Longitudinal Fluctuations in Relativistic Heavy Ion Collisions

Abstract

Collisions of relativistic heavy-ions aim to create a unique state of matter, known as the Quark Gluon Plasma (QGP), where quarks and gluons are in a deconfined state. So far, one of the most suitable probes to study the properties of the QGP are the flow coefficients $\nu_{n}$, which quantify the anisotropic particle distributions in the final state, and that can be obtained via Fourier decomposition. These flow coefficients have been extensively studied. Moreover, it has been seen that the pseudorapidity of the final particle distribution in a heavy ion collision is sensitive to event-by-event longitudinal fluctuations, and the shape can be decomposed using Legendre polynomials, analogous to the Fourier decomposition in anisotropic flow. A significant asymmetry is observed, and can be quantified with the first order coefficient, $a_{1}$, from the longitudinal decomposition. The presence of a longitudinal asymmetry in nuclei-nuclei collisions does not agree with the boost invariance assumed in various models that describe the initial state of a relativistic heavy ion collision. The description of the asymmetry can reveal nontrivial information about the structure of the fireball in the longitudinal direction and hence can be used as a probe of the initial conditions of the evolution of the system. In this thesis, the longitudinal decomposition for Xe-Xe collisions at $\sqrt{s_{\text{NN}}}=5.44$ TeV and Pb-Pb collisions $\sqrt{s_{\text{NN}}}=5.02$ TeV from the LHC Run 2 data measured with the ALICE detector is presented and compared to models. Furthermore, during the Long Shutdown 2 (LS2), the ALICE detector went through several upgrades that allows it to collect in the LHC Run 3 and Run 4 about two order of magnitude more data compared to the LHC Run 2 and Run 1. The data collection rate increase required the development of a new software in ALICE, the Online-Offline Computing System (O$^2$), along which a new organized analysis system was developed, the Hyperloop train system. This thesis will also present the concept and successful development of the Hyperloop train system that has been actively used since June 2020 in ALICE.