arXiv subjects

Vincenzo Nugara

Publications and source records attributed to Vincenzo Nugara.

7 recordsLinked to original sources

arXiv2026

Bulk viscosity of a binary mixture: the role of the intra-species interaction

The bulk viscosity $ζ$ is a transport coefficient which is of central importance for various areas of modern physics. In particular, its determination for a mixture of more than one fluid is challenging, since it involves a complex interplay of multiple microscopic processes that operate on different time scales. Within the Chapman-Enskog framework, based on a series expansion of the Boltzmann distribution function, many previous works have derived the 1$^{\text{st}}$ order result for the $ζ$ of a mixture. However, such a result fails to reproduce relevant physical features of the system, especially when the masses of the two components are similar. In this work we improve the 1$^{\text{st}}$ order Chapman-Enskog result by deriving the $ζ$ at the 2$^{\text{nd}}$ order in the expansion. We show that this improved formula encodes many physical properties that the 1$^{\text{st}}$ order result misses: under specific conditions, the 2$^{\text{nd}}$ order result can be qualitatively and quantitatively very different from the 1$^{\text{st}}$ order one. Moreover, this result is compared against the $ζ$ evaluated within the Green-Kubo formalism, by means of a numerical solution of the Relativistic Boltzmann equation. The agreement with respect to this benchmark is significantly improved when moving from the 1$^{\text{st}}$ to the 2$^{\text{nd}}$ order CE result.

hep-ph↗

arXiv2026

Non-equilibrium Dynamical Attractors and Thermalisation of Charm Quarks in Nuclear Collisions at the LHC Energy

We study the non-equilibrium dynamics, thermalisation and attractor behaviour of charm quarks in a longitudinally expanding Quark-Gluon Plasma within the Relativistic Boltzmann Transport approach in 1+1D Bjorken expansion. Considering both a strong AdS/CFT coupling scenario with constant $2πT D_s=1$ and a temperature-dependent diffusion coefficient $D_s^\text{lQCD}(T)$ from the recent unquenched lattice QCD data, we analyse the evolution of effective temperature, momentum moments and distribution functions for different initial conditions, including FONLL and EPOS4HQ spectra. We find that charm quarks exhibit dynamical attractors; however, the temperature dependence of $D_s^\text{lQCD}(T)$ leads to significantly longer relaxation times compared to the strong coupling limit. While dynamical attractors occur within $\sim 1-1.5 \rm \,fm$ for $2πT D_s=1$, they are delayed to $\sim 5 \rm \,fm$ for $D_s^\text{lQCD}(T)$, becoming comparable to the lifetime of the Quark-Gluon Plasma phase in ultra-relativistic collisions. This indicates that charm quarks may not fully thermalise, especially in small systems such as peripheral or light-ion collisions. We further show that, for $D_s^\text{lQCD}(T)$, the deviation from equilibrium becomes as large as $δf_{HQ}/f_{eq} \sim p_T^β\sim \mathcal{O}(1)$ already at $p_T\simeq 3\rm\, GeV$, rising with $β\sim 4.5$, thus questioning the applicability of viscous hydrodynamics to charm dynamics.

hep-ph↗

arXiv2026

Far-from-Equilibrium Attractors and Universality in Ultra-Relativistic Heavy-Ion Collisions within Relativistic Kinetic Theory

This PhD Thesis is devoted to the study of the emergence of attractors, universality and collectivity in ultra-relativistic collisions by means of relativistic kinetic theory. After an introduction about Quantum Chromodynamics (QCD), Quark-Gluon Plasma (QGP) and the importance of heavy-ion collisions to investigate both, we give an overview about the two main models able to describe the hot QCD matter collective behaviour, namely kinetic theory and hydrodynamics. Afterwards, the Relativistic Boltzmann Transport (RBT) model, which has been employed to obtain most part of the results of this thesis, is carefully described, from the numerical and physical perspectives. The study of attractors and universality proceeds then by starting from a simple one-dimensional massless model, moving to increasingly more complex scenarios, involving the full 3+1D setup, non-conformal systems and realistic event-by-event fluctuations. Particular attention is paid to the physical scales which govern the system collectivity and their interplay. We show that a very good description of collective behaviour can be carried out by means of a few variables which characterise the systems under study.

