Evidence for the Lorentz-Lorenz Effect in Low-Energy Pion-Nucleus Scattering

We point out the relatively clean geometrical aspect of the Lorentz-Lorenz effect and use it to extract the strength of the effect from an analysis of the elastic scattering of 50-MeV ${\pi }^{+}$ on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. We find that for a finite-range $\pi -N$ interaction there is strong evidence for the effect with a value $0.2<~\xi <~0.6$.