Proton spin, axial anomaly and the Cheshire cat principle

The proton matrix element of the flavor-singlet axial current (FSAC), often referred to as 'proton spin', is discussed in terms of the chiral bag model with due account of the axial anomaly and quantum number fractionization. We argue that independently of the 'confinement' or bag radius R, the FSAC matrix element remains small, say, less than 15%. We suggest this as an intricate manner in which the Cheshire cat principle manifests itself or can break down. (orig.)