Event-by-Event Identified Particle Ratio Fluctuations in Pb–Pb Collisions with ALICE

ALICE is the dedicated heavy-ion experiment among the experiments at the LHC at CERN. It is, in particular, designed to exploit the physics of strongly interacting matter. The theory of strong interactions, Quantum Chromodynamics (QCD), predicts that at sufficiently high energy densities nuclear matter transforms into a deconfined state of quarks and gluons. One of the possible signatures of a transition between hadronic and partonic phases is the enhancement of fluctuations of the number of particles in the hadronic final state of relativistic heavy-ion collisions. \\ \\ The observable $\nu_{\rm{dyn}}$, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of particle ratios: $K/\pi$, $p/\pi$ and $K/p$. It reflects deviations of the particle number distributions from those of a statistical distribution and also provides insight into the correlation between particle pairs. The $\nu_{\rm{dyn}}$ fluctuation measure was previously studied at the Super Proton Synchrotron (SPS) and at the Relativistic Heavy-Ion Collider (RHIC) in Pb--Pb and Au--Au collisions, respectively. The ALICE detector is ideally suited to extend these measurements to higher collision energies. In particular, the excellent charged-particle tracking and particle identification (PID) capabilities in the central barrel of the detector allow for a precise and differential event-by-event analysis at mid-rapidity and low transverse momentum. \\ \\ The standard approach of finding the moments is to count the number of particles event by event. However, this approach introduces difficulties such as incomplete particle identification due to overlapping energy loss (dE/dx) distribution functions in the Time Projection Chamber (TPC), which can be taken care of by either selecting suitable phase-space regions or by using additional detector information. These procedures reduce overall phase-space coverage and detection efficiencies. The present study is based on a novel experimental technique, the so-called Identity Method, which overcomes such limitations. The method follows a probabilistic approach using the inclusive dE/dx distributions measured in the TPC, and determines the moments of the multiplicity distributions by an unfolding procedure. Therefore, understanding and calibration of the dE/dx measurements in the TPC plays a significant role in the study of the $\nu_{\rm{dyn}}$ observable. \\ \\ The dE/dx in the TPC is derived from the pulse height distributions of charged-particle tracks traversing the TPC. The so-called "Ion-tail" and "Common-mode" effects produce a significant deterioration in the measurement of dE/dx, and thus the PID performance of the TPC. Therefore, these effects need to be understood and corrected. The correction of this problem requires a good understanding of the TPC signal shape. To study this, the so-called "black events" with a non-zero-suppressed baseline from the TPC laser calibration system is used. The characteristic signal shape is then crosschecked with three-dimensional Garfield simulations for the first time. Following the signal shape analysis an offline correction procedure is developed, where a significant improvement in the dE/dx resolution and thus the PID quality of the TPC is achieved. Moreover, these effects are implemented in the current simulation framework of the ALICE detector. \\ \\ The Identity Method employs fits to inclusive dE/dx distributions as an input for the calculation of the moments of particle multiplicity distributions. Therefore, a good understanding of the detector response of the TPC is required. For this, clean particle samples, which are retrieved from the decays with displaced vertices (pions from $K_{S}^{0}$ decays, protons from $\varLambda$ decays and electrons from photon conversions), are used. Additionally, for a better description of the detector response, a generalised Gauss function is chosen as fit function. It is the first time the TPC detector response is described with a generalised Gauss function. Based on the detector response functions obtained, an elaborate fitting procedure is developed to parametrize the TPC PID. \\ \\ The Identity Method is successfully applied to ALICE data. The first results on the identified-particle-ratio fluctuations in Pb--Pb collisions at $\sqrt{s_{\rm{NN}}}$=2.76TeV as a function of centrality and pseudorapidity, as well as a comparison to calculations with the HIJING and AMPT event generators are presented. The results for $p/\pi$ changes sign from positive to negative towards more peripheral collisions which indicates an increasing correlation between protons and pions. This behavior is not reproduced by the HIJING and AMPT models. On the other hand, for the most central collisions a change of sign for $p/\pi$ and $K/p$ observed compared to the measurements at lower energies from CERN-SPS and BNL-RHIC. A more detailed analysis of fluctuations with charge and species specific pairs is required to fully characterize the particle production dynamics in Pb--Pb collisions and understand, in particular, the origin of the sign changes reported in this thesis.