ELSEVIER Nuclear Physics A721 (2003) 835-838c www.elsevier.com/locate/npe Chiral dynamics and pionic 1s states of Pb and Sn isotopes*

Recent accurate data on Is states of 7 ~- bound to P b [l] and Sn [2] isotopes have set new standards and constraints for the detailed analysis of s-wave pion-nucleon interac-tions. This topic has a long history [3] culminating in various attempts to understand the notorious missing repulsion in the.rr-nucleus interaction: the standard ansatz for the (energy independent) s-wave pion-nucleus optical potential, given in terms of the em-pirical threshold T N amplitudes times densities pp,n and supplemented by sizable double-scattering corrections, still misses the observed repulsive interaction by a large amount. This problem has traditionally been circumvented on purely phenomenological grounds by introducing an extraordinarily large repulsive real part (Re&) in the p 2 terms of the.rr-nucleus potential. The arbitrariness of this procedure is of course unsatisfactory. In recent papers [4,5] we have re-investigated this issue from the point of view of the distinct explicit energy dependence of the pion-nuclear polarization operator [4] in a cal-culation based on systematic in-medium chiral perturbation theory [5,6]. Ref. [4] has also clarified the relationship to a working hypothesis launched previously [7,8]: that the extra repulsion needed in the s-wave pion-nucleus optical potential at least partially reflects the tendency toward chiral symmetry restoration in a dense medium. To leading order, thi