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Recent results on particle production in hadronic collisions at the LHC, from proton-proton (pp) to nucleus-nucleus (AA), challenge the traditional paradigm of hadronization as a universal late-time process which can be factorized from the partonic description of the rest of the event. If the violation of this description in nuclear collisions has been accepted for long -- with several hadronic observables finding a natural interpretation assuming some form of recombination of partons from a deconfined medium -- this same occurrence came as a surprise in proton-proton collisions. Actually, strong indications that hadronization cannot be simply described by universal fragmentation fractions/functions were already found in fixed-target experiments, but limited to specific kinematic regions close to the beam rapidity, so that a complete change of paradigm did not look necessary. Here we show that a wide set of observables finds a natural interpretation assuming that in all high-energy hadronic collisions, from pp to AA, a deconfined medium is formed, acting as a reservoir in which a parton can undergo local color neutralization (LCN) through recombination with an opposite color charge. Hadronization, belonging to the non-perturbative domain of QCD, will always, inevitably, require some amount of educated modelling. However, some commong features to the various proposed model in the literature can be identified, which look necessary in order to describe the most recent data. Here we decide to focus on heavy-flavor (HF) production, since in this case the origin of at least one of the consituent quarks of the final hadron is well known and attributed to an initial hard partonic process., Comment: Contribution to the LHCP2024 Conference

QTRAJ is a computer code that simulates the propagation of quarkonium in the quark-gluon plasma (QGP) based on the quantum-trajectory algorithm. This algorithm solves a master equation in which the quarkonium is treated as an open quantum system (OQS). A major advantage of this approach is that it turns a 3D spatial evolution for a density matrix into a 1D Schr\"odinger equation for a wavefunction with a non-hermitian Hamiltonian, drastically reducing the computational cost. So far, the interaction implemented in the master equation was obtained within the framework of potential non-relativistic QCD (pNRQCD), and restricted to the regime $rT \ll 1$, where $r$ is the size of the color dipole and $T$ is the temperature. In the environment produced in heavy-ion collisions (HIC's) this limit is accurate for $\Upsilon(1S)$, but the applicability to other quarkonium states is dubious. In the present study we generalize the above approach, extending it to the regime $rT\!\sim\! 1$ in the one-gluon exchange approximation, with proper Hard Thermal Loop (HTL) resummation of medium effects. This is done by implementing new jump operators connecting different color states of the $Q\bar Q$ pair and expanding them in plane waves, giving rise to a variation of the algorithm present in QTRAJ 1.0. Here we provide an overview of this approach comparing the $rT \ll 1$ and $rT\sim 1$ cases, and we discuss prospects for phenomenological application to excited states of bottomonium., Comment: Proceedings of the Conference QNP 2024

We solve a Boltzmann equation for massless quark and gluon fluids in a transversally homogeneous, longitudinally boost-invariant expansion. Quarks can be out of chemical equilibrium and the relaxation times of the two species are assumed to be connected by Casimir scaling. We numerically calculate moments of the distribution functions, identifying their early- and late-time attractors and reconstructing also the full distributions. These attractors appear when the system is still far from local thermalization, before hydrodynamics traditionally would be expected to apply. We also analyze the evolution of entropy production for different initial momentum anisotropies and quark abundances., Comment: Proceeedings of the 10th International Conference on Quarks and Nuclear Physics (QNP2024)

We exactly solve a Boltzmann equation that describes the dynamics of coupled massless quark and gluon fluids undergoing transversally homogeneous longitudinal boost-invariant expansion. We include a fugacity parameter that allows quarks to be out of chemical equilibrium and we account for the different collision rates of quarks and gluons, which are related by Casimir scaling. Based on these assumptions, we numerically determine the evolution of a large set of moments of the quark and gluon distribution functions and reconstruct their entire distribution functions. We find that both late and early-time attractors exist for all moments of the distribution functions containing more than one power of the squared longitudinal momentum. These attractors emerge long before the system reaches the regime where hydrodynamic approximations apply. In addition, we discuss how the shear viscous corrections and entropy density of the fluid mixture evolve and consider the properties of their respective attractors. Finally, the entropy production is also investigated for different initial values of momentum anisotropy and quark abundance.

This paper is a write-up of the ideas that were presented, developed and discussed at the fourth International Workshop on QCD Challenges from pp to AA, which took place in February 2023 in Padua, Italy. The goal of the workshop was to focus on some of the open questions in the field of high-energy heavy-ion physics and to stimulate the formulation of concrete suggestions for making progresses on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.