arXiv subjects

Marco Ruggieri

Publications and source records attributed to Marco Ruggieri.

At least 19 recordsLinked to original sources

arXiv2026

Elliptic flow of charm quarks produced in the early stage of pA collisions

We investigate the build-up of elliptic flow of charm quarks produced in the early pre-equilibrium stage of high-energy proton--nucleus collisions. The initial stage is modeled within the Color Glass Condensate framework as an evolving glasma, initialized through the McLerran--Venugopalan model. Subnucleonic fluctuations have been implemented as constituent-quark hotspots for both the proton and the nuclear participants. Charm quarks are propagated in the evolving non-Abelian background by solving the relativistic Wong equations for their coordinates, momenta, and color charges. First, we compute the nuclear modification factor of charm quarks, finding a slight migration towards higher $p_T$ states in agreement with previous results in the literature. Then, we focus on the azimuthal anisotropies acquired through the interaction with glasma fields. We find that glasma-induced momentum anisotropies are efficiently transmitted to heavy quarks within $\tau \sim 0.4~\mathrm{fm/c}$, leading to a sizeable charm-quark $v_2$, with a magnitude that increases with the strength of the initial fields and with the number of nuclear participants. Remarkably, we show that the early-stage contribution alone can account for a significant fraction of the experimentally observed $J/\psi$ elliptic flow in p-Pb collisions, indicating that pre-hydrodynamic dynamics can play a non-negligible role in the final-state heavy-flavor collectivity, especially in small systems.

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arXiv2025

QCD axions and domain walls in dense matter under compact stellar conditions

In compact stellar environments, the stability of dense QCD matter requires the simultaneous fulfillment of charge neutrality and beta equilibrium. In this work, we study how temperature and finite chemical potential affect QCD topology and axion properties within this medium, analyzing both cases with and without the charge neutrality condition. Our results show that the topological susceptibility and axion properties are highly sensitive to the critical behavior of the chiral phase transition in both cases. In particular, the axion mass is strongly suppressed near the transition, while the axion self-coupling constant develops a pronounced peak whose magnitude depends on the temperature and density of the medium. Remarkably, around the critical point at $T\simeq70$ MeV and $\mu\simeq346$ MeV, the self-coupling constant is enhanced by more than a factor of seven compared to its vacuum value, a feature that to the best of our knowledge has not been reported in previous studies. Such a strong amplification at the phase boundary indicates that axion-mediated interactions could play an important role in shaping the structure and stability of compact stars, with potential implications for their evolution and observable astrophysical signatures.

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arXiv2025

Entropy from decoherence: a case study using glasma-based occupation numbers

We compute the entropy-per-particle, $S/N$, produced by the decoherence of a coherent state interacting with an environment, using an analytical open quantum system approach. The coherent state considered is characterized by occupation numbers borrowed from the glasma fields produced in the early stages of high-energy nuclear collisions. The environment is modeled as the vacuum, and decoherence arises from the interaction of the state with vacuum fluctuations. We describe the system-environment interaction via a phase-damping model, which represents continuous measurements on the system without altering its energy or particle number. Starting from the occupation numbers typical of the Glasma in high-energy proton-nucleus and nucleus-nucleus collisions, we find that the final $S/N$ after decoherence is lower than that of a two-dimensional thermal bath of ultrarelativistic gluons, except for proton-nucleus collisions at small values of $g\mu$. Our results indicate that quantum decoherence alone does not generate sufficient entropy to transform the initial coherent state into a thermalized gluon bath.

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arXiv2025

Anisotropic fluctuations of momentum and angular momentum of heavy quarks in the pre-equilibrium stage of pA collisions at the LHC

We simulate the real-time evolution of the $SU(3)$-glasma generated in the early stages of high-energy proton-nucleus collisions, employing classical lattice gauge theory techniques. Our setup incorporates a realistic modeling of the proton's internal structure and includes longitudinal fluctuations in the initial state, enabling the study of genuinely non-boost-invariant collision dynamics. Focusing on the momentum and angular momentum anisotropies of heavy quarks in the infinite mass limit, we find that the system retains significant anisotropy well beyond the characteristic timescale $\tau = 1/Q_s$. This persistence of anisotropy is further confirmed in the more realistic, non-boost-invariant scenario, across a range of fluctuation amplitudes. These findings pave the way for future investigations involving dynamical heavy quarks and more quantitative initializations of the glasma.

