Inclusive $J/\psi$ and $\psi'$ production in $\pi^-$-nucleus interactions at $\sqrt{s} \simeq 26$ GeV

Charmonium production by 350~GeV/$c$ negative pions incident on tungsten, copper and silicon has been studied in the WA92 experiment. \jp\ and \pp\ mesons are identified through their decays into muon pairs and are selected in the Feynman-$x$ range $x_F > 0$. Inclusive \jp\ and \pp\ cross-sections and \jp\ differential cross-sections as functions of $x_F$ and ${p_T}^2$ are measured. Assuming the relation $\sigma = \msz \, A^\alpha$ between the nuclear cross-section, $\sigma$, and the atomic mass, $A$, of a target nucleus a fit to \jp\ data for different targets yields $\alpha = 0.87 \pm 0.05 \pm 0.04$. Taking $\alpha=0.87$, the cross-sections per nucleon for \jp\ and \pp\ are $\mszj = 216 \pm 5 \pm 13 \; \mbox{nb}$ and $\mszpp = 28 \pm 8 \pm 11 \; \mbox{nb}$, and their ratio is $\mszpp / \mszj = 0.13 \pm 0.05 \pm 0.01 $. The measurements of charmonium production are compared with results on open-charm production previously obtained in the same experiment. The ratio between the \jp\ cross-section and the charmed-meson cross-section, relating to the same experimental conditions, is found to be $\mszj / \sigma_{0}(D\bar{D}) = (1.75 \pm 0.05 \pm 0.15) \times 10^{-2}.$