Heavy-quarkonium suppression in p-A collisions from parton energy loss in cold QCD matter

Abstract: The effects of parton energy loss in cold nuclear matter on heavy-quarkonium suppression in p-A collisions are studied. It is shown from first principles that at large quarkonium energy E and small production angle in the nucleus rest frame, the medium-induced energy loss scales as E. Using this result, a phenomenological model depending on a single free parameter is able to reproduce J/psi and Upsilon suppression data in a broad xF-range and at various center-of-mass energies. These results strongly support energy loss as the dominant effect in heavy-quarkonium suppression in p-A collisions. Predictions for J/psi and Upsilon suppression in p-Pb collisions at the LHC are made. It is argued that parton energy loss scaling as E should generally apply to hadron production in p-A collisions, such as light hadron or open charm production.

• The paper identifies parton energy loss as the main mechanism causing heavy-quarkonium suppression in p-A collisions.
• The authors develop a phenomenological model with one free parameter that accurately reproduces suppression data for J/ψ and Υ production.
• The study provides predictive insights for LHC p-Pb collisions, offering experimental validation of initial-state energy loss effects.

Analysis of Heavy-Quarkonium Suppression in Proton-Nucleus Collisions from Parton Energy Loss in Cold QCD Matter

The discussed paper addresses the intriguing phenomenon of heavy-quarkonium suppression in proton-nucleus (p--A) collisions, attributing it mainly to parton energy loss in cold nuclear matter. This study is significant as it provides insights into the nuclear modification patterns essential for understanding quark-gluon plasma (QGP) effects in nucleus-nucleus (A--A) collisions. The work, conducted by Francois Arleo and Stephane Peigne, presents a comprehensive evaluation of parton energy loss mechanisms and their implications for heavy-quarkonium production, with particular emphasis on the role of energy scaling.

Key Insights and Methodology

1. Energy Loss Mechanism: The paper identifies parton energy loss as a central mechanism for heavy-quarkonium suppression in p--A collisions. It posits that, at large transverse momenta and small production angles, the energy loss scales linearly with the energy of the quarkonium. This conclusion emerges from first-principle calculations, highlighting the processes of induced gluon radiation in a cold nuclear medium.
2. Phenomenological Model: Utilizing a phenomenological model with a single free parameter, the authors successfully reproduce empirical suppression data for both $J/\psi$ and $\Upsilon$ production. This underscores the robustness and predictive power of their model across different systems and energy scales.
3. Numerical Results: The model effectively describes quarkonium suppression data across a range of Feynman variables ($x_F$) and center-of-mass energies, supporting the energy loss as the dominant quarkonium suppression effect in p--A collisions. Crucially, it predicts substantial suppression ratios consistent with experimental findings.
4. Predictions for LHC: The paper makes specific predictions for quarkonium suppression in p--Pb collisions at the Large Hadron Collider (LHC). These predictions are valuable for designing future experiments and interpreting results in terms of initial-state effects independent of QGP formation.

Implications and Future Directions

This research provides a notable theoretical framework that not only clarifies the heavy-quarkonium suppression observed in p--A collisions but also extends its utility to predict outcomes at collider experiments like the LHC. Given the accuracy in matching experimental results, the findings offer a foundation for refined studies on other hadronic processes in proton-nucleus interactions, such as light hadron production.

1. Theoretical Advancement: The paper advances our comprehension of cold nuclear matter effects, essential for disentangling them from hot nuclear matter effects present in A--A collisions. It suggests avenues for defining the relative contributions of different initial-state effects, such as shadowing and nuclear absorption.
2. Experimental Validation: The predictions set the stage for experimental validation at high-energy colliders. Future research could focus on comparative analyses of suppression data across different systems, further constraining and testing the model parameters.
3. Broadening Applications: While the paper concentrates on quarkonium suppression, the insights gleaned from it can be extrapolated to study other processes such as open charm and light hadron production under similar conditions. This aligns with the broader research objective of characterizing nuclear modifications across various collision types.

The paper thus serves as a critical milestone in nuclear physics, shedding light on the complexities of quarkonium production and its suppression due to initial-state energy loss effects, and underscores the need for continued experimental and theoretical exploration in this arena.