Multiplicity-dependent inclusive J/$ψ$ production at forward rapidity in pp collisions at $\mathbf{\sqrt{s} = 13}$ TeV

TL;DR

This work measures inclusive forward $J/\psi$ production in pp collisions at $\sqrt{s}=13$ TeV as a function of forward charged-particle multiplicity, using $J/\psi \to \mu^+\mu^-$ in $2.5<y<4$ and forward multiplicity from V0C in $-3.7<\eta<-1.7$. The analysis employs detailed event-, muon-, and dimuon selections, MC-driven multiplicity calibration, and mass-fit extractions with acceptance corrections to obtain relative yields, while thoroughly assessing systematic uncertainties. The results show a steeper-than-linear dependence of forward $J/\psi$ yields on forward multiplicity, similar to midrapidity measurements, and robust model comparisons reveal that EPOS4HQ and 3-Pomeron CGC describe the data well, whereas PYTHIA with color reconnection underestimates the trend. The findings support a nuanced interplay of hard and soft processes in charmonium production and motivate future measurements with improved prompt/non-prompt separation (via the MFT) and autocorrelation-aware estimators such as flattenicity. The study provides constraints on production mechanisms at forward rapidity, informing both NRQCD-based and CGC-inspired descriptions of quarkonium in high-energy hadronic collisions.

Abstract

This paper presents a study of the inclusive forward $J/ψ$ yield as a function of forward charged-particle multiplicity in pp collisions at $\sqrt{s} =13$ TeV using data collected by the ALICE experiment at the CERN LHC. The results are presented in terms of relative $J/ψ$ yields and relative charged-particle multiplicities with respect to these quantities obtained in inelastic collisions having at least one charged particle in the pseudorapidity range $|η| < 1$. The $J/ψ$ mesons are reconstructed via their decay into $μ^+ μ^-$ pairs in the forward rapidity region ($2.5 < y < 4$). The relative multiplicity is estimated in the forward pseudorapidity range which overlaps with the $J/ψ$ rapidity region. The results show a steeper-than-linear increase of the $J/ψ$ yields versus the multiplicity. They are compared with previous measurements and theoretical model calculations.

Figures (4)

• Figure 1: The V0C distributions in minimum-bias events in data, PYTHIA 8.2 Monash MC Sjostrand:2014zea and EPOS3 MC Werner:2013tya. The chosen multiplicity classes are indicated by alternating gray and white areas. The percentile ranges corresponding to the multiplicity classes (from low to high multiplicities) are 70-100%, 50-70%, 40-50%, 30-40%, 20-30%, 15-20%, 10-15%, 5-10%, 1-5%, 0.5-1%, 0.25-0.5%, 0-0.25%. The bottom panel shows the ratio between data and MC.
• Figure 2: The dimuon invariant-mass distribution in the 5-10% multiplicity class. Each dimuon is weighted with the inverse of the detector acceptance and efficiency as a function of the dimuon $p_{\rm T}$ and $y$. The data are fitted to a combination of double-sided Crystal Ball (CB2) functions for the $\rm J/\psi$ and $\psi(2S)$ signals, and a Variable-Width Gaussian function for the background. Only statistical uncertainties are shown. The bottom panel shows the ratio between the data and the fit.
• Figure 3: Forward and midrapidity $\rm J/\psi$ relative yields versus the relative charged-particle multiplicity. The relative forward $\rm J/\psi$ yields as a function of the relative forward charged-particle multiplicity are shown as red circles. The data of midrapidity $\rm J/\psi$ and forward rapidity $\rm J/\psi$ versus midrapidity multiplicity are taken from Refs. ResultsMidrap and ResultsForward, respectively. The error bars and boxes represent the statistical and systematic uncertainties, respectively. The dotted line represents the diagonal.
• Figure 4: $\rm J/\psi$ relative yield as a function of the relative multiplicity in the V0C acceptance $-3.7<\eta<-1.7$ compared with PYTHIA 8.3 Bierlich:2022pfr, EPOS4HQ Zhao:2023ucp and two 3-Pomeron CGC model predictions Levin:2019fvbGotsman:2020ubn. The dotted line represents the diagonal.