Jet physics in heavy-ion collisions (1302.2579v2)

Abstract: Jets are expected to play a prominent role in the ongoing efforts to characterize the hot and dense QCD medium created in ultrarelativistic heavy ion collisions. The success of this program depends crucially on the existence of a full theoretical account of the dynamical effects of the medium on the jets that develop within it. By focussing on the discussion of the essential ingredients underlying such a theoretical formulation, we aim to set the appropriate context in which current and future developments can be understood.

Jet Physics in Heavy-Ion Collisions

The paper of jet physics within the context of heavy-ion collisions, as explored by Mehtar-Tani, Milhano, and Tywoniuk, provides significant insights into the complex interplay of energetic partons with the quark-gluon plasma (QGP). Their paper addresses the theoretical underpinnings necessary to understand jet dynamics influenced by the medium created in ultrarelativistic collisions, such as those performed at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).

The production of a quark-gluon plasma in these conditions has been convincingly established through various experimental signatures, laying the groundwork for further exploration into the properties of QCD matter under extreme conditions. Two aspects prominently discussed in the paper include the suppression of high transverse momentum ($p_T$) particles and the evidence of collective behavior indicative of deconfined constituents within the plasma.

Key Components

To evaluate the properties of jets in heavy-ion environments, several factors must be considered:

1. Jet Quenching: This phenomenon refers to the energy loss of high-energy partons traversing the QGP, manifesting as a suppression of high-$p_T$ particles. Calculations of such energy loss involve understanding the medium-induced gluon radiation processes, primarily accounting for the Landau-Pomeranchuk-Migdal (LPM) effect, which results from destructive interference between successive gluon emissions.
2. Nuclear Modification Factors: The comparison of jet production in nucleus-nucleus (A+B) collisions to that in proton-proton (p+p) collisions is often quantified using the nuclear modification factor $R_{AA}$, which measures deviations from binary scaling expectations. Such measurements are crucial for identifying the modifications induced by the QGP.
3. Transverse Momentum Broadening: This is quantified through the transport parameter $\hat{q}$, measuring the average transverse momentum squared transferred to a parton per unit path length in the medium. It reflects the 2-dimensional nature of collision dynamics affecting partons.
4. Radiative Corrections and Thermal Mass: Adjustments to the emission rates and thermal propagators encode modifications in self-energies of the propagating partons, crucial for accurately predicting the spatial evolution of jets.

The theoretical description relies heavily on the factorization of short and long-distance physics in QCD, utilizing perturbative QCD for the hard scatterings and effective models for describing the subsequent interactions with the medium.

Implications and Future Directions

This comprehensive survey of jet-medium interactions establishes that jets serve not only as probes of the QGP's properties but also as testing grounds for QCD under extreme conditions. The adaptability of jet observables offers pathways to extract medium parameters like $\hat{q}$ or to paper modified fragmentation functions, which become altered by the jet’s passage through the medium.

The paper further speculates on the role of color coherence and color flow alteration, where the medium can cause jets to lose their coherent structure, thereby impacting hadronization patterns. Such decoherence effects, significant for understanding both initial-state parton distributions and final-state interactions, highlight critical areas for future experimental and theoretical work.

In conclusion, understanding jet phenomena in heavy-ion collisions delivers profound implications for our comprehension of QCD matter's microscopic properties. Progress in this domain necessitates both refined theoretical models and precise experimental data to elucidate the complex nature of quark-gluon plasma.