arXiv subjects

G. Lugones

Publications and source records attributed to G. Lugones.

At least 19 recordsLinked to original sources

arXiv2026

Finite-Size Effects on the Critical End Point of Magnetized Quark Matter in the Nonlocal PNJL Model

We investigate finite-size effects in the $T$-$\mu$ phase diagram of magnetized quark matter within the framework of a nonlocal extension of the Polyakov--Nambu--Jona-Lasinio (PNJL) model. Finite-size corrections are incorporated through the multiple reflection expansion (MRE) formalism, which describes a spherical quark droplet of radius $R$ and modifies the density of states by including surface and curvature contributions. We consider two-flavor quark matter at finite temperature and chemical potential in the presence of a uniform magnetic field with strengths ranging from $eB=0$ to $1$ GeV$^{2}$, and droplet radii from $R=3$ fm to the bulk limit. The nonlocal PNJL (nlPNJL) model naturally reproduces both magnetic catalysis at low temperatures and inverse magnetic catalysis near the chiral transition, in agreement with lattice QCD results. We analyze the chiral condensate, the traced Polyakov loop, the normalized quark condensate, and the corresponding susceptibilities. We find that finite-size effects do not modify the overall structure of the phase diagram, and that the coincidence of the chiral restoration and deconfinement transitions persists for all magnetic field strengths and system sizes explored, within the present implementation in which finite-size corrections are restricted to the fermionic sector. However, the critical end point (CEP) is notably shifted as a function of both the magnetic field strength and the system size: it moves toward higher chemical potentials and lower temperatures as the system size decreases, an effect that is significantly amplified by strong magnetic fields. Our results have potential implications for the physics of phase conversion in compact stars and for the interpretation of relativistic heavy-ion collision experiments.

hep-ph↗

arXiv2025

Self-bound quark stars with a first-order two-to-three flavor phase transition

We investigate self-bound quark stars in a flavor-dependent quark-mass density-dependent model with an excluded-volume correction. We chart the parameter space at zero pressure to identify self-bound regimes, including parametrizations in which self-bound two-flavor matter undergoes a genuine first-order $ud \to uds$ transition at finite pressure. We construct cold, $\beta$-equilibrated stellar sequences and compute the corresponding global properties (mass-radius relation, tidal deformability, and moment of inertia). For a wide region of the model parameter space, we find that the onset of a $uds$ core occurs before the maximum-mass configuration is reached, yielding self-bound hybrid stars that follow the typical strange-quark-star sequence morphology but develop a characteristic kink at $p_c=p_{\rm tr}$ along the stellar curves. The excluded-volume parameter $\kappa$ controls the stiffness of the equation of state and thus masses, radii, tidal deformabilities, and moments of inertia; intermediate repulsion typically reconciles $M_{\max}\!\gtrsim\!2\,M_\odot$ with current astrophysical constraints. We further identify two equation-of-state-insensitive trends: dimensionless moment of inertia versus compactness and gravitational versus baryonic compactness. These results provide model-guided priors and tools for discriminating between hadronic and self-bound equations of state with multimessenger data.

astro-ph.HE↗

arXiv2025

The sharpness of the quark-hadron transition and the properties of hybrid stars

We investigate the effects of the sharpness of the phase transition between hadronic matter and quark matter on various properties of neutron stars. We construct hybrid equations of state by combining a hadronic model with a quark model using a Gaussian function. This approach introduces a smooth transition characterized by two parameters: one representing the overpressure relative to the first-order phase transition point, and the other related to the range over which the hybrid region extends in baryon chemical potential. We find that the sharpness of the phase transition significantly influences the equation of state, which can deviate by several tens of $\text{MeV fm}^{-3}$ from the one with a sharp first-order transition. The speed of sound exhibits diverse behaviors, including drastic drops, pronounced peaks, and oscillatory patterns, depending on the sharpness parameters. In terms of stellar structure, while the maximum neutron star mass remains largely unaffected by the sharpness of the phase transition, the stellar radii can vary significantly. Smoother transitions lead to a leftward shift (up to 1 km) of the mass-radius curve segment corresponding to hybrid stars. The tidal deformability decreases with smoother transitions, especially for higher-mass stars. Our results are quite general and do not qualitatively depend on the specific hadronic and quark matter models employed. In fact, the hybrid equation of state and stellar properties derived from microscopic models of quark-hadron pasta phases display the same behavior as described above.

nucl-th↗

arXiv2025

Dark interactions in neutron star interiors: the interplay of baryons, dark matter, and dark energy

