th / 0 30 80 30 v 1 1 1 A ug 2 00 3 Testing Chiral Dynamics in Pionic Atoms

The energy dependence of chirally expanded πN isoscalar and isovector amplitudes b 0 (E) and b 1 (E) respectively, for zero-momentum off shell pions near threshold, is tested by imposing the minimal substitution requirement E → E − V c on the properly constructed pion optical potential used in a large-scale fit to 100 pionic-atom data across the periodic table which also include the recently established 'deeply bound' pionic atoms of Pb and Sn. This fit cannot be reconciled with the well known free-space values of the πN threshold amplitudes. In contrast, introducing the empirically known energy dependence for on-shell pions leads to a better fit and to satisfactory values for the πN threshold amplitudes. The difference between these two approaches is briefly discussed.