hep-ph↗

arXiv2025

Shear viscosity of a binary mixture for a relativistic fluid at high temperature

The determination of the shear viscosity is a central topic in various areas of modern physics. In particular, it is often necessary to evaluate the shear viscosity $η$ of fluids made up of more than one species, all interacting with different cross sections. Since it may be difficult to extract information on the interaction among different species, various combinations of the viscosities of the individual components are often used. We work in the Chapman-Enskog framework and investigate on binary mixtures, by comparing such single component combinations with a full 2-component formalism: we find that, in most cases, the full viscosity is well approximated by a weighted linear average of the single component viscosities, although this result is far from being general. Moreover, we validate our 2-component Chapman-Enskog results for $η$ by comparing them with an independent numerical simulation of the Boltzmann equation, which estimates the shear viscosity via a Green-Kubo formula, in the case of a quasi-particle system that reproduces lattice QCD thermodynamics. We see that the temperature dependence of $η/s$ of such system of quarks and gluons is not well described by combinations of the individual components, highlighting the importance of inter-species scattering.

hep-ph↗

arXiv2025

Knudsen number and universal behavior of collective flows in conformal and non-conformal systems

We investigate the role of the Knudsen number (Kn) as a scaling parameter governing the emergence of collective behavior in relativistic heavy-ion collisions. Using the Relativistic Boltzmann Transport approach, we explore different initial conditions for both conformal (massless) and non-conformal (massive) systems with a constant specific shear viscosity $η/s$. Observables such as the time evolution of anisotropic flow coefficients collapse onto universal curves for fixed classes of Knudsen number, when using a scaled time variable accounting for the system size and the speed of sound $c_s$. More differential quantities, such as $v_n(p_T/\langle E_T\rangle)$, show a larger sensitivity to $c_s$. We also study events with fluctuating initial profiles from the \trento\ model, simulating collision systems from O-O to Pb-Pb at RHIC and LHC energies. Universal scaling at a given Kn value also holds in these event-by-event simulations, suggesting that the Knudsen number provides a unified criterion for classifying collectivity across different systems, including small systems where thermalisation may not be fully realised.

hep-ph↗

arXiv2025

Far-from-equilibrium attractors with Full Relativistic Boltzmann approach in 3+1 D: moments of distribution function and anisotropic flows $v_n$

We employ the Full Relativistic Boltzmann Transport approach for a conformal system in 3+1D to study the universal behaviour in moments of the distribution function and anisotropic flows. We investigate different transverse system sizes $R$ and interaction strength $η/s$ and identify universality classes based upon the interplay between $R$ and the mean free path; we show that each of this classes can be identified by a particular value of the opacity $\hat γ$, which has been previously introduced in literature. Our results highlight that, at early times, the inverse Reynolds number and momentum moments of the distribution function display universal behaviour, converging to a 1D attractor driven by longitudinal expansion. This indicates that systems of different sizes and interaction strengths tend to approach equilibrium in a similar manner. We provide a detailed analysis of how the onset of transverse flow affects these moments at later times. Moreover, we investigate the system size and $η/s$ dependence for the harmonic flows $v_2$, $v_3$, $v_4$ and their response functions, along with the impact of the $η/s$ and the system transverse size on the dissipation of initial azimuthal correlations in momentum space. Finally, we introduce the normalised elliptic flow $v_2/v_{2,eq}$, showing the emergence of attractor behaviour in the regime of large opacity. These results offer new insights into how different systems evolve towards equilibrium and the role that system size and interaction play in this process.

hep-ph↗

arXiv2023

Far-from-equilibrium attractors with Full Relativistic Boltzmann approach in boost-invariant and non-boost-invariant systems

We study the universal behavior associated with a Relativistic Boltzmann Transport (RBT) approach with the full collision integral in 0+1D conformal systems. We show that all momentum moments of the distribution function exhibit universal behavior. Furthermore, the RBT approach allows to calculate the full distribution function, showing that an attractor behavior is present in both the longitudinal and transverse momentum dependence. We compare our results to the far-from-equilibrium attractors determined with other approaches, such as kinetic theory in Relaxation Time Approximation (RTA) and relativistic hydrodynamic theories, both in their viscous (DNMR) an anisotropic (aHydro) formulations, finding a very similar evolution, but an even faster thermalization in RBT for higher order moments. For the first time, we extended this analysis also to study the attractor behavior under a temperature-dependent viscosity $η/s(T)$, accounting also for the rapid increase toward the hadronic phase. We find that a partial breaking of the scaling behavior with respect to $τ/τ_{eq}$ emerges only at $T \approx T_c$ generating a transient deviation from attractors; interestingly this in realistic finite systems may occur around the freeze-out dynamics. Finally, we investigate for the first time results beyond the boost-invariant picture, finding that also in such a case the system evolves toward the universal attractor. In particular, we present the forward and pull-back attractors at different space-time rapidities including rapidity regions where initially the distribution function is even vanishing.

hep-ph↗