A Monte Carlo Model for 'Jet Quenching'

TL;DR

JEWEL 1.0 introduces a Monte Carlo framework that couples a perturbative final-state parton shower to a QCD medium by comparing the splitting probability with a density-dependent scattering probability $S_a(Q_i^2,Q_f^2)$ in each jet fragment. The study explores elastic (collisional) energy loss and, via a phenomenological enhancement of splitting functions, radiative energy loss within a common multi-particle final-state framework. Vacuum benchmarks validate the baseline shower and hadronisation against LEP data, while medium extensions demonstrate observable differences in jet fragmentation and jet shapes. The results suggest that the $n$-jet fraction and jet-shape observables can help disentangle collisional and radiative mechanisms and guide future developments, including more realistic geometry and inelastic processes.

Abstract

We have developed the Monte Carlo simulation program JEWEL 1.0 (Jet Evolution With Energy Loss), which interfaces a perturbative final state parton shower with medium effects occurring in ultra-relativistic heavy ion collisions. This is done by comparing for each jet fragment the probability of further perturbative splitting with the density-dependent probability of scattering with the medium. A simple hadronisation mechanism is included. In the absence of medium effects, we validate JEWEL against a set of benchmark jet measurements. For elastic interactions with the medium, we characterise not only the medium-induced modification of the jet, but also the jet-induced modification of the medium. Our main physics result is the observation that collisional and radiative medium modifications lead to characteristic differences in the jet fragmentation pattern, which persist above a soft background cut. We argue that this should allow to disentangle collisional and radiative parton energy loss mechanisms by measuring the n-jet fraction or a class of jet shape observables.

Figures (10)

• Figure 1: The thrust, thrust major and thrust minor ($T_\text{r}=(T,T_\text{maj},T_\text{min})$) distributions for $\sqrt{s}=\unit[200]{GeV}$$e^+e^-\to q\, \bar{q}\to X$ collisions. Data of the ALEPH Collaboration Heister:2003aj are compared to simulations of Jewel: i) parton level after parton shower evolved down to $Q_0 = \unit[1]{GeV}$, ii) hadron level after parton shower evolution followed by hadronisation ($Q_0 = \unit[1]{GeV}$).
• Figure 2: The jet rates as a function of jet resolution scale $y_{\rm cut}$ in $\sqrt{s}=\unit[200]{GeV}$$e^+e^-\to q\, \bar{q}\to X$ collisions. Left hand side: Simulation of Jewel with and without hadronisation for evolution down to $Q_0=\unit[1]{GeV}$. Right hand side: Data of the Aleph collaboration Heister:2003aj compared to simulations of Jewel with hadronisation.
• Figure 3: The inclusive distribution ${\rm d} N_{\rm ch}/{\rm d} \xi$, $\xi = \ln \left[ E_\text{jet}/p_\text{hadron} \right]$ of charged hadrons in $e^+e^-\to q \bar{q} \to X$ events at $\sqrt{s}=\unit[200]{GeV}$. Data of the ALEPH Collaboration Heister:2003aj are compared to simulations of Jewel: i) parton level after parton shower evolved down to $Q_0 = \unit[1]{GeV}$, ii) hadron level after parton shower evolution to $Q_0 = \unit[1]{GeV}$ followed by hadronisation.
• Figure 4: The average parton energy loss ${\rm d} E/{\rm d} x$ of a quark of energy $E$, undergoing multiple elastic collisions over a path length $L = \unit[1]{fm}$ in a thermal medium of temperature $T$. Elastic collisions are described by the infra-red regulated partonic cross sections of equation (\ref{['3.5']}) (case I) and equation (\ref{['3.6']}) (case II).
• Figure 5: Distribution of energy loss $\Delta E$ for different parameter choices (top panels) and the probability for an energy loss smaller than $\Delta E$ after passage of a medium of length $L$ (bottom panels).