The impact of energy exchange among the relevant fluid components: baryonic matter, fermionic dark matter (DM), and dark energy (DE) on the internal structure of neutron stars is investigated. Using a representative DM mass $m_{\chi} = 10$ GeV and a barotropic DE relation, we add source terms Qi to the Tolman-Oppenheimer-Volkoff equations and examine three cases: (i) noninteracting fluids (Model I), (ii) fully interacting baryon plus DM and DM with DE fluids with optional DM self repulsion (Model II), and (iii) a unified dark sector coupled to baryons (Model III). Two effects dominate: softening by massive, pressure-poor DM, and additional softening/ and binding from DE with negative pressures. Model I isolates these baselines. In Model II, exchange terms self regulate, making the mass radius curves nearly independent of the coupling parameter $\alpha$ for nearly five orders of magnitude. Model III breaks this $\alpha$ degeneracy: a sizable vacuum fraction suppresses the baryonic pressure gradient, reducing both the maximum mass and radii, whereas a pure-DM core is less sensitive. We outline when dark interactions can leave observable, macroscopic imprints.

astro-ph.HE↗

arXiv2025

Scaling and universality in strange quark stars

We derive scaling laws that connect certain macroscopic observables of strange quark stars with key microscopic properties of self-bound quark matter, such as the energy per baryon at zero pressure and the strength of repulsive interactions. We also identify universal relations linking global properties of strange quark stars - specifically, their moment of inertia, tidal deformability, and both gravitational and baryonic compactness. Remarkably, these relations hold for two substantially different microscopic models - the quark-mass density-dependent model with excluded-volume corrections and the vector MIT bag model - underscoring their robust, model-independent nature. We demonstrate that the universal relations for strange quark stars differ significantly from those previously established for neutron stars composed of hadronic matter, thus enabling discrimination between the two types of objects without requiring detailed knowledge of their equations of state. Moreover, observational constraints on the maximum mass of compact stars could place bounds on both the depth of quark-matter self-binding and the strength of quark repulsive interactions.

nucl-th↗

arXiv2024

Impact of dark energy on the structure of neutron stars: The vacuum case

The potential role of a cosmic vacuum dark component in the properties of neutron stars is investigated. It is assumed that the static, spherically symmetric distribution of matter within neutron stars is supported by two distinct components: ordinary matter and a vacuum fluid. For normal matter we use a set of state-of-the-art nuclear matter equations of state, each grounded in nuclear physics experiments. The vacuum energy component is inhomogeneously distributed within the star and obeys the standard equation of state ($p_v = -\epsilon_v$). This is characterized by an energy density fraction $y = \epsilon_m/ (\epsilon_m + \epsilon_v)$, which we model as either a constant or radius-dependent. Our findings reveal that the inclusion of vacuum energy significantly affects the mass-radius relationships in neutron stars, influencing both the maximum achievable masses and the qualitative form of these relationships. Some constraints from current multimessenger observational data limiting the amount of vacuum energy within neutron stars are also discussed.

hep-ph↗

arXiv2024

Surface and curvature tensions of cold dense quark matter: a term-by-term analysis within the Nambu-Jona-Lasinio model

In this paper, we conduct a thorough investigation of the surface and curvature tensions, $\sigma$ and $\gamma$, of three-flavor cold quark matter using the Nambu-Jona-Lasinio (NJL) model with vector interactions. Our approach ensures both local and global electric charge neutrality, as well as chemical equilibrium under weak interactions. By employing the multiple reflection expansion formalism to account for finite size effects, we explore the impact of specific input parameters, particularly the vector coupling constant ratio $\eta_V$, the radius $R$ of quark matter droplets, as well as charge-per-baryon ratio $\xi$ of the finite size configurations. We focus on the role of the contributions of each term of the NJL Lagrangian to the surface and curvature tensions in the mean field approximation. We find that the total surface tension exhibits two different density regimes: it remains roughly constant at around $100 \, \mathrm{MeV \, fm^{-2}}$ up to approximately $2-4$ times the nuclear saturation density, and beyond this point, it becomes a steeply increasing function of $n_B$. The total surface and curvature tensions are relatively insensitive to variations in $R$ but are affected by changes in $\xi$ and $\eta_V$. We observe that the largest contribution to $\sigma$ and $\gamma$ comes from the regularized divergent term, making these quantities significantly higher than those obtained within the MIT bag model.

