

===Chiral bag model===
The ''chiral bag model''<ref>{{cite journal
 |author1-link=Gerald E. Brown  |first1=Gerald E. |last1=Brown
 |author2-link=Mannque Rho  |first2=Mannque |last2=Rho
 |date=March 1979
 |title=The little bag
 |journal=[[Physics Letters B]]
 |volume=82  |issue=2  |pages=177–180
 |doi=10.1016/0370-2693(79)90729-9
 |bibcode=1979PhLB...82..177B
}}</ref><ref>{{cite journal
 |last1=Vepstas |first1=L.
 |last2=Jackson |first2=A. D.
 |last3=Goldhaber |first3=A. S.
 |year=1984
 |title=Two-phase models of baryons and the chiral Casimir effect
 |journal=[[Physics Letters B]]
 |volume=140  |issue=5–6  |pages=280–284
 |bibcode=1984PhLB..140..280V
 |doi=10.1016/0370-2693(84)90753-6
}}</ref> merges the ''MIT bag model'' and the ''skyrmion model''. In this model, a hole is punched out of the middle of the skyrmion and replaced with a bag model. The boundary condition is provided by the requirement of continuity of the [[axial vector current]] across the bag boundary.

Very curiously, the missing part of the topological winding number (the baryon number) of the hole punched into the skyrmion is exactly made up by the non-zero [[vacuum expectation value]] (or [[spectral asymmetry]]) of the quark fields inside the bag. {{As of|2017}}, this remarkable trade-off between [[topology]] and the [[spectrum of an operator]] does not have any grounding or explanation in the mathematical theory of [[Hilbert space]]s and their relationship to [[geometry]].

Several other properties of the chiral bag are notable: It provides a better fit to the low-energy nucleon properties, to within 5–10%, and these are almost completely independent of the chiral-bag radius, as long as the radius is less than the nucleon radius. This independence of radius is referred to as the ''Cheshire Cat principle'',<ref>{{cite journal
 |last1=Vepstas |first1=L.
 |last2=Jackson |first2=A. D.
 |year=1990
 |title=Justifying the chiral bag
 |journal=[[Physics Reports]]
 |volume=187  |issue=3  |pages=109–143
 |bibcode=1990PhR...187..109V
 |doi=10.1016/0370-1573(90)90056-8
}}</ref> after the fading of [[Lewis Carroll]]'s [[Cheshire Cat]] to just its smile. It is expected that a first-principles solution of the equations of QCD will demonstrate a similar duality of quark–[[meson]] descriptions.