arXiv subjects

R. M. Aguirre

Publications and source records attributed to R. M. Aguirre.

17 recordsLinked to original sources

arXiv2026

Effects of isospin imbalance on the chiral phase transition within a Chiral Dual Partner Model

The chiral symmetry is a property of the fundamental theory of the strong interaction that is relevant for the hadronic physics. It is expected that at sufficiently high temperature and matter density, this symmetry becomes manifest. Within the Chiral Dual Partner Model the chiral transformation is implemented in such a way that a fermion mass term is allowed if the parity partner of each baryon is included in the framework. This model is used here to study possible manifestations of the chiral symmetry in dense nuclear matter, assuming constant isospin fraction. It is found that the onset of the odd parity baryons is associated with two branches of thermodynamical instabilities, one of them leads to a first order phase transition for temperatures below $T_c\simeq 11$ MeV. An analysis of these instabilities in the phase space is given, and neutron star matter is considered as a special case.

nucl-th↗

arXiv2026

Effects of equilibrium coexisting phases in the first-order chiral transition within the Linear sigma model with quarks

The first order chiral phase transition for quark matter with flavor imbalance is studied using the Linear sigma model with quarks, also known as Quark-meson model. Special attention is paid to the role of the scalar isovector meson. The general consensus presently is that the chiral transition changes from a smooth crossover to first-order at low temperatures. This transition is assumed to be discontinuous, with unstable or metastable intermediate states. However, if multiple charges are simultaneously conserved the system could undergo a continuous change through a coexistence of equilibrium states. Under such assumption the bulk properties are analyzed and several remarkable effects for the speed of sound and the susceptibilities are stressed.

hep-ph↗

arXiv2025

Coexisting phases in the chiral transition within the Linear sigma model with quarks

It is believed at present that the chiral transition changes from a smooth crossover to a first-order transition at low temperatures and high densities. Such regime is commonly analyzed using effective models since first principle calculations, as in lattice arrangements, are not feasible. This transition is assumed to be discontinuous, with unstable or metastable intermediate states. However, if multiple charges are simultaneously conserved the system could undergo a continuous change through a coexistence of equilibrium states. This type of transition has multiple manifestations, as in the nuclear liquid-gas transition causing the spinodal fragmentation. The coexistence of phases in the chiral transition is studied here for quark matter assuming the conservation of the isospin composition. Using the Linear sigma model with quarks several remarkable effects are found and discussed.

nucl-th↗

arXiv2024

Pseudoscalar mesons from a PNJL model at zero temperature

We study pseudoscalar $π$, $K$ and $η$ meson properties, such as masses and couplings, in dense matter at zero temperature. We use a recently proposed phenomenological quark model, known as the PNJL0, which takes into account the confinement/deconfinement phase transition by means of the traced Polyakov loop ($Φ$) which serves as an order parameter at zero temperature. We consider two different scenarios, namely, symmetric quark matter with equal chemical potentials for all the flavors, and the beta equilibrated matter. In the latter case the hadron-quark phase transition is implemented by a two model approach. For the hadron side we use a relativistic mean-field model with density dependent couplings. We show that $Φ$ induces abrupt changes in the mesons properties with gap sizes regulated by the phenomenological gluonic sector of the model.

hep-ph↗

arXiv2023

Effects of the anomalous magnetic moments of the quarks on the neutral pion properties within a SU(2) Nambu-Jona Lasinio model

The properties of the neutral pion in quark matter under the influence of an external magnetic field are studied. The effects of the anomalous magnetic moments (AMM) of the quarks at finite density is considered. The inclusion of the AMM into the NJL model gives rise to additional magnetic effects. In particular the Dirac sea produce new divergences in the vacuum contributions, which depend explicitly on the magnetic field. An improper treatment of these contributions is the source of unphysical results, as emphasized in recent investigations. The pion polarization function is evaluated in the random phase approximation using analytic regularization and a subtraction scheme to deal with such divergencies. This procedure is combined with the standard three momentum cutoff, and reduces to it for vanishing magnetic intensity. The pion mass and coupling constant are evaluated for a wide range of magnetic intensity and baryonic density.