nucl-th↗

arXiv2024

Strange quark stars: the role of excluded volume effects

We study cold strange quark stars employing an enhanced version of the quark-mass density-dependent model which incorporates excluded volume effects to address non-perturbative QCD repulsive interactions. We provide a comparative analysis of our mass formula parametrization with previous models from the literature. We identify the regions within the parameter space where three-flavor quark matter is more stable than the most tightly bound atomic nucleus (stability window). Specifically, we show that excluded volume effects do not change the Gibbs free energy per baryon at zero pressure, rendering the stability window unaffected. The curves of pressure versus energy density exhibit various shapes -- convex upward, concave downward, or nearly linear -- depending on the mass parametrization. This behavior results in different patterns of increase, decrease, or constancy in the speed of sound as a function of baryon number density. We analyze the mass-radius relationship of strange quark stars, revealing a significant increase in maximum gravitational mass and a shift in the curves towards larger radii as the excluded volume effect intensifies. Excluded volume effects render our models compatible with all modern astrophysical constraints, including the properties of the recently observed low-mass compact object HESSJ1731.

nucl-th↗

arXiv2023

Excluded volume effects in the quark-mass density-dependent model: implications for the equation of state and compact star structure

We present a significant extension of the quark mass density-dependent model (QMDDM), initially revised in our prior study (Lugones and Grunfeld, Phys. Rev. D 107, 043025 (2023)), where thermodynamic inconsistencies were addressed. Our current work enriches the QMDDM by incorporating excluded volume effects, as a step towards a more realistic representation of the quark matter equation of state (EOS) at zero temperature. We introduce the concept of ``available volume'' in the Helmholtz free energy formulation, accounting for the space excluded by each quasiparticle due to its finite size or repulsive interactions. We present a methodology to modify the EOS for point-like particles, allowing for a simple and direct incorporation of excluded volume effects. This is first addressed in a simple one-flavor model and then extended to a more realistic three-flavor system, incorporating both mass and volume dependencies on the baryon number density. We examine various ansatzes for the excluded volume, ultimately adopting one that aligns with the asymptotic freedom behavior of Quantum Chromodynamics (QCD). The EOS for electrically neutral systems in chemical equilibrium is computed, focusing on self-bound and hybrid matter scenarios. We show that the incorporation of excluded volume effects renders the EOS stiffer and that excluded volume effects are essential to align the mass-radius relation of self-bound and hybrid stars with modern astrophysical constraints.

nucl-th↗

arXiv2023

Hybrid stars with reactive interfaces: analysis within the Nambu-Jona-Lasinio model

It has been shown recently that quark-hadron conversions at the interface of a hybrid star may have a key role on the dynamic stability of the compact object. In this work we perform a systematic study of hybrid stars with reactive interfaces using a model-agnostic piecewise-polytropic hadronic equation of state and the Nambu-Jona-Lasinio model for three-flavor quark matter. For the hadronic phase we use a soft, an intermediate and a stiff parametrization that match at $1.1 n_0$ {with predictions} based on chiral effective field theory (cEFT) interactions. In the NJL Lagrangian we include scalar, vector and 't Hooft interactions. The vector coupling constant $g_{v}$ is treated as a free parameter. We also consider that there is a split between the deconfinement and the chiral phase transitions which is controlled by changing the conventional value of the vacuum pressure $-\Omega_{0}$ in the NJL thermodynamic potential by $-\left(\Omega_{0}+\delta \Omega_{0}\right)$, being $\delta \Omega_{0}$ a free parameter. We analyze the mass-radius ($M$-$R$) relation in the case of rapid ($\tau \ll 1 \, \mathrm{ms}$) and slow ($\tau \gg 1 \, \mathrm{ms}$) conversions, being $\tau$ the reaction timescale. In the case of slow interface reactions we find $M$-$R$ curves with a cusp at the maximum mass point where a pure hadronic branch and a slow-stable hybrid star (SSHS) branch coincide. We find that the length of the slow-stable branch grows with the increase of the transition density and the energy density jump at the hadron-quark interface. We calculate the tidal deformabilities of SSHSs and analyse them in the light of the GW170817 event.

nucl-th↗

arXiv2022

Cold dense quark matter with phenomenological medium effects: a self-consistent formulation of the quark-mass density-dependent model

