Elastic scattering of π + and π − from Ca 40 at 64.8 MeV

Angular distributions of ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ elastic scattering on $^{40}\mathrm{Ca}$ at 64.8 MeV are presented. The data are analyzed using a first-order, nonlocal optical model based on a Kisslinger $t$ matrix with an off-shell form factor. This model is unable to fit the data for any value of the off-shell parameter $\ensuremath{\alpha}$ unless the nuclear matter radius is allowed to vary from that derived from electron scattering. The resulting best-fit radii are as much as 0.40 fm larger than the electron scattering value for ${\ensuremath{\pi}}^{+}$ and 0.55 fm larger for ${\ensuremath{\pi}}^{\ensuremath{-}}$. For a given $\ensuremath{\alpha}$ the ${\ensuremath{\pi}}^{\ensuremath{-}}$ best fit radius is also consistently larger by about 0.2 fm than that for ${\ensuremath{\pi}}^{+}$. In addition, the fits show no significant preference for any value of $\ensuremath{\alpha}$ between 400 and 900 MeV/c. Inclusion of the normally omitted second-order potential terms in the optical model does not significantly alter the results.NUCLEAR REACTIONS ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ elastic scattering from $^{40}\mathrm{Ca}$ at 64.8 MeV; angular distributions, $15\ifmmode^\circ\else\textdegree\fi{}l{\ensuremath{\theta}}_{\mathrm{lab}}l127\ifmmode^\circ\else\textdegree\fi{}$; optical model analysis.