Study of energy flow fluctuations within jets at heavy-ion collisions with ALICE
Authors:   , Pliatskas Stylianidis Christos 
Published: January 2025

This dissertation advances the understanding of the strong nuclear force under extreme conditions akin to those in the early universe by analyzing data from the ALICE detector at CERN's Large Hadron Collider (LHC). It leverages high-energy collisions of fundamental particles to study Quantum Chromo-Dynamics (QCD), the theory governing the strong force, where partons (quarks, antiquarks, and gluons) interact via color charge. A key feature of QCD is parton confinement into color-neutral hadrons, the particles observed in nature.

In particle collisions, partons scatter, producing particle showers through QCD radiation, a process culminating in hadronization. Since detectors measure hadrons rather than partons directly, jet-finding algorithms are employed to reconstruct parton-level interactions from collimated sprays of particles. These algorithms are critical in both proton-proton (pp) and heavy-ion collisions.

Heavy-ion collisions, involving large nuclei such as lead, create conditions of extreme temperature and density, leading to the formation of a Quark-Gluon Plasma (QGP). This state of deconfined quarks and gluons behaves like a near-perfect liquid and alters the structure of jets passing through it, a phenomenon known as jet quenching. This dissertation introduces and evaluates a novel jet observable, jet energy flow, defined as the difference in transverse momentum between small and large jets, to probe these interactions.

By analyzing pp and Pb-Pb (heavy-ion) collisions, this study characterizes how jet energy flow is modified by the QGP. The results demonstrate a monotonic decrease in energy flow with increasing jet radius in pp collisions, confirming prior observations that most jet energy resides in a compact core near the jet axis. In Pb-Pb collisions, jets exhibit narrower energy profiles and enhanced energy loss, indicative of the QGP's effects.

The sensitivity of the jet energy flow observable to QGP-induced modifications was evaluated using the JEWEL Monte Carlo model, which simulates jet-medium interactions. Notably, the recoil-inclusive and recoil-exclusive modes of JEWEL provide distinct predictions for energy flow behavior, highlighting the observable's potential for probing medium effects.

The findings suggest a strong agreement between the JEWEL recoil-exclusive mode and experimental measurements, although discrepancies remain. These discrepancies underscore the need for further refinement of theoretical models and more extensive measurements, particularly at larger jet radii, to capture medium-induced radiation redistribution.

In conclusion, the jet energy flow observable represents a significant advancement in the study of jet quenching. Its ability to capture event-by-event fluctuations offers unique insights into QGP dynamics, complementing existing jet substructure measurements. These results pave the way for future investigations into the strong nuclear force and the properties of the QGP, contributing to a deeper understanding of high-energy nuclear interactions and the early universe.