Charmonia production in hadron colliders and the extraction of colour-octet matrix elements

TL;DR

The study addresses the puzzle of prompt charmonium production in hadron colliders by incorporating the NRQCD color-octet mechanism into a Monte Carlo framework with initial-state radiation. It implements the color-octet channels in PYTHIA, fits Tevatron data to extract NRQCD matrix elements, and shows that accounting for ISR lowers the required matrix elements, particularly the combined 1S0(8) and 3P0(8) pieces. The authors then provide LHC predictions for direct J/psi and psi' production using the tuned matrix elements and examine the impact of different PDFs on the pT spectra. The work demonstrates a practical approach to testing NRQCD universality across processes and offers a fast simulation tool for future collider studies, with implications for resolving tensions with photoproduction data and guiding experimental analyses at the LHC.

Abstract

We present the results of our analysis on charmonia ($J/ψ$ and $ψ'$) hadroproduction taking into account higher-order QCD effects induced by initial-state radiation in a Monte Carlo framework, with the colour-octet mechanism implemented in the event generation. We find that those colour-octet matrix elements extracted so far from Fermilab Tevatron data for both $J/ψ$ and $ψ'$ production have to be lowered significantly. We finally make predictions for charmonia production at the LHC, presenting a code for a fast simulation with PYTHIA based on the colour-octet model.

Figures (5)

• Figure 1: Curves obtained from PYTHIA (not fit) including the colour-octet mechanism for prompt $J/\psi$(left) and $\psi'$(right) production at the Tevatron using the same parameters as in Ref. [17]. The solid line corresponds to initial- and final-state radiation off and the dotted line to initial- and final-state radiation on. The MRSD0 parton distribution function and $M_c=1.48$ GeV were employed as in [17]. All curves are multiplied by the corresponding muon branching fraction of charmonium.
• Figure 2: Two-parameter fits to the experimental Tevatron data using CSM + COM, where initial- and final-state radiation were incorporated via PYTHIA generation. a) ( left) $J/\psi$ and b) ( right) $\psi'$ production. The different contributions are shown separately i) dotted line: CSM, ii) dashed line: $^1S_0^{(8)}$+$^3P_J^{(8)}$, iii) dot-dashed line: $^3S_1^{(8)}$, iv) solid line: all contributions.
• Figure 3: ( left) Different contributions to $J/\psi$ hadroproduction using the CTEQ PDF. The curves are labelled in the same way as in Fig. 2. ; ( right) Our prediction for prompt $J/\psi$ direct production at the LHC using PYTHIA with the colour-octet matrix elements from Table 1: a) dotted line: CTEQ 2L, b) dot-dashed line: MRSD0, and c) solid line: GRV HO. A rapidity cut ${\mid}y{\mid}<2.5$ was required.
• Figure 4: $J/\psi$ (solid line) and ${\psi}'$ (dash-dotted line) direct production at the LHC according to the colour-octet ME's reported in Tables 1 and 2 using the CTEQ PDF. The cut ${\mid}y{\mid}<2.5$ was required in both cases. Both curves are multiplied by the respective branching fraction of charmonium into muons.
• Figure 5: Histograms for $BF(J/\psi{\rightarrow}{\mu}^+{\mu}^-){\times} d{\sigma}/dp_t(pp{\rightarrow}{J/\psi}X)$ obtained from PYTHIA using the CTEQ PDF at 14 TeV center-of-mass energy. Solid line: all contributions; dashed line: $gg$ contribution.