Jet physics in heavy-ion collisions

TL;DR

This paper reviews the theoretical framework for jet modification in ultrarelativistic heavy-ion collisions, focusing on how a deconfined QCD medium induces radiation and decoheres color within perturbative QCD. It develops the LPM-based picture of radiative energy loss and an in-medium branching formalism, highlighting the roles of the transport coefficient $\\hat q$, the branching time $t_{br}$, and the decoherence parameter $\\Delta_{med}$, and discusses two main shower-modeling approaches: rate equations and vacuum-like evolution with medium-modified splitting kernels. It then connects these mechanisms to jet quenching observables—$R_{AA}$, di-jet asymmetry, and jet substructure—showing qualitative agreement with LHC data while outlining theoretical uncertainties and challenges. The goal is to provide a coherent, perturbative framework for extracting medium properties from jet measurements and guiding future theory and experiments in jet tomography of the QGP.

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.

Figures (5)

• Figure 1: Diagrams contributing to the dipole scattering rate in Eq. (\ref{['eq:DipoleCrossSection']}) (one must also add the complex conjugate diagrams).
• Figure 2: Left: Square of the gluon emission amplitude in medium. Right: Structure of the squared amplitude after integrating out all transverse momenta.
• Figure 3: Graphical illustration of the equation (\ref{['eq:Sigma1a']}). The thick wavy lines represent the probability ${\@fontswitch{}{\mathcal{}} P}$ for transverse momentum broadening, the black dot is the splitting probability ${\@fontswitch{}{\mathcal{}} K}$, and the circled cross is the cross section of the hard process producing a gluon of momentum $p_0$.
• Figure 4: The characteristic regimes of radiation in media: the 'dipole' regime, $r_\perp \ll Q_s^{-1}$ (left) and the 'decoherence' regime, $r_\perp \gg Q_s^{-1}$ (right). Figure taken from Ref. MehtarTani:2011gf.
• Figure 5: The soft gluon emission spectrum off the quark constituent of a singlet antenna with opening angle $\theta_{12} = 0.2$, according to Eq. (\ref{['eq:SpectrumCoherentSoft']}), in the presence of a medium with $\Delta_\text{med} = 0.5$ (solid line). Here $\bar{\alpha} \equiv \alpha_s C_F/\pi$. On average vacuum radiation is confined within $\theta < \theta_{12}$, while the medium-induced radiation is radiated at $\theta > \theta_{12}$. The limit of opaque medium, given by $\Delta_\text{med} = 1$, is marked by the dashed line. Figure taken from Ref. MehtarTani:2011tz.