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 ππ amplitude and other terms originating from the chiral lagrangian partners. While the individual terms depend strongly on the off-shell extrapolation of the ππ amplitude, the sum is model independent within the Olson and Turner family of chiral lagrangians keeping ξ+4η constant, as previously found for the πN →ππN and pion double-charge exchange in nuclei, and vanishes in the limit of mπ→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. ((orig.))