ADependence of Charm Production

The prompt neutrino (${\nu }_e+{\overline{\nu }}_e$) 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 $A^{\alpha }$, where $\alpha =0.75\mathrm{}\pm 0.05$. No significant variation of $\alpha$ with neutrino energy is observed.