We revisit the quark-mass density-dependent model -- a phenomenological equation of state for deconfined quark matter in the high-density low-temperature regime -- and show that thermodynamic inconsistencies that have plagued the model for decades, can be solved if the model is formulated in the canonical ensemble instead of the grand canonical one. Within the new formulation, the minimum of the energy per baryon occurs at zero pressure, and the Euler's relation is verified. Adopting a typical mass-formula, we first analyze in detail a simple model with one particle species. We show that a ``bag'' term that produces quark confinement naturally appears in the pressure (and not in the energy density) due to density dependence of the quark masses. Additionally, the chemical potential gains a new term as in other models with quark repulsive interactions. Then, we extend the formalism to the astrophysically realistic case of charge-neutral three-flavor quark matter in equilibrium under weak interactions, focusing on two different mass formulae: a flavor dependent and a flavor blind one. For these two models, we derive the equation of state and analyze its behavior for several parameter choices. We systematically analyze the parameter space and identify the regions corresponding to self-bound 2-flavor and 3-flavor quark matter, hybrid matter and causal behavior.

nucl-th↗

arXiv2021

Vector interactions inhibit quark-hadron mixed phases in neutron stars

We investigate the surface tension $σ$ and the curvature energy $γ$ of quark matter drops in the MIT bag model with vector interactions. Finite size corrections to the density of states are implemented by using the multiple reflection expansion (MRE) formalism. We find that $σ$ and $γ$ are strongly enhanced by new terms arising from vector interactions. With respect to the noninteracting case they are increased by a large factor, which can be as high as $\sim 10$ when the vector coupling constant $g$ varies within the range used in the literature. This behavior may have major consequences for the hadron-quark mixed phase speculated to exist at neutron star (NS) interiors, which may be totally suppressed or have its extension substantially reduced.

nucl-th↗

arXiv2021

Electroweak decay of quark matter within dense astrophysical combustion flames

We study the weak interaction processes taking place within a combustion flame that converts dense hadronic matter into quark matter in a compact star. Using the Boltzmann equation we follow the evolution of a small element of just deconfined quark matter all along the flame interior until it reaches chemical equilibrium at the back boundary of the flame. We obtain the reaction rates and neutrino emissivities of all the relevant weak interaction processes without making any assumption about the neutrino degeneracy. We analyse systematically the role the initial conditions of unburnt hadronic matter, such as density, temperature, neutrino trapping and composition, focusing on typical astrophysical scenarios such as cold neutron stars, protoneutron stars, and post merger compact objects. We find that the temperature within the flame rises significantly in a timescale of 1 nanosecond. The increase in $T$ strongly depends on the initial strangeness of hadronic matter and tends to be more drastic at larger densities. Typical final values range between $20$ and $60 \, \mathrm{MeV}$. The nonleptonic process $u + d \rightarrow u + s$ is always dominant in cold stars, but in hot objects the process $u + e^{-} \leftrightarrow d + {ν_e}$ becomes relevant, and in some cases dominant, near chemical equilibrium. The rates for the other processes are orders of magnitude smaller. We find that the neutrino emissivity per baryon is very large, leading to a total energy release per baryon of $2-60 \, \mathrm{MeV}$ in the form of neutrinos along the flame. We discuss some astrophysical consequences of the results.

nucl-th↗

arXiv2021

Surface and curvature properties of charged strangelets in compact objects

Droplets of absolutely stable strange quark matter (strangelets) immersed in a lepton background may be the energetically preferred composition of strange star crusts and of the interior of a new class of stars known as strangelet dwarfs. In this work we calculate the surface tension $σ$ and the curvature coefficient $γ$ of charged strangelets as a function of the baryon number density, the temperature, the chemical potential of trapped neutrinos, the strangelet size, the electric potential and the electric charge at their boundary. Strange quark matter in chemical equilibrium and with global electric charge neutrality is described within the MIT bag model. We focus on three different astrophysical scenarios, namely cold strange stars, proto strange stars and post merger strange stars. Finite size effects are implemented within the multiple reflection expansion framework. We find that $σ$ decreases significantly as the strangelet's boundary becomes more positively charged. This occurs because $σ$ is dominated by the contribution of $s$ quarks which are the most massive particles in the system. Negatively charged $s$-quarks are suppressed in strangelets with a large positive electric charge, diminishing their contribution to $σ$ and resulting in smaller values of the total $σ$. We verify that the more extreme astrophysical scenarios, with higher temperatures and higher neutrino chemical potentials, allow higher positive values of the strangelet's electric charge at the boundary and consequently smaller values of $σ$. In contrast, $γ$ is strongly dominated by the density of light ($u$ and $d$) quarks and is quite independent of the charge-per-baryon ratio, the temperature and neutrino trapping. We discuss the relative importance of surface and curvature effects as well as some astrophysical consequences of these results.

