Abstract
Charmonium is a valuable probe in heavy-ion collisions to study the properties of the quark gluon plasma, and is also an interesting probe in small collision systems to study cold nuclear matter effects, which are also present in large collision systems. With the recent observations of collective behavior of produced particles in small system collisions, measurements of the modification of charmonium in small systems have become increasingly relevant. We present the results of $J/\psi$ measurements at forward and backward rapidity in various small collision systems, $p$$+$$p$, $p$$+$Al, $p$$+$Au and $^3$He$+$Au, at $\sqrt{s_{_{NN}}}$=200 GeV. The results are presented in the form of the observable $R_{AB}$, the nuclear modification factor, a measure of the ratio of the $J/\psi$ invariant yield compared to the scaled yield in $p$$+$$p$ collisions. We examine the rapidity, transverse momentum, and collision centrality dependence of nuclear effects on $J/\psi$ production with different projectile sizes $p$ and $^3$He, and different target sizes Al and Au. The modification is found to be strongly dependent on the target size, but to be very similar for $p$$+$Au and $^{3}$He$+$Au. However, for 0%--20% central collisions at backward rapidity, the modification for $^{3}$He$+$Au is found to be smaller than that for $p$$+$Au, with a mean fit to the ratio of $0.89\pm0.03$(stat)${\pm}0.08$(syst), possibly indicating final state effects due to the larger projectile size.
Highlights
The cross section for production of charmonium in proton collisions with heavy nuclei is strongly modified relative to that in p + p collisions
The ratio of invariant yields between the forward and backward rapidity regions is presented in the bottom panel, where the systematic uncertainty due to the J/ψ polarization cancels in the ratio
The new p + Au results are found to agree within uncertainties with the previous
Summary
The cross section for production of charmonium in proton collisions with heavy nuclei is strongly modified relative to that in p + p collisions. A major motivation for this work is to study CNM effects that can modify charm production in p + A collisions, which include modification of the nuclear-parton-distribution functions (nPDFs) in a nucleus [1,2], initial state parton energy loss [3], breakup of the forming charmonium in collisions with target nucleons [4,5], coherent gluon saturation [6,7], and transverse momentum broadening [8] These mechanisms are generally expected to act in the early stages of the collision, and effect either the production rates of charm quarks or their propagation through the nucleus.
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