arXiv subjects

Gabriel S. Denicol

Publications and source records attributed to Gabriel S. Denicol.

At least 19 recordsLinked to original sources

arXiv2026

Effective kinetic theory description of the magnetic field-induced anisotropic gluon pressure during pre-equilibrium in heavy-ion collisions

We develop an effective kinetic description for the interaction of gluons and magnetic fields during the pre-equilibrium stage of relativistic heavy-ion collisions. For this purpose, we formulate the Boltzmann-Vlasov equation with the interaction term modeled by the effect of the magnetic field on the elements of an electrically charged and colored dipole originating from the quantum fluctuation of gluons into quark-antiquark pairs. We find the numerical solution of the collisionless Boltzmann-Vlasov equation with a time-dependent magnetic field profile to determine the time evolution of the directional pressures. The presence of the magnetic field tames the growth of the transverse to longitudinal pressure ratio compared with the case in the absence of the magnetic field; however, the system does not isotropize since collisions are not included. We study the cases of initial gluon distributions with longitudinal anisotropies, as well as the case of an initial isotropic gluon distribution. In both cases, we find that the magnetic field produces a non-monotonic early-time response for large values of the initial magnetic field strength.

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arXiv2026

Thermal boundaries for relativistic fluids without a conserved charge

We study an ultrarelativistic fluid with no conserved particle number in a planar slab geometry bounded by two parallel, thermally conducting plates held at fixed (possibly different) temperatures. The fluid-wall interaction is described within kinetic theory by modeling the fluid as self-interacting radiation: a gas of bosons emitted and absorbed by the walls according to black-body laws, while mutual collisions enforce local equilibration. From this microscopic setup we derive effective boundary conditions for the hydrodynamic fields, finding that the walls behave as modified absorbing boundaries (and not as thermostats). In particular, the fluid temperature at the boundary does not generally coincide with the wall temperature. We prove that the resulting initial-boundary-value problem is well posed. When the plates are held at different temperatures, the system develops a uniform steady flow from the hotter to the colder wall, with energy density equal to the arithmetic mean of the corresponding black-body energy densities in the limit of a small temperature difference.

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arXiv2026

Breakdown of Hydrodynamic Universality in Neutron Star Oscillations

Hydrodynamic universality refers to the property that different formulations of relativistic dissipative hydrodynamics yield identical predictions in the asymptotic long-wavelength regime. We show that neutron-star oscillations need not reach this regime. Because the finite stellar radius limits the accessible wavelengths and causality imposes a lower bound on microscopic relaxation times, the separation between microscopic and macroscopic scales can become insufficient for hydrodynamic universality to emerge. Comparing linear oscillations in relativistic Navier--Stokes and Israel--Stewart theories, we find that realistic bulk viscosities can produce sizeable modifications of the oscillation spectrum, even at the longest wavelengths. These results identify neutron-star oscillations as a direct probe of microscopic nonequilibrium dynamics beyond the leading hydrodynamic description.

gr-qc↗

arXiv2026

Breakdown of Smooth Shock Solutions in Transient Relativistic Hydrodynamics

In this work, we demonstrate that shock solutions in the Israel-Stewart framework lose regularity once the shock velocity reaches a critical value, and a discontinuity emerges in the solution, which can be interpreted as a second shock wave. This subshock arises as a consequence of the finite speed of information propagation inherent to the Israel-Stewart theory. We then perform numerical simulations to confirm the breakdown of solution continuity. Subsequently, we propose two regularization procedures to extend the domain of continuous shock solutions. The first employs a third-order extension, which introduces new kinetic fields, while the second incorporates a small numerical bulk viscosity. Both methods effectively increase the maximum propagation speed of the Israel-Stewart theory and extend the range over which regular shock solutions exist. Thus, we confirm that this loss of regularity is a direct consequence of the Israel-Stewart framework. These results suggest that Israel-Stewart theory may not provide an adequate description of ultra-relativistic shock waves.

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arXiv2026

Momentum anisotropy from Resistive Magnetohydrodynamics

We derive relativistic resistive magnetohydrodynamics framework for a two-component ultrarelativistic plasma of massless, oppositely charged particles directly from the Boltzmann-Vlasov equation using the 14-moment approximation. The resulting second-order equations couple the net-charge diffusion current to the shear-stress tensor through the electric field, with all transport coefficients given in closed microscopic form. In the homogeneous limit, the charge-current dynamics is well described by relaxation-type Ohm's law for moderate field strengths, while large viscosity drives the system into an underdamped oscillatory regime absent from the standard Israel-Stewart formulation. Most strikingly, a purely electric field generates sizable momentum anisotropy even without any underlying flow gradient. Under Bjorken expansion this field-induced anisotropy persists but becomes subleading to the hydrodynamic expansion source.