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arXiv2025

Assessing the Reconstruction of the Critical Line in the QCD Phase Diagram from Imaginary to Real Chemical Potential

We test a technique adopted in the lattice simulations framework, to reconstruct the chiral-phase boundary at real chemical potential, $\mu$, via extrapolation from imaginary $\mu$. We use a low-energy effective model, the Quark-Meson model, both in the mean-field approximation and within the Functional Renormalization Group, the latter in the Local Potential Approximation. The model provides results both for real and imaginary values of $\mu$, thus a direct comparison can be performed between the prediction of the model for real values of $\mu$ and the ones obtained via extrapolation from the results at imaginary $\mu$. We compute an effective convergence radius for the reconstruction technique, $\mu_\mathrm{conv}$, and find $\mu_\mathrm{conv}\approx146$ MeV. This value sustains the validity of the reconstruction technique also for finite and moderate values of the chemical potential. On the other hand, within our model, $\mu_\mathrm{conv}$ is quite smaller then the value of $\mu$ where we find the actual critical endpoint. Near this point of the phase diagram, we find a discrepancy between the actual phase boundary and the one pbtained via extrapolatin of $\approx150\%$. Therefore, our results show that the location of the critical endpoint obtained via reconstruction from imaginary $\mu$ should be considered with due caution

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arXiv2025

Topological Susceptibility in the Superconductive Phases of Quantum Chromodynamics: a Dyson-Schwinger Perspective

We test non-perturbative gluon propagators recently studied in the literature, by computing the topological susceptibility, $\chi$, of the superconductive phases of Quantum Chromodynamics at high density. We formulate the problem within the High-Density Effective Theory, and use the 2-particle irreducible formalism to compute the effective potential of the dense phases. We focus on superconductive phases with two and three massless flavors. Within this formalism, we write a Dyson-Schwinger equation in the rainbow approximation for the anomalous part of the quark propagator in the superconductive phases, in which the non-perturbative gluon propagator plays its role. We complete the model by adding a $U(1)_A$-breaking term whose coupling is fixed perturbatively at large quark chemical potential. We then use the effective potential to compute $\chi$ in the superconductive phases. We finally discuss implications of the results for the axion mass in superdense phases of Quantum Chromodynamics.

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arXiv2024

Dynamics of Hot QCD Matter 2024 -- Hard Probes

The hot and dense QCD matter, known as the Quark-Gluon Plasma (QGP), is explored through heavy-ion collision experiments at the LHC and RHIC. Jets and heavy flavors, produced from the initial hard scattering, are used as hard probes to study the properties of the QGP. Recent experimental observations on jet quenching and heavy-flavor suppression have strengthened our understanding, allowing for fine-tuning of theoretical models in hard probes. The second conference, HOT QCD Matter 2024, was organized to bring the community together for discussions on key topics in the field. This article comprises 15 sections, each addressing various aspects of hard probes in relativistic heavy-ion collisions, offering a snapshot of current experimental observations and theoretical advancements. The article begins with a discussion on memory effects in the quantum evolution of quarkonia in the quark-gluon plasma, followed by an experimental review, new insights on jet quenching at RHIC and LHC, and concludes with a machine learning approach to heavy flavor production at the Large Hadron Collider.

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arXiv2024

Melting of $c \bar c$ and $b \bar b$ pairs in the pre-equilibrium stage of proton-nucleus collisions at the Large Hadron Collider

We study the melting of $c\bar c$ and $b\bar b$ pairs in the early stage of high-energy proton-nucleus collisions. We describe the early stage in terms of an evolving $SU(3)$ glasma stage, that is dominated by intense, out-of-equilibrium gluon fields. On top of these fields, we liberate heavy quark-antiquark pairs, whose constituents are let evolve according to relativistic kinetic theory coupled to the gluon fields. We define a pair-by-pair probability that the pair melts during the evolution, which we relate to the fluctuations of the color charges induced by the interaction of the quarks with the gluon fields. We find that color decorrelation is the main melting mechanism within the pre-equilibrium stage. Moreover, we estimate that within $\Delta \tau\approx0.4-0.5$ fm/c after the formation time of the pairs, about $50\%$ of $c\bar c$ and $b\bar b$ pairs are melted.

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arXiv2024

Topological susceptibility and axion potential in two-flavor superconductive quark matter

We study the potential of the axion, $a$, of Quantum Chromodynamics, in the two-flavor color superconducting phase of cold and dense quark matter. We adopt a Nambu-Jona-Lasinio-like model. Our interaction contains two terms, one preserving and one breaking the $U(1)_A$ symmetry: the latter is responsible of the coupling of axions to quarks. We introduce two quark condensates, $h_L$ and $h_R$, describing condensation for left-handed and right-handed quarks respectively; we then study the loci of the minima of the thermodynamic potential, $\Omega$, in the $(h_L,h_R)$ plane, noticing how the instanton-induced interaction favors condensation in the scalar channel when the $\theta-$angle, $\theta=a/f_a$, vanishes. Increasing $\theta$ we find a phase transition where the scalar condensate rotates into a pseudo-scalar one. We present an analytical result for the topological susceptibility, $\chi$, in the superconductive phase, which stands both at zero and at finite temperature. Finally, we compute the axion mass and its self-coupling. In particular, the axion mass $m_a$ is related to the full topological susceptibility via $\chi=m_a^2 f_a^2$, hence our result for $\chi$ gives an analytical result for $m_a$ in the superconductive phase of high-density Quantum Chromodynamics.

