A Dependence of Charm Production

The prompt neutrino (${\ensuremath{\nu}}_{e}+{\overline{\ensuremath{\nu}}}_{e}$) flux produced by the interaction of 400-GeV protons with targets of beryllium, copper, and tungsten has been measured in a beam-dump experiment at Fermilab. With the assumption that these neutrinos arise from charm decay, the cross section for charm production by protons on nuclei varies as ${A}^{\ensuremath{\alpha}}$, where $\ensuremath{\alpha}=0.75\ifmmode\pm\else\textpm\fi{}0.05$. No significant variation of $\ensuremath{\alpha}$ with neutrino energy is observed.