Multiplicity dependent $J/\psi$ and $\psi(2S)$ production at forward and backward rapidity in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV

TL;DR

This paper measures how the production of $J/ar{J}$ and $ar{ ext{psi}}(2S)$ charmonium states varies with event multiplicity in proton-proton collisions at 200 GeV, providing insights into the role of multiparton interactions.

Contribution

It presents the first measurements of self-normalized $J/ar{J}$ yields and $ar{ ext{psi}}(2S)$ to $J/ar{J}$ ratios at RHIC energies, and compares them with PYTHIA simulations including MPI effects.

Findings

Data agree with LHC measurements and MPI-including PYTHIA models.

The $ar{ ext{psi}}(2S)$ to $J/ar{J}$ ratio shows weak dependence on multiplicity.

Results support MPI as a key factor in charmonium production.

Abstract

The $J/\psi$ and $\psi(2S)$ charmonium states, composed of $c\bar{c}$ quark pairs and known since the 1970s, are widely believed to serve as ideal probes to test quantum chromodynamics in high-energy hadronic interactions. However, there is not yet a complete understanding of the charmonium-production mechanism. Recent measurements of $J/\psi$ production as a function of event charged-particle multiplicity at the collision energies of both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC) show enhanced $J/\psi$ production yields with increasing multiplicity. One potential explanation for this type of dependence is multiparton interactions (MPI). We carry out the first measurements of self-normalized $J/\psi$ yields and the $\psi(2S)$ to $J/\psi$ ratio at both forward and backward rapidities as a function of self-normalized charged-particle multiplicity in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV. In addition, detailed {\sc pythia} studies tuned to RHIC energies were performed to investigate the MPI impacts. We find that the PHENIX data at RHIC are consistent with recent LHC measurements and can only be described by {\sc pythia} calculations that include MPI effects. The forward and backward $\psi(2S)$ to $J/\psi$ ratio, which serves as a unique and powerful approach to study final-state effects on charmonium production, is found to be less dependent on the charged-particle multiplicity.