arXiv subjects

M. C. Ruivo

Publications and source records attributed to M. C. Ruivo.

35 records · Page 2Linked to original sources

arXiv2006

Behavior of the topological susceptibility at finite T and $μ$ and signs of restoration of chiral symmetries

We investigate the possible restoration of chiral and axial symmetries across the phase transition at finite temperature and chemical potential, by analyzing the behavior of several physics quantities, such as the quark condensates and the topological susceptibility, the respective derivatives in order to chemical potential, and the masses of meson chiral partners. We discuss whether only chiral symmetry or both chiral and axial symmetries are restored and what is the role of the strange quark. The results are compared with recent lattice results.

hep-ph↗

arXiv2005

Effects of the restoration of U$_A$(1) symmetry on pseudoscalar meson observables

We investigate the restoration of chiral SU$(3)\otimes$SU$(3)$ and axial U$_A$(1) symmetries, at finite temperature and density, in the framework of the three flavor Nambu-Jona-Lasinio model with anomaly. We implement a temperature (density) dependence of the anomaly coefficient motivated by lattice results for the topological susceptibility and we discuss the restoration of symmetries by analyzing the behavior of the mesonic chiral partners and of the mixing angles. The results indicate that the axial part of the symmetry is restored before the possible restoration of the full U(3)$\otimes$U(3) chiral symmetry can occur.

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arXiv2005

Analysis of U$_A$(1) symmetry-breaking and restoration effects on the scalar-pseudoscalar spectrum

We explore patterns of effective restoration of the chiral U$_A$(1) symmetry using an extended three-flavor Nambu-Jona-Lasinio model that incorporates explicitly the axial anomaly through the 't Hooft interaction, and assuming that the coefficient of the anomaly term is temperature and density dependent. The special case of explicit breaking of chiral symmetry without U$_A$(1) anomaly is also considered, since this scenario can provide additional information allowing to understand the interplay between the U$_A$(1) anomaly and (spontaneous) chiral symmetry breaking effects. The pseudoscalar and scalar sectors are analyzed in detail bearing in mind the identification of chiral partners and the study of its convergence. We also concentrate on the behavior of the mixing angles that give us relevant information on the issue under discussion. In the region of temperatures (densities) studied, we do not observe signs indicating a full restoration of U(3)$\otimes$U(3) symmetry as, for instance, the degeneracy of both $a_0$ and $f_0$ mesons with the pion. As we work in a real world scenario ($m_u=m_d<<m_s$), we only observe the return to symmetries of the classical QCD Lagrangian in the non-strange sector.

hep-ph↗

arXiv2005

Restoration of U$_A$(1) symmetry and meson spectrum in hot or dense matter

We explore the effects of breaking and restoration of chiral and axial symmetries using an extended three-flavor Nambu-Jona-Lasinio model that incorporates explicitly the axial anomaly through the 't Hooft interaction. We implement a temperature (density) dependence of the anomaly coefficient motivated by lattice results for the topological susceptibility. The spectrum of scalar and pseudoscalar mesons is analyzed bearing in mind the identification of chiral partners and the study of its convergence. We also concentrate on the behavior of the mixing angles that give us relevant information on the issue under discussion. The results suggest that the axial part of the symmetry is restored before the possible restoration of the full U(3)$\otimes$U(3) chiral symmetry might occur.

hep-ph↗

arXiv2004

Effective restoration of the U_A(1) symmetry with temperature and density

We investigate the full U(3)$\otimes$U(3) chiral symmetry restoration, at finite temperature and density, on the basis of a quark model which incorporates the most relevant properties of QCD in this context: explicit and spontaneous breaking of chiral symmetry and axial U$_A$(1) symmetry breaking. A specific lattice-inspired behavior of the topological susceptibility, combined with the convergence of chiral partners, signals the onset of an effective chiral symmetry restoration. The results suggest that the axial part of the symmetry is restored before the possible restoration of the full U(3)$\otimes$U(3) chiral symmetry can occur. This conclusion is valid in the context of both finite temperature and density.

hep-ph↗

arXiv2004

Phase transitions and behavior of kaons in hot and dense matter

We study phase transitions and the behavior of kaons in hot and dense matter, giving special attention to the role of strange quarks. At T=0, it is found that the fraction of the strange valence quarks affects the energy per particle of the system, without changing the nature of the phase transition, and, on the other side, it has meaningful effects on the behavior of kaons and antikaons masses. The phase behavior of kaons in the $T-ρ$ plane is analyzed in connection with the chiral phase transition.

hep-ph↗

arXiv2004

Is the U$_A$(1) symmetry restored at finite temperature or density?

We investigate the full U(3)$\otimes$U(3) chiral symmetry restoration, at finite temperature and density, on the basis of the three flavor Nambu-Jona-Lasinio model with the anomaly term given by the 't Hooft interaction. We implement a temperature (density) dependence of the anomaly coefficient motivated by lattice results for the topological susceptibility. The results suggest that the axial part of the symmetry is restored before the possible restoration of the full U(3)$\otimes$U(3) chiral symmetry can occur.

hep-ph↗

arXiv2004

Effective restoration of chiral and axial symmetries at finite temperature and density

The effective restoration of chiral and axial symmetries is investigated within the framework of the SU(3) Nambu-Jona-Lasinio model. The topological susceptibility, modeled from lattice data at finite temperature, is used to extract the temperature dependence of the coupling strength of the anomaly. The study of the scalar and pseudoscalar mixing angles is performed in order to discuss the evolution of the flavor combinations of $q \bar q$ pairs and its consequences for the degeneracy of chiral partners. A similar study at zero temperature and finite density is also realized.

