A Hybrid Strong/Weak Coupling Approach to Jet Quenching (1405.3864v3)

Abstract: We propose and explore a new hybrid approach to jet quenching in a strongly coupled medium. The basis of this phenomenological approach is to treat physics processes at different energy scales differently. The high-$Q^2$ processes associated with the QCD evolution of the jet from production as a single hard parton through its fragmentation, up to but not including hadronization, are treated perturbatively. The interactions between the partons in the shower and the deconfined matter within which they find themselves lead to energy loss. The momentum scales associated with the medium (of the order of the temperature) and with typical interactions between partons in the shower and the medium are sufficiently soft that strongly coupled physics plays an important role in energy loss. We model these interactions using qualitative insights from holographic calculations of the energy loss of energetic light quarks and gluons in a strongly coupled plasma, obtained via gauge/gravity duality. We embed this hybrid model into a hydrodynamic description of the spacetime evolution of the hot QCD matter produced in heavy ion collisions and confront its predictions with jet data from the LHC. The holographic expression for the energy loss of a light quark or gluon that we incorporate in our hybrid model is parametrized by a stopping distance. We find very good agreement with all the data as long as we choose a stopping distance that is comparable to but somewhat longer than that in ${\cal N}=4$ supersymmetric Yang-Mills theory. For comparison, we also construct alternative models in which energy loss occurs as it would if the plasma were weakly coupled. We close with suggestions of observables that could provide more incisive evidence for, or against, the importance of strongly coupled physics in jet quenching.

A Hybrid Strong/Weak Coupling Approach to Jet Quenching: An Expert Overview

In this paper, the authors propose a novel hybrid framework for analyzing jet quenching phenomena in heavy ion collisions. Their approach uniquely combines aspects of both perturbative and non-perturbative QCD physics by leveraging techniques from the gauge/gravity duality to address processes occurring at different energy scales.

Summary and Methodology

The paper explores jet quenching—a significant energy loss experienced by high-energy partons as they traverse the quark-gluon plasma created in heavy ion collisions. This phenomenon has been observed at both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).

Hybrid Model Design:

1. Perturbative Treatment: The authors treat the high-$Q^2$ processes, which involve the initial production and fragmentation of the jet, as perturbative and use the DGLAP evolution to describe them. This accounts for the hard scattering events and parton showering in a well-defined theoretical framework.
2. Strongly Coupled Dynamics: For the interactions between the propagating partons and the QCD plasma, which involves momentum scales corresponding to the plasma temperature, they employ insights from holographic computations. Within a strongly coupled regime, they model energy loss using dynamics derived from gauge/gravity duality, focusing on the behavior of light quarks and gluons.

Implementation: The authors integrate this hybrid model into a hydrodynamic description of the space-time evolution of the hot QCD matter generated in experiments. They conduct Monte Carlo simulations to predict observable outcomes, leading to comparisons with empirical data on transverse momentum distributions, jet asymmetries, and fragmentation functions.

Key Results and Analysis

The hybrid model yields several predictions in line with experimental data:

• The model accurately describes the centrality-dependent suppression of jet yields, quantified by the nuclear modification factor $R_{AA}$, as seen in LHC data.
• It reproduces the dijet asymmetry observed in heavy ion collisions, consistent with momentum imbalances caused by different parts of the jet experiencing varying degrees of energy loss.
• The model also provides plausible explanations for the fragmentation function ratios, showing how medium-modified jets differ from their vacuum counterparts.

Parameters and Comparison:

The hybrid model utilizes one principal parameter related to the stopping distance of partons in the plasma, adjusted to fit experimental data. The authors contrast their approach with alternate models assuming weakly coupled plasmas, demonstrating their hybrid model's advantages in accounting for the complex interactions within the medium.

Theoretical and Practical Implications

• Theoretical Understanding: The research bridges a gap in theoretical modeling by effectively combining weakly and strongly coupled aspects of QCD, offering a more comprehensive framework for jet quenching.
• Future Insights: The hybrid approach could advance the understanding of QCD matter under extreme conditions by exploring how partonic interactions and energy loss vary with changes in the mediums’ properties.

Closing Remarks and Prospects

Future developments in this research could involve refining the model's assumptions, particularly concerning medium-induced splitting and transverse momentum broadening. Additionally, efforts could focus on integrating more realistic hydrodynamic profiles and enhancing the treatment of hadronization to improve correlation with experimental data. Robust experimental probes capable of distinguishing between the energy loss mechanisms of quarks and gluons would provide invaluable insights into the underlying physics, potentially confirming the strong coupling dynamics identified in the hybrid model.

Ultimately, this paper lays the groundwork for future explorations of jet quenching and related phenomena, highlighting the sophisticated interplay between different QCD regimes in nuclear collisions, and suggesting pathways for more precise, experimentally verified models.