[Paper Review] Nuclear modification of the J/Psi transverse momentum distributions in high energy pA and AA collisions

Dmitri E. Kharzeev, E.M. Levin+1 more|arXiv (Cornell University)|May 7, 2012

High-Energy Particle Collisions Research5 citations

TL;DR

This paper evaluates nuclear modification of J/ψ transverse momentum spectra in pA and AA collisions at RHIC and LHC energies using the QCD dipole model, resumming initial-state cold nuclear matter (CNM) effects to all orders in αₛ²A¹ᐟ³. It finds that CNM effects alone cannot explain J/ψ suppression at semi-hard transverse momenta, implying significant final-state effects are required—especially at LHC energies where CNM suppression increases but experimental data show less suppression, exacerbating the discrepancy.

ABSTRACT

We evaluate the transverse momentum spectrum of J/Psi (up to semi-hard momenta) in pA and AA collisions taking into account only the initial state effects, but resumming them to all orders in α_s^2 A^{1/3}. In our previous papers we noticed that cold nuclear matter effects alone could not explain the experimental data on rapidity and centrality dependencies of the J/Psi yield in AA collisions indicating the existence of an additional suppression mechanism. Our present calculations indicate that the discrepancy persists and even increases at semi-hard transverse momenta, implying a significant final state effect on J/Psi production in this kinematical domain. The QCD dipole model we employ is only marginally applicable for J/Psi production at mid-rapidity at RHIC energies but its use is justified in the forward rapidity region. At LHC energies we can quantitatively evaluate the magnitude of cold nuclear matter effects in the entire kinematical region of interest. We present our calculations of J/Psi transverse momentum spectra in pA and AA collisions at LHC and RHIC energies.

Motivation & Objective

• To quantify initial-state cold nuclear matter (CNM) effects on J/ψ production in high-energy pA and AA collisions.
• To assess whether CNM effects alone can explain the observed J/ψ suppression in experiments at RHIC and LHC.
• To evaluate the role of final-state effects by comparing theoretical CNM predictions with experimental data on transverse momentum spectra.
• To extend the QCD dipole model to include all-order resummation of multiple gluon exchanges in nuclei for J/ψ production.
• To test the applicability of the dipole model in both mid-rapidity and forward rapidity regions at different energies.

Proposed method

• Uses the QCD dipole model to describe J/ψ production via multiple gluon exchanges in the t-channel, consistent with C- and P-parity quantum numbers.
• Resums all-order multiple interactions in the nucleus using the saturation scale Qₛ, with scaling behavior dσ/d²p⊥d²b⊥ ∝ Qₛ⁴/p⊥⁶ in the semi-hard regime.
• Expresses pA and AA cross sections as a convolution of pp cross sections and nuclear-dependent scattering factors to isolate CNM effects.
• Applies the quasi-classical approximation and includes low-x evolution effects to generalize results across kinematic regions.
• Uses a phenomenological model for the scattering amplitude to compute transverse momentum spectra numerically.
• Evaluates the longitudinal form factor 𝒫ₗ to assess coherence effects, using a hard-sphere nucleus model to derive 𝒫ₗ(p⊥) dependence.

Experimental results

Research questions

• RQ1 Can cold nuclear matter effects alone explain the observed J/ψ suppression in pA and AA collisions at RHIC and LHC energies?
• RQ2 How does the nuclear modification of J/ψ transverse momentum spectra evolve with energy and centrality in the semi-hard momentum region?
• RQ3 To what extent does the QCD dipole model accurately describe J/ψ production in different rapidity regions at RHIC and LHC?
• RQ4 What is the quantitative magnitude of initial-state CNM effects on J/ψ spectra at LHC energies compared to RHIC?
• RQ5 Why does the discrepancy between CNM predictions and experimental data on J/ψ suppression increase at semi-hard transverse momenta?

Key findings

• Initial-state CNM effects, resummed to all orders in αₛ²A¹ᐟ³, lead to a significant suppression of J/ψ at semi-hard transverse momenta, especially at LHC energies.
• The calculated CNM suppression increases at LHC energies compared to RHIC, contradicting experimental data which show less suppression at LHC.
• The discrepancy between theoretical CNM predictions and experimental data persists and grows at semi-hard p⊥, indicating a strong need for final-state effects to explain the data.
• The QCD dipole model is quantitatively applicable at LHC energies across the full kinematic region, but only marginally valid at RHIC mid-rapidity.
• The longitudinal form factor 𝒫ₗ shows that coherence is lost at high p⊥, leading to breakdown of geometric scaling and transition to hard dynamics.
• The model predicts reasonable agreement with RHIC dAu data on J/ψ production, but fails to describe LHC AA data, highlighting the need for additional suppression mechanisms.