Differential and Direct Differential Elastic Scattering Cross Sections for Electrons and Potassium Atoms

In a modulated crossed-beam experiment we have elastically scattered unpolarized electrons of 3.3 eV energy from spin-polarized K atoms. A measurement of the polarization of the scattered electron yields $\frac{{|f(\ensuremath{\theta})|}^{2}}{\ensuremath{\sigma}(\ensuremath{\theta})}$. Our measured values of $\frac{{|f|}^{2}}{\ensuremath{\sigma}(\ensuremath{\theta})}$ show a significant angular shift relative to the theoretical curve in forward-angle scattering ($\ensuremath{\theta}={20}^{\ensuremath{\circ}}\ensuremath{-}40$\ifmmode^\circ\else\textdegree\fi{}). This shift is not apparent in measurements of the differential scattering cross section $\ensuremath{\sigma}(\ensuremath{\theta})$ over the same angular range, which, however, do show a significant angular shift in the range $\ensuremath{\theta}={50}^{\ensuremath{\circ}}\ensuremath{-}120$\ifmmode^\circ\else\textdegree\fi{}.