hep-ph↗

arXiv2004

Two photon decay of $π^0$ and $η$ at finite temperature and density

A comparative study of the anomalous decays $π^0, η\toγγ$, at finite temperature and at finite density, is performed in the framework of the three--flavor Nambu--Jona-Lasinio. The similarities and differences between both scenarios are discussed. In both cases the lifetimes of these mesons decrease significantly at the critical point, although this might not be sufficient to observe enhancement of these decays in heavy-ion collisions.

hep-ph↗

arXiv2004

Anomalous decay of pion and eta at finite temperature

We study the anomalous decays $π^0, η\toγγ$ in the framework of the three--flavor Nambu--Jona-Lasinio [NJL] model at finite temperature. The similarities and differences between these results and those obtained at zero temperature and finite density are discussed. In both cases the lifetimes of these mesons decrease significantly at the critical point, although this might not be sufficient to observe enhancement of these decays in heavy-ion collisions.

hep-ph↗

arXiv2004

Pseudoscalars Mesons in Hot, Dense Matter

Phase transitions in hot and dense matter and the in--medium behavior of pseudoscalar mesons ($π^{\pm}, π^0, K^{\pm}, K^0 ,\bar K^0,η{and} η' $) are investigated, in the framework of the three flavor Nambu--Jona-Lasinio model, including the 't Hooft interaction, which breaks the $U_A(1)$ symmetry. Three different scenarios are considered: zero density and finite temperature, zero temperature and finite density in quark matter with different degrees of strangeness, and finite temperature and density. At T=0, the role of strange valence quarks in the medium is discussed, in connection with the phase transition and the mesonic behavior. It is found that the appearance of strange quarks, above certain densities, leads to meaningful changes in different observables, especially in matter with β$ --equilibrium. The behavior of mesons in the $T-ρ$ plane is analyzed in connection with possible signatures of restoration of symmetries.

hep-ph↗

arXiv2004

Charged pions and kaons weak leptonic decays at finite density

The weak leptonic decays $π^\pm\toμ^\pmν_μ(\barν_μ), π^\pm\to e^\pmν_e(\barν_{e}), K^\pm\toμ^\pmν_μ(\barν_μ)$ and $K^\pm\to e^\pm ν_e(\barν_{e})$ are investigated within the framework of the SU(3) Nambu-Jona-Lasinio model, in the vacuum and in quark matter in $β$ equilibrium. It is found that, at low densities, the decay rates of the charge multiplets, that are decoupled in this kind of flavor asymmetric matter, are affected differently. As the density increases, these decay rates become closer and show a tendency to vanish. This shows that weak interactions are strongly influenced by the medium as a consequence of restoration of chiral symmetry.

hep-ph↗

arXiv2003

Anomalous decay of pion and eta at finite density

We study the anomalous decays $π^0, η\toγγ$ in the framework of the three--flavor Nambu--Jona-Lasinio [NJL] model, in the vacuum and in quark matter in $β$ --equilibrium. It is found that the behavior of the relevant observables essentially reflects a manifestation of the partial restoration of chiral symmetry, in non strange and strange sectors. The probability of such decays decreases with density, showing that anomalous mesonic interactions are significantly affected by the medium.

hep-ph↗

arXiv2003

$η- η'$ in a hot and dense medium

The behavior of $η$ and $η'$ in hot strange quark matter in weak equilibrium with temperature, is investigated within the SU(3) Nambu-Jona-Lasinio [NJL] model. Possible manifestations of restoration of symmetries, by temperature or density, in the behavior of $η$ and $η'$ are discussed. The role played by the combined effect of temperature and density in the nature of the phase transition and meson behavior is also analyzed.

hep-ph↗

arXiv2003

Pseudoscalar neutral mesons in hot and dense matter

The behavior of neutral pseudoscalar mesons $π^0, η$ and $η'$ in hot and dense matter is investigated, in the framework of the three flavor Nambu-Jona-Lasinio model. Three different scenarios are considered: zero density and finite temperature, zero temperature and finite density in a flavor asymmetric medium with and without strange valence quarks, and finite temperature and density. The behavior of mesons is analyzed in connection with possible signatures of restoration of symmetries. In the high density region and at zero temperature it is found that the mass of the $η'$ increases, the deviation from the mass of the $η$ being more pronounced in matter without strange valence quarks.

hep-ph↗

arXiv2001

Pseudoscalar Mesons in Asymmetric Matter

The behavior of kaons and pions in hot non strange quark matter, simulating neutron matter, is investigated within the SU(3) Nambu-Jona-Lasinio [NJL] and in the Enlarged Nambu-Jona-Lasinio [ENJL] (including vector pseudo-vector interaction) models. At zero temperature, it is found that in the NJL model, where the phase transition is first order, low energy modes with K-, Pi+ quantum numbers, which are particle-hole excitations of the Fermi sea, appear. Such modes are not found in the ENJL model and in NJL at finite temperatures. The increasing temperature has also the effect of reducing the splitting between the charge multiplets.

hep-ph↗

arXiv1993

Strong and Radiative Meson Decays in a Generalized Nambu--Jona-Lasinio Model

We investigate strong and radiative meson decays in a generalized Nambu--Jona-Lasinio model. The one loop order calculation provides a satisfactory agreement with the data for the mesonic spectrum and for radiative decays. Higher order effects for strong decays of $ρ$ and $K^*$ are estimated to be large. We also discuss the role of the flavour mixing determinantal interaction.

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