Nonlinear meson dynamics and binding corrections in low energy pion-nucleus scattering

Phenomenological fits to low-energy $\pi$-nucleus scattering have uniformly required a greatly enhanced $s$-wave repulsion in the optical potential, for which no satisfactory theoretical explanation has been given. Evaluation of nonlinear terms in field-theoretic Lagrangians with broken chiral symmetry and inclusion of necessary binding effects leads to a substantial, repulsive ${\rho }^2\mathrm{}\left(r\right)\mathrm{}$ potential, even on an isospin zero target. This potential may account for the previously unexplained effect.