hep-th↗

arXiv2026

Radial Oscillations of Viscous Stars at Finite Temperature

We study the radial oscillation spectrum of relativistic stars within Israel-Stewart and Navier-Stokes theories, extending previous analyses to include heat diffusion and a thermodynamically consistent finite-temperature equation of state. The inclusion of heat flux gives rise to a distinct thermal sector in the mode spectrum, whose structure closely mirrors the dispersion relations of an infinite dissipative fluid. Within Israel-Stewart theory, the thermal modes transition from purely damped to propagating behavior above a critical overtone number, providing a finite-size realization of relativistic second sound in compact stars. Remarkably, the finite stellar geometry can push even the fundamental thermal mode into the propagating regime -- a feature with no continuum analogue. For the class of equations of state considered here, where finite-temperature corrections enter as controlled, Sommerfeld-type perturbations of a cold polytrope, the thermal sector couples only weakly to the ordinary fluid oscillation spectrum, with the coupling being of second order in a suitable temperature parameter. We further show that the discrete stellar spectrum is well captured by an analytic ansatz constructed from the flat-spacetime dispersion relations, with the star's finite radius discretizing the continuous mode structure. Our results complete the analysis of radial oscillations of viscous stars by incorporating the last remaining dissipative degree of freedom within the Israel-Stewart framework.

gr-qc↗

arXiv2026

Transport coefficients of chiral fluid dynamics using low-energy effective models

We investigate the first-order transport coefficients of a fluid made of quasiparticles with a temperature-dependent mass extracted from chiral models. We describe this system using an effective kinetic theory, given by the relativistic Boltzmann equation coupled to a temperature-dependent background field determined from the thermal masses. We then simplify the collision term using the relaxation time approximation and implement a Chapman-Enskog expansion to calculate all first-order transport coefficients. In particular, we compute the bulk and shear viscosities using thermal masses extracted from the linear sigma model coupled with constituent quarks and the NJL model.

hep-ph↗

arXiv2026

The shape of transverse momentum spectra in hybrid hydrodynamic models

We study the scaled transverse momentum spectra over a wide parameter space of state-of-the-art hydrodynamic simulation models in order to learn what information can be obtained from the shape of identified-particle spectra -- previously observed to be surprisingly universal across centrality and collision systems in both experimental data and hydrodynamic simulations. We study its sensitivity to each of 17 model parameters in the context of 4 different models for particlization when switching from the hydro description to the kinetic theory afterburner. We find that the strongest sensitivity is to parameters relating to bulk viscosity, free-streaming time, and the $\texttt{T$_\mathrm{R}$ENTo}$ nucleon width parameter $w$. However, we find that the model generally has surprisingly little flexibility in describing the scaled spectrum observable, despite the large number of parameters. Within this small range of parameter dependence, we further find significant tension in a simultaneous description of momentum-integrated observables. In particular, while the mean transverse momentum prefers a large value of the nucleon width parameter $w$, a small value is required to obtain scaled spectra that are consistent with experimental measurements. We speculate on the origin of these model tensions and possible missing physics in the commonly-used $\texttt{T$_\mathrm{R}$ENTo}$+free streaming+hydro+afterburner simulation model.

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arXiv2026

Radial Oscillations of Viscous Neutron Stars: Zero Diffusion Case

The spectrum of radial oscillations of neutron stars is systematically studied within two frameworks of viscous relativistic hydrodynamics: the relativistic Navier-Stokes and Israel-Stewart theories. A correspondence is established between the discrete stellar eigenmodes and the continuous dispersion relation of perturbations around a homogeneous fluid, providing a basis for interpreting our numerical results. We analyze the Newtonian limit and assess the impact of relativistic corrections, such as the gravitational redshifting of microscopic relaxation timescales. We show that bulk viscosity can significantly affect the behavior of both hydrodynamic and nonhydrodynamic fundamental modes, and that, depending on the magnitude of the viscous effects, it is the nonhydrodynamic mode that becomes unstable beyond the turning point in a sequence of equilibrium configurations. These results provide a useful step toward systematic studies of neutron star quasinormal modes in the presence of viscosity.

gr-qc↗

arXiv2026

Universality of scaled particle spectra in ultrarelativistic heavy-ion collisions

We study the transverse momentum spectra of identified particles in ultrarelativistic collisions of large and small collision systems. In order to isolate information contained in the momentum dependence, we propose to scale the spectra by the total particle number and mean transverse momentum -- global quantities which are already well studied. We observe an interesting, nearly universal, centrality-independent shape in the scaled spectra, similar to scalings that have been studied previously. This scaling behavior breaks down at large transverse momentum and for very small systems, such as those produced in p-p collisions. We perform hybrid hydrodynamic simulations and show that, in these simulations, a centrality-independent shape is a consequence of an event-by-event independence. Our results motivate further theoretical and experimental investigations of the regime of validity of this scaling phenomenon and their physical interpretation at different collision energies and systems.