hep-ph↗

arXiv2023

Influence of hyperon-hyperon interaction on the properties of neutron stars

The properties of neutron stars are studied in a composite model of the strong interaction. In the regime of low to medium baryonic densities a covariant hadronic model is adopted which includes an exclusive channel for the hyperon-hyperon interaction mediated by hidden strangeness mesons, which in turn couple to other mesons through polynomial vertices. The new coupling constants are subject to phenomenological constraints. The presence of free quarks in the core of the star is considered by using the Nambu-Jona Lasinio model supplemented with a vector interaction. The deconfinement process is described by a continuous coexistence of phases. Several structure parameters of neutron stars, such as mass-radius relation, moment of inertia, tidal deformability, and the propagation of nonradial f and g-modes within the relativistic Cowling approximation are studied. The predictions of the model are in good agreement with recent observational data, in particular the maximum inertial mass is greater than the observational lower limit of two solar masses.

nucl-th↗

arXiv2022

Hyperons, deconfinement and the speed of sound in neutron stars

The effects of the presence of hyperons and a phase transition to deconfined quark matter on the speed of sound in neutron stars is investigated. For this purpose a composite description consisting of a model of the covariant field theory of hadrons and one for unbound quarks are used. A phase transition with continuous and monotonous variation of the equation of state is assumed. The predictions are contrasted with recent observational data on isolated neutron stars as well as on binary systems. Only one candidate is finally obtained from six different descriptions. According to the present calculations the onset of the hyperons causes the equilibrium speed of sound to exceed the conformal limit. Qualitative agreement with recent work about the influence of the speed of sound on the g-modes of oscillation in neutron stars is obtained.

nucl-th↗

arXiv2020

Regularization of the Nambu-Jona Lasinio model under a uniform magnetic field and the role of the anomalous magnetic moments

The vacuum contribution to quark matter under a uniform magnetic field within the SU(3) version of the Nambu and Jona-Lasinio model is studied. The standard regularization procedure is examined and a new prescription is proposed. For this purpose analytic regularization and a subtraction scheme are used to deal with divergencies depending on the magnetic intensity. This scheme is combined with the standard three momentum cutoff recipe, and reduces to it for vanishing magnetic intensity. Furthermore, the effects of a direct coupling between the anomalous magnetic moments of the quarks and the magnetic field is considered. Single particle properties as well as bulk thermodynamical quantities are studied for a configuration of matter found in neutron stars. A wide range of baryonic densities and magnetic intensities are examined at zero temperature.

hep-ph↗

arXiv2019

Vacuum effects on the properties of nuclear matter under an external magnetic field

The effects of the Dirac sea of the nucleons are investigated within a covariant model of the hadronic interaction. We extend the usual Mean Field Approximation and present a procedure to deal with divergences which are proportional to polynomials on the magnetic field intensity. For this purpose a nucleon propagator is used which takes accounts of the full effect of the magnetic field as well as the presence of the anomalous magnetic moments of both protons and neutrons. We examine single particle properties and bulk thermodynamical quantities and conclude that within a reasonable range of densities and magnetic intensities the effects found are moderate.

nucl-th↗

arXiv2019

In medium properties of $K^0$ and $ϕ$ mesons under an external magnetic field

The one loop polarization insertions for the kaon and the $ϕ$ mesons are evaluated in a hadronic medium as functions of the field intensity and the baryonic density. For this purpose an effective chiral model of the hadronic interaction is used, supplemented with a phenomenological $K-ϕ$ interaction. The propagators of the charged particles include the full effect of the coupling to the magnetic field which induces the quantum Landau levels. Consequently, the vacuum diagrams are defined within a zeta function regularization scheme. The effective masses as well as the decay widths are introduced and examined as functions of matter density and the magnetic intensity.

nucl-th↗

arXiv2017

Modification of the masses of the lightest neutral mesons in a hadronic medium under an external magnetic field

The effective masses of the neutral mesons in a hadronic medium and under an external magnetic field are evaluated as functions of the baryonic density and the field intensity. For this purpose the meson polarization is evaluated in the one loop approximation using a Quantum Hadrodynamics model which includes pions, sigma, omega, and rho mesons. The propagators of the baryons include the full effect of the coupling to the magnetic field through their charges and their anomalous magnetic moments. Within the range of magnetic intensities considered here $10^{17}$ G $< B < 10^{19}$ G, the dependence on B is moderate for the pion and the longitudinal component of the omega meson, and negligible for the remaining mesons.

nucl-th↗

arXiv2016

Measurements of fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets in $pp$ collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector

Fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets are reported, using an integrated luminosity of 20.3 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for $t\bar{t}$ events with at least one additional $b$-jet is measured to be 950 $\pm$ 70 (stat.) $^{+240}_{-190}$ (syst.) fb in the lepton-plus-jets channel and 50 $\pm$ 10 (stat.) $^{+15}_{-10}$ (syst.) fb in the $e μ$ channel. The cross-section times branching ratio for events with at least two additional $b$-jets is measured to be 19.3 $\pm$ 3.5 (stat.) $\pm$ 5.7 (syst.) fb in the dilepton channel ($e μ$,\,$μμ$, and \,$ee$) using a method based on tight selection criteria, and 13.5 $\pm$ 3.3 (stat.) $\pm$ 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 $\pm$ 0.33 (stat.) $\pm$ 0.28 (syst.)\% for the ratio of $t\bar{t}$ production with two additional $b$-jets to $t\bar{t}$ production with any two additional jets. All measurements are in good agreement with recent theory predictions.

hep-ex↗

arXiv2010

Finite temperature collective modes in a two phase coexistence region of asymmetric nuclear matter

The relation between collective modes and the phase transition in low density nuclear matter is examined. The dispersion relations for collective modes in a linear approach are evaluated within a Landau-Fermi liquid scheme by assuming coexisting phases in thermodynamical equilibrium. Temperature and isospin composition are taken as relevant parameters. The in-medium nuclear interaction is taken from a recently proposed density functional model. We found significative modifications in the energy spectrum, within certain range of temperatures and isospin asymmetry, due to the separation of matter into independent phases. We conclude that detailed calculations should not neglect this effect.

nucl-th↗

arXiv2006

Landau parameters for isospin asymmetric nuclear matter based on a relativistic model of composite and finite extension nucleons

We study the properties of cold asymmetric nuclear matter at high density, applying the quark meson coupling model with excluded volume corrections in the framework of the Landau theory of relativistic Fermi liquids. We discuss the role of the finite spatial extension of composite baryons on dynamical and statistical properties such as the Landau parameters, the compressibility, and the symmetry energy. We have also calculated the low lying collective eigenfrequencies arising from the collisionless quasiparticle transport equation, considering both unstable and stable modes. An overall analysis of the excluded volume correlations on the collective properties is performed.

nucl-th↗

arXiv2005

Collective modes in strange and isospin asymmetric hadronic matter

We study the propagation of non-strange and strange meson modes in hadronic matter considering both isospin and strangeness mixings induced by quantum fluctuations in the medium. Baryons are described using the Quark Meson Coupling model extended to include interactions of strange quarks. In particular we evaluate the dependence of the meson masses on the baryonic density, the strangeness fraction and the isospin asymmetry of the medium. We have found a considerable admixture of strangeness and isospin in the sigma-mode in the high density regime.

nucl-th↗

arXiv2003

Neutron star structure in a quark model with excluded volume correction

We study the effects of the finite size of baryons on the equation of state of homogeneous hadronic matter. The finite extension of hadrons is introduced in order to improve the performance of field theoretical models at very high densities. We simulate the in-medium averaged baryon-baryon strong repulsion at very short distances by introducing a Van der Waals like normalization of the fields. This is done in the framework of the Quark Meson Coupling model, that allows to take care of the quark structure of baryons. Since within this model the confinement volume evolves with the fields configuration, the treatment is not equivalent to a simple hard-core potential. We investigate the phase transition to quark matter and the structure of neutron stars. We have found significant corrections at high densities.

nucl-th↗

arXiv1999

Equation of State of Hypermatter in beta Equilibrium in a Quark Model with Excluded Volume Correction

We study the effects of the finite size of the baryons on the equation of state of homogeneous hadronic matter with hyperons. The finite extension of hadrons is introduced in order to improve the perfomance of field theoretical models at very high densities, simulating the short-range hard-core repulsive part of the baryon-baryon potential. We choose the Quark Meson Coupling model to describe the baryon dynamics because this model provides a density dependent radius of the bag. In addition we calculate finite size corrections for the Zimanyi-Moszkowski model in order to make a definite comparison. In both cases finite size effects are implemented through a Van der Waals like correction in the normalization of the baryon fields. In this approach we investigate $β$-stable matter in conditions that could be found in the interior of massive stars. We find that the excluded volume corrections contribute significantly for densities above twice the symmetric nuclear matter saturation density, although for the quark model the results strongly depends upon the equilibrium conditions for the confinement volume immersed in a dense medium.

nucl-th↗