[Paper Review] Forward $J/ψ$ and $D$ meson nuclear suppression at the LHC

B. Ducloué, T. Lappi+1 more|arXiv (Cornell University)|Dec 14, 2016

High-Energy Particle Collisions Research19 references4 citations

TL;DR

This paper revisits forward J/ψ and D meson nuclear suppression in proton-lead collisions at the LHC using the color glass condensate (CGC) framework with improved nuclear geometry treatment via the optical Glauber model. By deriving the nucleus dipole cross section from the proton fit to HERA data, the model reduces nuclear suppression and achieves significantly better agreement with ALICE and LHCb data than previous CGC calculations.

ABSTRACT

Using the color glass condensate formalism, we study the nuclear modification of forward $J/ψ$ and $D$ meson production in high energy proton-nucleus collisions at the LHC. We show that relying on the optical Glauber model to obtain the dipole cross section of the nucleus from the one of the proton fitted to HERA DIS data leads to a smaller nuclear suppression than in the first study of these processes in this formalism and a better agreement with experimental data.

Motivation & Objective

• To address the discrepancy between earlier CGC calculations and LHC data on forward J/ψ and D meson suppression in p-A collisions.
• To investigate how nuclear geometry effects influence the initial saturation scale and resulting nuclear modification factors.
• To test whether the optical Glauber model provides a more accurate description of nuclear dipole cross sections than simple A^{1/3} scaling.
• To improve theoretical predictions for forward quarkonium and open charm production in high-energy p-A collisions using a consistent CGC framework.
• To validate the model against ALICE and LHCb data for J/ψ and D⁰ mesons at √sNN = 5 TeV.

Proposed method

• Uses the dilute-dense limit of the color glass condensate (CGC) framework to describe forward J/ψ and D meson production in p-A collisions.
• Applies the running-coupling Balitsky-Kovchegov (BK) equation to evolve the dipole correlator in rapidity, with initial conditions fitted to HERA deep inelastic scattering (DIS) data.
• Employs the optical Glauber model to relate the nucleus dipole cross section to the proton cross section, avoiding arbitrary scaling assumptions.
• Calculates the nuclear modification factor R_pA by normalizing p-A cross sections to pp cross sections, canceling normalization uncertainties.
• Uses the color evaporation model (CEM) for J/ψ production and fragmentation functions for D⁰ meson production, with non-perturbative parameters canceled in R_pA.
• Performs numerical solutions of the BK equation with initial conditions derived from the MV model parametrization, incorporating impact parameter averaging.

Experimental results

Research questions

• RQ1 Does the optical Glauber model for nuclear geometry lead to reduced nuclear suppression in forward J/ψ and D meson production compared to simple A^{1/3} scaling?
• RQ2 Can the improved treatment of the initial saturation scale in the nucleus reduce the discrepancy between CGC predictions and LHC data?
• RQ3 How does the choice of initial condition for the nucleus dipole cross section affect the predicted R_pA for forward J/ψ and D⁰ mesons?
• RQ4 To what extent do nuclear geometry effects explain the smaller-than-expected suppression observed in ALICE and LHCb data?
• RQ5 Is the CGC framework with Glauber-based nuclear scaling capable of describing both J/ψ and D meson data consistently at high transverse momentum?

Key findings

• The optical Glauber model leads to a smaller initial saturation scale for the lead nucleus compared to simple A^{1/3} scaling, reducing nuclear suppression.
• The resulting nuclear modification factor R_pA for forward J/ψ production shows significantly better agreement with ALICE and LHCb data than in the first CGC study of this process.
• For D⁰ mesons, the same approach yields a reduced suppression and improved agreement with LHCb data in the 2.5 < Y < 4 rapidity window.
• The uncertainty bands in R_pA account for variations in charm quark mass (1.2–1.5 GeV) and factorization scale (Q between M⊥/2 and 2M⊥), yet the results remain consistent with data.
• The model predicts that R_pA approaches unity at large transverse momentum, consistent with experimental observations and previous findings in single-inclusive hadron production.
• The improved agreement is primarily due to a more realistic nuclear saturation scale, not new parameters, as the initial proton condition is fixed by HERA DIS data.