Gluon Radiation and Parton Energy Loss

TL;DR

Hard partons traversing spatially extended matter lose energy through medium-induced gluon radiation, altering their fragmentation patterns in high-energy collisions. The authors synthesize a comprehensive framework based on Wilson lines in the eikonal limit and extend it to non-eikonal trajectories with path-integral methods and the non-Abelian Furry approximation, enabling calculation of the medium-induced gluon spectrum. They analyze two main regimes—the dipole (multiple soft scattering) and opacity (few hard scatterings)—and introduce the transport coefficient hat{q} and the saturation scale Q_s as central medium parameters, including their evolution in expanding media. The work connects the radiation spectrum to observable consequences such as quenching weights and medium-modified fragmentation functions, providing tools to quantify jet quenching in RHIC and LHC environments.

Abstract

The propagation of hard partons through spatially extended matter leads to medium-modifications of their fragmentation pattern. Here, we review the current status of calculations of the corresponding medium-induced gluon radiation, and how this radiation affects hadronic observables at collider energies.

Figures (10)

• Figure 1: (a) Dependence of the medium-induced radiative energy loss $\langle \Delta E\rangle$ on the in-medium pathlength $L$ for $E = 100$ GeV. The transport coefficient is defined here as $\frac{\hat{q}}{2} = n_0\, C$. (b) Double logarithmic presentation of (a). Figure taken from Wiedemann:2000tf.
• Figure 2: The energy distribution of radiated gluons $\omega \frac{dI}{d\omega}$ for different values of the kinematical constraint $R = \omega_c\, L$. Figure taken from Salgado:2003gb.
• Figure 3: The fraction of the total radiative energy loss $\Delta E/E$ emitted outside a jet cone of fixed angle $\Theta$. Here, the transport coefficient is defined as $\hat{q} = 2\, n_0\, C$. Figure taken from Wiedemann:2000tf.
• Figure 4: LHS: The medium-induced gluon energy distribution radiation for a dynamically expanding collision regions (\ref{['4.27']}) with expansion parameter $\alpha =$ 0, 0.5, 1.0 and 1.5. The value of the transport coefficient $\hat{q}_0$ is taken at initial time $\xi_0$. RHS: The same gluon radiation spectrum with parameters rescaled according to (\ref{['4.32']}). Figure taken from Salgado:2003gb.
• Figure 5: Estimate of the transport coefficient as a function of the energy density $\epsilon$. Figure taken from Baier:2002tc.