Differential cross sections of $J/\psi$ and ${\psi }^{\prime }$ in 800 GeV/$c\hspace{0.5em}p$-Si interactions

We present the $x_F$ and $p_T$ differential cross sections of $J/\psi$ and ${\psi }^{\prime }$, respectively, in the ranges $-0.05<\mathrm{}{x}_F<0.25\mathrm{}$ and $p_T<3.5\mathrm{}\hspace{0.5em}\mathrm{GeV}/c$. The data samples are constituted by about 12 000 $J/\psi$ and 200 ${\psi }^{\prime }$ produced in proton-silicon interactions at 800 GeV/$c$ and decaying into opposite sign muons. The $x_F$ and $p_T$ distributions are compared with recent results from experiments E789 at the same energy and to leading order QCD predictions using the MRS D0 parametrization for the parton structure function. The measured shapes of the differential cross sections, except for the $d\sigma {/dx}_F$ at small $x_F,$ agree very well with the prediction, even though their value is quite a bit larger than the prediction. We also present the $\cos \theta$ differential cross section of the $J/\psi$ which indicates unpolarized production in contrast with color octet models predictions.