Lorentz-Lorenz effect andρ2terms in pion-nucleus scattering

Recent results indicate that terms in the optical potential nonlinear in the matter density $\rho$ are important in $\pi$-nucleus scattering. It does not seem possible to identify these terms with a single physical effect. In particular, the contribution to the real part of the optical potential due to true meson absorption may be of the same order as that of the Lorentz-Lorenz effect.