PION CLOUD CONTRIBUTION TO THE S-WAVE REPULSION IN PIONIC ATOMS

The nuclear pion cloud contribution to the pion self-energy for pionic atoms is evaluated and one finds large cancellations between terms involving the pi pi amplitude and other terms originating from the chiral lagrangian partners. While the individual terms depend strongly on the off-shell extrapolation of the pi pi amplitude, the sum is model independent within the Olson and Turner family of chiral lagrangians keeping xi + 4 eta constant, as previously found for the pi N --> pi pi N and pion double-charge exchange in nuclei, and vanishes in the limit of m(pi) --> 0. One finds a small net repulsion which is however too small to account for the ''missing'' s-wave repulsion. A revision of the present status of the problem of the ''missing'' s-wave repulsion is done at the end.