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arXiv2024

$c {\bar c}$ and $b {\bar b}$ suppression in Glasma

This study investigates the evolution and dissociation dynamics of $c\bar{c}$ and $b\bar{b}$ pairs within the pre-equilibrium, gluon-dominated stage of high energy nuclear collisions. An attractive potential made of a perturbative Coulomb-like term and of a confining term is used to simulate the attractive strong force in the pairs. Besides, we implement the interaction of the pairs with the evolving Glasma fields by virtue of the Wong equations. The interaction with the classical color fields dominates the dynamics, causing an increase in pair separation and subsequent dissociation. The observed finite probability of dissociation for these states reveals the intricate interplay between QCD dynamics and the suppression of $c\bar{c}$ and $b\bar{b}$ states during the pre-equilibrium stage. The research highlights differences between $c\bar{c}$ and $b\bar{b}$ pairs, revealing the role of quark flavor in the dissociation process. Dissociation spectra analysis indicates a peak shift towards higher momentum, reflecting a slight energy gain by the pairs. This investigation provides valuable insights into the complex dynamics of $c\bar{c}$ and $b\bar{b}$ pairs in the Glasma, which may help in better interpretation of experimental results on further integration with subsequent phases of the created matter.

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arXiv2023

Functional Renormalization Group Study of Thermodynamic Geometry Around the Phase Transition of Quantum Chromodynamic

We investigate the thermodynamic geometry of the quark-meson model at finite temperature, $T$, and quark number chemical potential, $\mu$. We extend previous works by the inclusion of fluctuations exploiting the functional renormalization group approach. We use recent developments to recast the flow equation into the form of an advection-diffusion equation. We adopt the local potential approximation for the effective average action. We focus on the thermodynamic curvature, $R$, in the $(\mu,T)$ plane, in proximity of the chiral crossover, up to the critical point of the phase diagram. We find that the inclusion of fluctuations results in a smoother behavior of $R$ near the chiral crossover. Moreover, for small $\mu$, $R$ remains negative, signaling the fact that bosonic fluctuations reduce the capability of the system to completely overcome the fermionic statistical repulsion of the quarks. We investigate in more detail the small $\mu$ region by analyzing a system in which we artificially lower the pion mass, thus approaching the chiral limit in which the crossover is actually a second order phase transition. On the other hand, as $\mu$ is increased and the critical point is approached, we find that $R$ is enhanced and a sign change occurs, in agreement with mean field studies. Hence, we completely support the picture that $R$ is sensitive to a crossover and a phase transition, and provides information about the effective behavior of the system at the phase transition.

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arXiv2023

Heavy quark $\kappa$ and jet $\hat{q}$ transport coefficients in the Glasma early stage of heavy-ion collisions

We study the impact of the Glasma fields, used to describe the very early stage of heavy-ion collisions, on the transport of hard probes, namely heavy quarks and jets. We perform numerical simulations of the strong classical fields using techniques from real-time lattice gauge theory. The resulting fields are used as background for the classical transport of ensembles of particles, described by Wong's equations. For this purpose, we develop a numerical solver for the transport of the probes, based on colored particle-in-cell methods. We focus on the dynamics of heavy quarks and jets in the classical colored fields. To quantify the effect of the Glasma, we extract the momentum broadening of hard probes and evaluate the anisotropy transfer from the Glasma to the probes. Lastly, we evaluate the heavy quark $\kappa$ and jet $\hat{q}$ transport coefficients in the Glasma, which turn out to be large and exhibit a peak, irrespective of the particle initialization.

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arXiv2023

Thermalization and isotropization of heavy quarks in a non-Markovian medium in high-energy nuclear collisions

We study the isotropization and thermalization of heavy quarks in a non-Markovian medium in high energy nuclear collisions. In particular, we analyze the case of a non-stationary medium with a noise whose time-correlator decays as a power law (heavy tailed noise). We assume the correlations decay with an exponent $\beta-1$, $0\leq\beta<1$; we treat $\beta$ as a free parameter. We analyze the effect of memory on the thermalization and isotropization of heavy quarks in the medium via a generalized Langevin equation. In general, we find that memory slows down the dynamics of heavy quarks; moreover, thermalization and isotropization happen on the same time scale once a realistic initialization is considered. We also find that while the effect on charm quarks can be relevant, beauty quarks are hardly affected by memory in the quark-gluon plasma phase. Finally, we comment on the effect of memory on the estimate of $D_s$ of charm and beauty.

