Divergence of the quark self-energy in the second quantized chiral bag model

When summing over the intermediate quark states of a spherical cavity, the quark self-energy of the chiral bag model, in lowest order of the pion coupling, is shown to generate a series of terms, each one growing linearly with the angular variable κ. However, there is a cancellation between terms for different κ, which finally leads to an overall linearly divergent series.