The quark-gluon plasma (QGP) produced in ultrarelativistic heavy-ion collisions provides a unique environment in which to study strongly interacting matter under extreme conditions. Heavy quarkonium states constitute particularly valuable probes of this medium, as their production and survival are modified by interactions with the QGP. In particular, the suppression of quarkonium yields has long been regarded as an important signature of deconfinement, although a quantitative description requires accounting for both dissociation and recombination during the dynamical evolution of the collision.
In recent years, the open quantum system approach has provided a systematic framework for describing quarkonium evolution in the QGP. In this framework, the heavy quark-antiquark pair is treated as a quantum subsystem interacting with the surrounding medium, whose degrees of freedom act as an environment. Effective field theory (EFT) techniques can then be employed to exploit the hierarchy of energy scales and derive master equations governing the reduced density matrix of the heavy-quark system.
Most applications of this framework have focused on the evolution of quarkonium in the deconfined phase. However, after the QGP hadronizes, quarkonium states continue to propagate through a gas of interacting hadrons until kinetic freeze-out. Interactions during this hadronic stage may further modify quarkonium abundances and should therefore be quantified for a complete description of quarkonium production in heavy-ion collisions.
The main objective of this Master's thesis is to develop an open quantum system description of quarkonium in the hadron gas phase and to derive the corresponding master equation using effective field theory techniques. Particular attention will be devoted to the scattering of pions with quarkonium and to processes connecting quarkonium states with open-heavy-flavor DD-meson pairs, including quarkonium dissociation into DD mesons and the inverse recombination processes. The resulting master equation will provide a framework to compute the corresponding transition rates and to quantify how the hadronic phase affects the final quarkonium yields. Ultimately, this work aims to determine the importance of hadronic interactions for the interplay between quarkonium suppression and recombination throughout the full evolution of a heavy-ion collision.