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arXiv2023

Exploring the axion potential and axion walls in dense quark matter

We study the potential of the Quantum Chromodynamics axion in hot and/or dense quark matter, within a Nambu-Jona-Lasinio-like model that includes the coupling of the axion to quarks. Differently from previous studies, we implement local electrical neutrality and $\beta-$equilibrium, which are relevant for the description of the quark matter in the core of compact stellar objects. Firstly we compute the effects of the chiral crossover on the axion mass and self-coupling. We find that the low energy properties of axion are very sensitive to the phase transition of Quantum Chromodynamics, in particular, when the bulk quark matter is close to criticality. Then, for the first time in the literature we compute the axion potential at finite quark chemical potential and study the axion domain walls in bulk quark matter. We find that the energy barrier between two adjacent vacuum states decrease in the chirally restored phase: this results in a lower surface tension of the walls. Finally, we comment on the possibility of production of walls in dense quark matter.

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arXiv2023

Simulating jets and heavy quarks in the Glasma using the colored particle-in-cell method

We explore the impact of strong classical color fields, which occur in the earliest stages of heavy-ion collisions and are known as the Glasma, on the classical transport of hard probes, namely heavy quarks and jets. To achieve this, we simulate SU(3) color fields using classical real-time lattice gauge theory and couple them to an ensemble of test particles whose dynamics are described by Wong's equations. We provide an overview of how classical color algebras are constructed and introduce a method to generate random classical SU(3) color charges. We extensively test our numerical particle solver in the limits of infinitely massive heavy quarks and ultra-relativistic light-like jets and obtain excellent quantitative agreement with previous studies. Going towards realistic masses and initial moment, we extract longitudinal and transverse momentum broadening for heavy quarks and jets. The resulting accumulated momenta and the anisotropy of these dynamical hard probes exhibit deviations from limiting scenarios, showing that the full dynamics have a significant effect.

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arXiv2022

Anisotropic fluctuations of angular momentum of heavy quarks in the Glasma

We study the evolution of the angular momentum of the heavy quarks in the very early stage of high energy nuclear collisions, in which the background is made of evolving Glasma fields. Given the novelty of the problem, we limit ourselves to the use of toy heavy quarks with a large, unphysical mass, in order to implement the kinetic equations for the angular momentum in the non-relativistic limit. We find that as a consequence of the anisotropy of the background fields, angular momentum fluctuations are also anisotropic: we understand this in simple terms relating the fluctuations of the angular momentum, $L$, to those of linear momentum. While orbital angular momentum diffuses and develops substantial fluctuations and anisotropies, the spin does not. Hence, we can identify the fluctuations of $L$ with those of the total angular momentum $J=L + S$. Therefore, our study suggests that the total angular momentum of the heavy quarks in the early stage of high energy nuclear collisions will present anisotropic fluctuations.

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arXiv2022

Dynamics of Hot QCD Matter -- Current Status and Developments

The discovery and characterization of hot and dense QCD matter, known as Quark Gluon Plasma (QGP), remains the most international collaborative effort and synergy between theorists and experimentalists in modern nuclear physics to date. The experimentalists around the world not only collect an unprecedented amount of data in heavy-ion collisions, at Relativistic Heavy Ion Collider (RHIC), at Brookhaven National Laboratory (BNL) in New York, USA, and the Large Hadron Collider (LHC), at CERN in Geneva, Switzerland but also analyze these data to unravel the mystery of this new phase of matter that filled a few microseconds old universe, just after the Big Bang. In the meantime, advancements in theoretical works and computing capability extend our wisdom about the hot-dense QCD matter and its dynamics through mathematical equations. The exchange of ideas between experimentalists and theoreticians is crucial for the progress of our knowledge. The motivation of this first conference named "HOT QCD Matter 2022" is to bring the community together to have a discourse on this topic. In this article, there are 36 sections discussing various topics in the field of relativistic heavy-ion collisions and related phenomena that cover a snapshot of the current experimental observations and theoretical progress. This article begins with the theoretical overview of relativistic spin-hydrodynamics in the presence of the external magnetic field, followed by the Lattice QCD results on heavy quarks in QGP, and finally, it ends with an overview of experiment results.

nucl-th↗

arXiv2022

Momentum broadening of heavy quarks and jets in the Glasma from classical colored particle simulations

We investigate the effects of the pre-equilibrium Glasma stage of heavy-ion collisions on the broadening of momentum for heavy quarks and jets using classical colored particle simulations based on Wong's equations. Confirming previous studies, we find that these probes accumulate momentum on the order of the saturation scale $Q_s$ and that the color field of the Glasma induces an anisotropy with more broadening along the beam axis. For quark jets the anisotropy is more pronounced at lower energies.

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