nucl-th↗

arXiv2020

The role of quark matter surface tension in magnetars

In spite of its key role in compact star physics, the surface tension of quark matter is not well comprehended yet. In this work we analyze the behavior of the surface tension of three-flavor quark matter in the outer and inner core of cold deleptonized magnetars, proto magnetars born in core collapse supernovae, and hot magnetars produced in binary neutron stars mergers. We explore the role of temperature, baryon number density, trapped neutrinos, droplet size, and magnetic fields within the multiple reflection expansion formalism. Quark matter is described within the MIT bag model and is assumed to be in chemical equilibrium under weak interactions. We discuss some astrophysical consequences of our results.

nucl-th↗

arXiv2020

Discontinuity gravity modes in hybrid stars: assessing the role of rapid and slow phase conversions

Discontinuity gravity modes may arise in perturbed quark-hadron hybrid stars when a sharp density jump exists in the stellar interior and are a potential fingerprint to infer the existence of quark matter cores in compact objects. When a hybrid star is perturbed, conversion reactions may occur at the quark-hadron interface and may have a key role in global stellar properties such as the dynamic stability and the quasi-normal mode spectrum. In this work we study the role of the conversion rate at the interface. To this end, we first derive the junction conditions that hold at the sharp interface of a non-radially perturbed hybrid star in the case of slow and rapid conversions. Then, we analyse the discontinuity $g$-mode in both cases. For rapid conversions, the discontinuity $g$-mode has zero frequency because a displaced fluid element near the phase splitting surface adjusts almost immediately its composition to its surroundings and gravity cannot provide a buoyancy force. For slow conversions, a $g$-mode exists and its properties are analysed here using modern hadronic and quark equations of state. Moreover, it has been shown recently that in the case of slow conversions an extended branch of stable hybrid configurations arises for which $\partial M/ \partial ε_c <0$. We show that $g$-modes of the standard branch (that is, the one with $\partial M/ \partial ε_c > 0$) have frequencies and damping times in agreement with previous results in the literature. However, $g$-modes of the extended branch have significantly larger frequencies (in the range $1-2 \, \mathrm{kHz}$) and much shorter damping times (few seconds in some cases). We discuss the detectability of $g$-mode GWs with present and planned GW observatories.

astro-ph.HE↗

arXiv2018

Surface tension of hot and dense quark matter under strong magnetic fields

We study the surface tension of hot, highly magnetized three flavor quark matter droplets, focusing specifically on the thermodynamic conditions prevailing in neutron stars, hot lepton rich protoneutron stars and neutron star mergers. We explore the role of temperature, baryon number density, trapped neutrinos, droplet size and magnetic fields within the multiple reflection expansion formalism (MRE), assuming that astrophysical quark matter can be described as a mixture of free Fermi gases composed by quarks $u$, $d$, $s$, electrons and neutrinos, in chemical equilibrium under weak interactions. We find that the total surface tension is rather unaffected by the size of the drop, but is quite sensitive to the effect of baryon number density, temperature, trapped neutrinos and magnetic fields (specially above $eB \sim 5 \times 10^{-3} \mathrm{GeV}^2$). Surface tensions parallel and transverse to the magnetic field span values up to $\sim$ 25 MeV/fm$^2$. For $T \lesssim 100$ MeV the surface tension is a decreasing function of temperature but above 100 MeV it increases monotonically with $T$. Finally, we discuss some astrophysical consequences of our results.

astro-ph.HE↗

arXiv2018

Gravitational wave asteroseismology limits from low density nuclear matter and perturbative QCD

We investigate the fundamental mode of non-radial oscillations of non-rotating compact stars in general relativity using a set of equations of state (EOS) connecting state-of-the-art calculations at low and high densities. Specifically, a low density model based on the chiral effective field theory (EFT) and high density results based on perturbative Quantum Chromodynamics (QCD) are matched through different interpolating polytropes fulfilling thermodynamic stability and subluminality of the speed of sound, together with the additional requirement that the equations of state support a two solar mass star. We employ three representative models (EOS I, II and III) presented in Ref. [1] such that EOS I gives the minimum stellar radius, EOS II the maximum stellar mass, and EOS III the maximum stellar radius. Using this family of equations of state, we find that the frequency and the damping time of the $f$-mode are constrained within narrow quite model-independent windows. We also analyze some proposed empirical relations that describe the $f$-mode properties in terms of the average density and the compactness of the neutron star. We discuss the stringency of these constrains and the possible role of physical effects that cannot be encoded in a mere interpolation between low and high density EOSs.

astro-ph.HE↗