Fitting scenarios in a simple theoretical approach to1slevel shifts in pionic atoms

The Klein-Gordon version of a simple analytic model of the pionic atom due to M. Ericson is derived. The expected precision of the resulting expression for pionic atom $1s$ level shifts is about 10%. The model is well-suited for the qualitative comparison of theory, especially with regard to various parametric fitting strategies, and experiment in this case. It is found that a repulsive $\mathrm{Re}{B}_0$, comparable with $\mathrm{Im}{B}_0$ in magnitude, is required to fit the experimental ($T=0\mathrm{}$) data in the low $A$ region.