Searches for Path Length Dependent Jet Quenching in JETSCAPE

Abstract

The primary objective of this thesis is to research path length-dependent jet quenching in quark-gluon plasma (QGP) produced during ultra-relativistic heavy ion collisions at the Large Hadron Collider (LHC). This was done using the JETSCAPE framework, that allows for modular simulation of heavy-ion collisions, using the latest Monte Carlo simulators available for each stage of the process. This was done using two sets of modules, the AA hard tune and the PP19 tune. The AA hard tune simulated a heavy-ion collision with a hydrodynamic medium, alongside jets produced by Pythia to probe this medium. While the PP19 tune simulated proton-proton collisions, once again producing jets with Pythia. By investigating the position of the hard scatterings generated in the AA hard tune, under different trigger conditions, path length dependent jet quenching was observed. However, this was a much smaller effect than what was expected. As such two particle correlations to investigate the shapes of the jets as a function of path length was not possible. However, correlation functions for AA hard and PP19 tune were done regardless, as this is the first time something like this has been done in JETSCAPE. Comparing these revealed the effects of QGP in modifying the shape of the jet peaks, as well as correlations from the bulk medium.

Popular Abstract

Scientists at the Large Hadron Collider, (LHC), in CERN, have been able to produce the world's hottest droplet of liquid. The temperatures of this liquid exceeds trillions of Kelvins! That is over 100,000 times higher than the temperature at the Sun's core. This droplet of liquid is called the quark-gluon plasma or QGP for short. Quarks and gluons are some of the most fundamental particles in the universe. These make up the protons and neutrons, that are in the nucleus.

Scientists are able to produce QGP by colliding two heavy nuclei together. At the LHC these nuclei are traveling very fast, at 99.999992% the speed of light. At such high velocities special relativistic effects start to become significant, and from our perspective these nuclei appear to be completely flat, like two pancakes colliding head on. As a result of this collision, all of the quarks and gluons that are inside the protons and neutrons, burst forth forming what is known as the QGP.

Some of these quarks (or gluons) collide with each other at very high energies, and transfer a lot of momentum during the collision, causing them to shoot out in opposite directions. These quarks (or gluons) then traverse through the QGP, and upon exiting the liquid for a jet of particles called, unsurprisingly, a jet. These jets are very useful to scientists, as it allows them to study the QGP indirectly by measuring the difference between jets that travel through the QGP versus those that do not, such as jets produced in proton-proton collisions. This is necessary because the QGP is very short lived, and all we see is the detritus of such a high energy collision.

One of the consequences of traveling through the QGP is that, the jets loose some energy. This process is known as jet quenching. And it seems obvious that if a jet travels longer in the QGP it must interact more with the medium and therefore loose for energy. However, this effect has not been observed in experimental data yet. Some measurements suggest that there might be, what is known as, path length dependent jet quenching. However, theoretical models suggest that this might just be due to variations in the data, and not due to QGP. Thus, we decided to investigate this phenomenon further. By testing heavy-ion collision using a new framework called JETSCAPE. JETSCAPE uses the latest and greatest Monte Carlo simulations that are currently available to generate heavy-ion collisions. Using JETSCAPE we generated thousands of collision events, and analyzed the data using a technique called two particle correlations, in order to search for path length dependent jet quenching.

We were able to find signs of path length dependence, however these effect were much smaller than what he had hoped for. Thus, a thorough investigation of jet shape with respect to path length, was not possible. However, two particle correlations were done using JETSCAPE data, for two different types of collisions: i) heavy-ion collisions as before, and ii) proton-proton collisions. Comparing the results from these two revealed some key differences, that showcase the effects of jet modification.

This is promising, as it suggests that path length dependent jet quenching is likely a real phenomenon, but much smaller than we anticipate. Which might be why we haven't observed it in experimental data as current experimental observables aren't sensitive enough to path length. Thus, future studies can build on this and try to find better parameters which showcase more significant path length dependence. These parameters can then be used to guide analysis of experimental data.

• author: Chakraborty, Ayush
• supervisor: Alice Ohlson
• organization: Particle and nuclear physics; Department of Physics
• course: FYSK04 20242
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9182671
• date added to LUP: 2025-01-27 07:57:20
• date last changed: 2025-01-27 07:57:20