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

TL;DR

This study measures the kinematic distributions and nuclear effects of $J/ar{J}$ production in 920 GeV proton-nucleus collisions, revealing a transition from suppression to enhancement at negative Feynman-$x$ values, based on extensive data from the HERA-B experiment.

Contribution

First measurement of $J/ar{J}$ production in the negative $x_F$ region, providing new insights into nuclear effects at these kinematics.

Findings

Average nuclear suppression parameter $ar{ ext{alpha}}$ is 0.981 ± 0.015.

Nuclear suppression at high $x_F$ turns into enhancement at negative $x_F$.

Data collected from 2.4×10^5 $J/ar{J}$ mesons in multiple decay channels.

Abstract

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 $-0.34 < x_{F} < 0.14$ 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 $x_F$.