Modification of Jet Spectra with Respect to the Medium in JEWEL

Abstract

The goal of this thesis was to investigate the path-length dependence of jet suppression for jets propagating through a medium known as the quark–gluon plasma (QGP). The QGP is formed in collisions between two lead (Pb) ions, where the overlap region consists of participant nucleons distributed in an approximately elliptical geometry. Simulations of jet quenching in these collisions were performed using JEWEL together with the Glauber and Trajectum models. The former was used to describe the initial state, while the latter described the final state. Events containing jets were categorized according to their initial-state eccentricity ε2 and their final-state elliptic flow, quantified by the parameter q2. The jets were then further separated into in-plane and out-of-plane groups with respect to the short axis of the elliptical collision region. Across both models, out-of-plane jets were observed to be more suppressed than in-plane jets. This thesis successfully reproduced a measurement previously performed by the ALICE Collaboration at CERN. Furthermore, the results suggest that the observed path-length dependence is independent of the choice of models used, further validating its existence.

Popular Abstract

The main idea this thesis aims to demonstrate is path-length dependence in jet suppression. Simply put, this project will simulate sprays of particles losing energy the further they travel in a medium using an event-generating tool called JEWEL, in order to see if we can qualitatively reproduce a similar study done by the ALICE collaboration at CERN.
Imagine that you get into a classical food fight in the cafeteria. You spot your best friend hiding behind an overturned table across the hall, and like any good best friend, you pick up a large creamy apple pie and aim it straight at their exposed face. Consequently, your friend also picks up a pie of the same kind and spots your mischievous eyes! And you both hurl these two pies at each other, and as they go flying through the air, they collide in a glorious spray of cream and apple crumbs. This is what happens when two lead (Pb) ions, smash into each other. Since they travel at relativistic speeds, they look like pancakes, and their collision produces a glorious spray of particles we call jets.
Now you spot a spray bottle of whipped cream and stealthily make your way to the other end of the hall. You squeeze the trigger, but only a few creamy sprays come out. Opening the bottle, you find that there is a little block of thick, dense liquid like honey, clogging the little pipe connecting the contents to the nozzle. This liquid, known as quark-gluon plasma (aka QGP), is a state of matter that forms at extremely high energies in the Pb-Pb collision area. Imagine that this liquid acts like a viscous medium that the jets travel through, and these jets lose energy as they interact with the medium. Thereby, there are fewer jets that come out of the collision due to this clog, and this is known as jet quenching.
Each Pb ion consists of a nucleus, which is essentially a group of protons and neutrons. If you were shrunk down to atomic sizes, holding a nucleus would be akin to holding a fistful of marbles. When a collision occurs, the marbles taking part in the collision (called participant nucleons) form a region that looks like a standing almond. Jets coming out of the short horizontal axis of the almond are considered to be in-plane, and jets spraying out of the long axis are called out-of-plane. Since the long axis of the almond contains more QGP in comparison to the short axis, there is a lower number of out-of-plane jets than in-plane jets. This project demonstrates this effect and shows that it is only observable when considering a highly elliptical QGP shape.
There are three models taken into account and then embedded into JEWEL. The first is a simple model of the collision, where the individual nucleons (or marbles) do not move but stay still at a fixed eccentricity; therefore, it is not useful for our studies investigating path-length dependence. In reality, the marbles shuffle around, which is taken into consideration in the next model, called Glauber. The third and final model adds another layer of complexity by also looking at the final state of the collision. In this state, the collision region expands like a balloon due to the incredibly high energy and the fluid-like nature of the QGP.
In all the models, it was quite remarkable to see that there were more in-plane jets than out-of-plane ones, meaning that this thesis has successfully shown, via JEWEL, that jet suppression does indeed depend on the distance jets travel across the medium; jets traveling the long way through the medium are more suppressed than those taking a shortcut. Furthermore, by choosing events that are highly elliptical, we can observe that the significance of this analysis also lies in showing that one needs to consider the geometry of the QGP. Coming back to the analogy of the spray bottle, the clog should be more elliptical if you want to get back at your friend.

• author: Bentota Arachchige, Dahamvin
• supervisor: Alice Ohlson; Korinna Zapp
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Jet suppression, QGP, Path-length dependence
• language: English
• id: 9230541
• date added to LUP: 2026-06-04 08:18:10
• date last changed: 2026-06-04 08:18:10