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arXiv2025

Relativistic Dissipative Magnetohydrodynamics for accretion disks

We derive a relativistic magnetohydrodynamics (RMHD) theory for a dilute electron-ion gas governed by the Boltzmann-Vlasov equation, using the method of moments. This yields an extended MHD framework beyond standard astrophysical formulations, which typically include only the shear-stress component parllel to the magnetic field. We analyze our framework in the linear regime and show that it leads to the firehose instability when the bakground longitudinal pressure becomes large. In these extreme scenario, the transverse and semi-transverse shear-stress components become large and may play a role in accretion disk dynamics.

astro-ph.HE↗

arXiv2025

Relaxation Time Approximation for a multi-species relativistic gas

We generalize a recent prescription for the relaxation time approximation for the relativistic Boltzmann equation for systems with multiple particle species at finite temperature. This is performed by adding counter-terms to the traditional Anderson-Witting ansatz for each particle species. Our approach allows for the use of momentum-dependent relaxation times and the obedience of local conservation laws regardless of the definition of the local equilibrium state. As an application, we derive the first order Chapman-Enskog corrections to the equilibrium distribution and display results for the hadron-resonance gas. We also demonstrate that our collision term ansatz obeys the second law of thermodynamics.

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arXiv2025

Hydrodynamization and thermalization in heavy-ion collisions: a kinetic theory perspective

Understanding the applicability of fluid-dynamical models to describe the hot and dense matter produced in the early stages of hadronic collisions is a fundamental problem in the field. In particular, it is not clear to what degree this hydrodynamization process requires proximity to a local equilibrium state. In this contribution, we study this problem in kinetic theory considering an ultrarelativistic gas undergoing strong longitudinal expansion, assuming Bjorken flow. We solve the Boltzmann equation and verify that the system displays considerable deviations from local equilibrium, even though the energy-momentum tensor is well described by fluid dynamics. We further quantify this effect computing the emission of photons in the quark-gluon plasma and verify whether this deviation from equilibrium can be observed.

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arXiv2025

Probing the onset of collectivity via scaled particle spectra in ultrarelativistic nuclear collisions

We identify a novel scaling in the transverse momentum spectra of produced particles, obtained by removing the global scales of multiplicity and mean transverse momentum. Hydrodynamic simulations and experimental data reveal an almost universal scaled spectrum across centralities, systems, and even small systems, pointing to its origin in the collective, fluid-like dynamics of the QGP. Comparing this observable with Bayesian a priori distributions shows its independent constraining power on QCD transport properties, while also exposing limitations of current models. A detailed posterior analysis will be pursued in future work, opening a new avenue to refine our understanding of collectivity in heavy-ion collisions.

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arXiv2025

Causality and stability of magnetohydrodynamics for an ultrarelativistic locally neutral two-component gas

We investigate the causality and stability of the relativistic theory of magnetohydrodynamics derived in Phys. Rev. D 109, 096021 (2024) to describe a locally neutral two-component plasma of massless particles. We show that this formalism is linearly causal and stable around global equilibrium, for any value of the magnetic field and discuss its qualitative differences to the traditional Israel-Stewart formalism in the linear regime. Finally, we compare this framework with the magnetohydrodynamic model used in the study of astrophysical plasmas, in which only the longitudinal component of the shear-stress tensor is considered. We discuss the domain of applicability of this type of framework in the context of ultrarelativistic heavy-ion collisions.

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arXiv2025

Far from equilibrium hydrodynamics of nonthermal fixed points

Nonthermal fixed points are paradigmatic far-from-equilibrium phenomena of relevance to high-energy physics, cosmology, and cold atomic gases. We propose that, despite their intrinsically nonequilibrium nature, nonthermal fixed points give rise to hydrodynamic excitations otherwise known in the vicinity of thermal equilibrium. As a result, nonthermal fixed points can also be characterized by transport coefficients, such as a far-from-equilibrium, and therefore manifestly time-dependent, incarnation of shear viscosity. We corroborate our proposal with explicit studies using relativistic kinetic theory with binary collisions of massless particles in the 14-moment approximation and comparisons to QCD kinetic theory simulations.

hep-th↗

arXiv2025

Divergence and resummation of the moment expansion for an ultrarelativistic gas in Bjorken flow

In this letter, we demonstrate for the first time that the moment expansion for an ultrarelativistic gas undergoing Bjorken flow diverges. We then show how this series can be resummed using the Borel-Padé method and use this to determine the single-particle distribution function of the gas. Finally, we compare the exact resummed solution of the single-particle distribution function with solutions of the Boltzmann equation in the hydrodynamic limit and verify that the system displays considerable deviations from local equilibrium.

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arXiv2024

Branch-cut in the shear-stress response function of massless $λφ^4$ with Boltzmann statistics

Using an analytical result for the eigensystem of the linearized collision term for a classical system of massless scalar particles with quartic self-interactions, we show that the shear-stress linear response function possesses a branch-cut singularity that covers the whole positive imaginary semi-axis. This is demonstrated in two ways: (1) by truncating the exact, infinite linear system of linear equations for the rank-two tensor modes, which reveals the cut touching the origin; and (2) by employing the Trotterization techniques to invert the linear response problem. The former shows that the first pole tends towards the origin and the average separation between consecutive poles tends towards zero as power laws in the dimension of the basis. The latter allows one to obtain the response function in closed form in terms of Tricomi hypergeometrical functions, which possess a branch-cut on the above-mentioned semi-axis. This suggests that the presence of a cut along the imaginary frequency axis of the shear stress correlator, inferred from previous numerical analyses of weakly coupled scalar $λφ^4$ theories, does not arise due to quantum statistics but instead emerges from the fundamental properties of this system's interactions.

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