$K_S$ Regeneration on Electrons From 30-{GeV}/$c$ to 100-{GeV}/$c$: A Measurement of the $K^0$ Radius

The amplitude of ${K}_{S}$ regeneration by electrons, $\frac{{{f}_{21}}^{e}}{k}=\ensuremath{-}(\frac{\ensuremath{\alpha}}{3})〈{R}^{2}〉$ ($〈{R}^{2}〉$ is the ${K}^{0}$ charge radius), can be determined by comparing the rates of coherent (transmission) regeneration and of diffraction regeneration at ${q}^{2}=0$. We made a determination from 30 to 100 GeV/c, using a novel approach: Two distinct Pb regenerators, of optimized thicknesses, were exposed to a double beam, and interchanged every burst. We find $〈{R}^{2}〉=\ensuremath{-}(0.054\ifmmode\pm\else\textpm\fi{}0.026)$ ${\mathrm{fm}}^{2}$. The sign, magnitude, and $p$ independence agree with predictions.