CERN Accelerating science

Quantum Chromodynamics (QCD), the theory describing the interaction between quarks and gluons, suggests that the coupling strength of the strong interaction asymptotically decreases at large momentum scales. Hence, at large enough densities and/or temperatures – like the conditions present in the early Universe – the quarks and gluons composing hadrons can exist in a deconfined state of matter known as the Quark-Gluon Plasma (QGP). A Large Ion Collider Experiment (ALICE) at the Large Hadron Collider (LHC) and the Solenoidal Tracker at RHIC (STAR) experiment at the Relativistic Heavy Ion Collider (RHIC) have gathered experimental evidence of QGP formation generated through relativistic heavy ion collisions. The QGP lifetime is accepted to be on the order of $10^{-21}$ seconds, with a temperature reaching $10^{12}$ K, before the system cools and undergoes a phase transition, back into confined ground-state hadrons and hadronic resonances. Whether this transition from quark to hadron degrees of freedom occurs at a uniform temperature for all quark flavors remains a question of interest. Assuming a thermally equilibrated system, experimental particle yields from ALICE and STAR serve as anchors for the determination of common freeze-out parameters -- namely, the chemical freeze-out temperature ($T_\mathrm{ch}$) and the baryo-chemical potential ($\mu_\mathrm{B}$) -- in the QCD phase diagram through thermal fits within the Statistical Hadronization Model (SHM) framework. In this dissertation, I present evidence supporting a flavor-dependent chemical freeze-out in the crossover region of the QCD phase diagram via SHM calculations of the freeze-out parameters at ALICE and STAR at collision energies ranging from $\sqrt{s_{\rm{NN}}} = $ 11.5 GeV to 5.02 TeV. I also show the system size dependence of $T_\mathrm{ch}$ at vanishing $\mu_\mathrm{B}$ via the same approach in pp, pPb and PbPb collisions at LHC energies – supporting the applicability of the SHM in small collision systems and that flavor-dependent freeze-out parameters lead to a natural explanation of strangeness enhancement from small to large collision systems at ALICE.