The theory and phenomenology of perturbative QCD based jet quenching (1002.2206v3)

Abstract: The study of the structure of strongly interacting dense matter via hard jets is reviewed. High momentum partons produced in hard collisions produce a shower of gluons prior to undergoing the non-perturbative process of hadronization. In the presence of a dense medium this shower is modified due to scattering of the various partons off the constituents in the medium. The modified pattern of the final detected hadrons is then a probe of the structure of the medium as perceived by the jet. Starting from the factorization paradigm developed for the case of particle collisions, we review the basic underlying theory of medium induced gluon radiation based on perturbative Quantum Chromo Dynamics (pQCD) and current experimental results from Deep Inelastic Scattering on large nuclei and high energy heavy-ion collisions, emphasizing how these results constrain our understanding of energy loss. This review contains introductions to the theory of radiative energy loss, elastic energy loss, and the corresponding experimental observables and issues. We close with a discussion of important calculations and measurements that need to be carried out to complete the description of jet modification at high energies at future high energy colliders.

• The paper presents a detailed analysis of jet quenching in pQCD, emphasizing radiative energy loss calculations and their alignment with collider data.
• It compares multiple methodologies—including Higher Twist, AMY, and GLV/ASW frameworks—to model energy loss in dense QGP media.
• The findings provide significant constraints on QGP transport coefficients, advancing theoretical predictions for future high-energy experiments.

Overview of Jet Quenching in Perturbative QCD

The paper by Majumder and van Leeuwen explores the theoretical and phenomenological aspects of jet quenching in perturbative Quantum Chromodynamics (pQCD). It focuses on the paper of how jets—narrow cones of particles produced in high-energy processes—are modified as they pass through a dense medium, such as the Quark-Gluon Plasma (QGP) that might be created in heavy-ion collisions. Through their interactions with the medium, these jets lose energy, a process termed "jet quenching," providing insights into the medium's properties.

The research primarily deals with the suppression of high transverse momentum ($p_T$) hadrons and reconstructs the theory within the context of pQCD. The authors review the calculation methodologies for radiative energy loss and outline how these theoretical distributions align with experimental data from facilities like the Relativistic Heavy Ion Collider (RHIC) and, prospectively, the Large Hadron Collider (LHC).

Theoretical Framework

Several approaches to calculating jet energy loss are discussed, each providing different assumptions and methodologies:

1. Higher Twist (HT) Framework: This formalism focuses on multiple gluon emissions and the calculation of energy loss as a series in powers of $L$, relying on scattering within the medium. It uses factorization theorems to relate partonic cross-sections to hadronic observables.
2. AMY Approach (Arnold, Moore, and Yaffe): Rooted in Hard Thermal Loop (HTL) perturbation theory, this approach treats the medium as a quark-gluon plasma in thermal equilibrium, considering the interplay of collisional and radiative processes.
3. GLV (Gyulassy-Levai-Vitev) and ASW (Armesto-Salgado-Wiedemann) Approaches: These methods model the medium as a series of static scattering centers, focusing on gluon interactions and opacity expansions, incorporating LPM interference effects over multiple scatterings.
4. Transverse Momentum Diffusion and Elastic Energy Loss: Emphasizing that apart from radiative processes, the momentum exchange can result in significant modifications to the jet evolution, especially for heavy quarks due to the dead cone effect.

Strong Numerical Results

The authors highlight calculations that show strong alignment with experimental data, such as the suppression factors observed at RHIC. They point to the measurement of the nuclear modification factor ($R_{AA}$) and di-hadron correlations. There are critical discussions around path length dependencies and the role of LPM interference in jet quenching.

Practical and Theoretical Implications

One of the substantial implications of this work is its contribution to understanding the properties and QCD nature of the QGP. This includes establishing constraints on the transport coefficients like $\hat{q}$, which quantifies the transverse momentum diffusion per unit path length. The precise measurement and determination of these coefficients remain essential for characterizing the medium's properties through jet quenching studies.

Future Developments

Looking forward, this research provides a strong basis for quantitative assessments of jet quenching phenomena at the LHC, where energies and densities exceed those observed at RHIC. As experimental resolution evolves, it promises to sharpen the theoretical predictions further and aid in disentangling the intertwined contributions of radiative and collisional energy loss mechanisms, making substantial advancements in the field of high-energy nuclear physics.

In conclusion, this paper forms a cornerstone in the exploration of strongly interacting matter under extreme conditions, leveraging the precision of pQCD calculations to interpret complex experimental data. It builds a comprehensive understanding necessary for taking full advantage of new experimental data that will arise from future high-energy collider experiments.