The Pion-Nuclear Isoscalar s-Wave Interaction at Zero Energy

From an analysis of pionic atom data of the isoscalar (N = Z) nuclei we find for the first time a unique description of the s-wave interaction which is based mainly on experimental data. The isoscalar coefficients of the optical potential are 0 = - 0.0070(18)mπ-1 and 0 = [- 0.270(23) + i × 0.127(7)]mπ-4. These values refer to an optical potential expressed in terms of directly measured nuclear-charge distributions.