Kinematic distributions and nuclear effects of J/psi production in 920-GeV fixed-target proton-nucleus collisions

Measurements of the kinematic distributions of $J/psi$ mesons produced in $p-$C, $p-$Ti and $p-$W collisions at $sqrt{s}=41.6 mathrm{GeV}$ in the Feynman-$x$ region $-0.34 < x_{F} &lt; 0.14$ and for transverse momentum up to $p_T = 5.4 mathrm{GeV}/c$ are presented. The $x_F$ and $p_T$ dependencies of the nuclear suppression parameter, $alpha$, are also given. The results are based on $2.4 cdot 10^{5}$ $J/psi$ mesons in both the $e^+ e^-$ and $mu^{+}mu^{-}$ decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of $x_{F}$ for the first time. The average value of $alpha$ in the measured $x_{F}$ region is $0.981 pm 0.015$. The data suggest that the strong nuclear suppression of $J/psi$ production previously observed at high $x_F$ turns into an enhancement at negative $x_F$.