Differential and direct differential elastic scattering cross section 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 $\frac{{\left|f\right(\theta \left)\right|}^2}{\sigma \left(\theta \right)}$. Our measured values of $\frac{{\left|f\right|}^2}{\sigma \left(\theta \right)}$ show a significant angular shift relative to the theoretical curve in forward-angle scattering (θ=20°-40°). This shift is not apparent in measurements of the differential scattering cross section σ(θ) over the same angular range, which, however, do show a significant angular shift in the range θ=50°-120°.