S-wave optical potential in pionic atoms

We present a new formal approach for the calculation of the s-wave optical potential for pionic atoms which makes extensive use of the Lindhard function, its analytical properties, and its useful low density limit. The conventional parameters $b_0$ and $B_0$ are calculated and compared to results from empirical fits to pionic atom data. We show that the virtual excitation of ph-Δh components by the pion is the main source of the real part of $B_0$. Higher order effects from the polarization of the pion lead to some enhancement of Im$B_0$, providing values which are compatible with the empirical data. The real part of the potential suffers from larger uncertainties due to the off-shell extrapolation of the πN scattering amplitude.