arXiv subjects

M. Malheiro

Publications and source records attributed to M. Malheiro.

At least 73 records · Page 4Linked to original sources

arXiv2003

Role of the Coulomb and the vector-isovector $ρ$ potentials in the isospin asymmetry of nuclear pseudospin

We investigate the role of the Coulomb and the vector-isovector $ρ$ potentials in the asymmetry of the neutron and proton pseudospin splittings in nuclei. To this end, we solve the Dirac equation for the nucleons using central vector and scalar potentials with Woods-Saxon shape and $Z$ and $N-Z$ dependent Coulomb and $ρ$ potentials added to the vector potential. We study the effect of these potentials on the energy splittings of proton and neutron pseudospin partners along a Sn isotopic chain. We use an energy decomposition proposed in a previous work to assess the effect of a pseudospin-orbit potential on those splittings. We conclude that the effect of the Coulomb potential is quite small and the $ρ$ potential gives the main contribution to the observed isospin asymmetry of the pseudospin splittings. This isospin asymmetry results from a cancellation of the various energy terms and cannot be attributed only to the pseudospin-orbit term, confirming the dynamical character of this symmetry pointed out in previous works.

nucl-th↗

arXiv2003

Metastable strange matter and compact quark stars

Strange quark matter in beta equilibrium at high densities is studied in a quark confinement model. Two equations of state are dynamically generated for the {\it same} set of model parameters used to describe the nucleon: one corresponds to a chiral restored phase with almost massless quarks and the other to a chiral broken phase. The chiral symmetric phase saturates at around five times the nuclear matter density. Using the equation of state for this phase, compact bare quark stars are obtained with radii and masses in the ranges $R\sim 5 - 8$ km and $M\sim M_\odot$. The energy per baryon number decreases very slowly from the center of the star to the periphery, remaining above the corresponding values for the iron or the nuclear matter, even at the edge. Our results point out that strange quark matter at very high densities may not be absolutely stable and the existence of an energy barrier between the two phases may prevent the compact quarks stars to decay to hybrid stars.

astro-ph↗

arXiv2002

Hadronic Entropy Enhancement and Low Density QGP

Recent studies show that for central collisions the rising of the incident energy from AGS to RHIC decreases the value of the chemical potential in the Hadron-QGP phase diagram. Thus, the formation of QGP at RHIC energies in central collisions may be expected to occur at very small values of the chemical potential. Using many different relativistic mean-field hadronic models (RMF) at this regime we show that the critical temperature for the Hadron-QGP transition is hadronic model independent. We have traced back the reason for this and conclude that it comes from the fact that the QGP entropy is much larger than the hadronic entropy obtained in all the RMF models. We also find that almost all of these models present a strong entropy enhancement in the hadronic sector coming from the baryonic phase transition to a nucleon-antinucleon plasma. This result is in agreement with the recent data obtained in the STAR collaboration at RHIC where it was found a rich proton-antiproton matter.

hep-ph↗

arXiv2002

Dynamical nature of the nuclear pseudospin and its isospin asymmetry

Pseudospin symmetry in nuclei is investigated by solving the Dirac equation with Woods-Saxon scalar and vector radial potentials. We relate the pseudospin interaction with a pseudospin-orbit term in a Schroedinger-like equation for the lower component of the Dirac spinor. We show that this term gives a large contribution to the energy splittings of pseudospin partners, so that the near pseudospin degeneracy arises from a significant cancellation among the different terms in that equation. This is a manifestation of the dynamical character of this symmetry in the nucleus. We analyze the isospin dependence of the pseudospin symmetry in a nuclear isotope chain by including a vector-isovector potential V_rho and a Coulomb potential and conclude that V_rho gives the main contribution to the observed pseudospin isospin asymmetry.

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arXiv2002

Small quark stars in the chromodielectric model

Equations of state for strange quark matter in beta equilibrium at high densities are used to investigate the structure (mass and radius) of compact objects. The chromodielectric model is used as a general framework for the quark interactions, which are mediated by chiral mesons, $σ$ and $\vec π$, and by a confining chiral singlet dynamical field, $χ$. Using a quartic potential for $χ$, two equations of state for the same set of model parameters are obtained, one with a minimum at around the nuclear matter density $ρ_0$ and the other at $ρ\sim 5 ρ_0$. Using the latter equation of state in the Tolman-Oppenheimer-Volkoff equations we found solutions corresponding to compact objects with $R\sim 5 - 8$ km and $M\sim M_\odot$. The phenomenology of recently discovered X-ray sources is compatible with the type of quark stars that we have obtained.

hep-ph↗

arXiv2001

Deuteron Compton Scattering in Chiral Perturbation Theory

Compton scattering on the deuteron is studied in the framework of baryon chiral perturbation theory to third order in small momenta, for photon energies of order the pion mass. The scattering amplitude is a sum of one- and two-nucleon mechanisms with no undetermined parameters. Our results are in good agreement with the intermediate energy experimental data, and a comparison is made with the recent higher-energy data obtained at SAL.

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arXiv2001

Pseudospin symmetry as a relativistic dynamical symmetry in the nucleus

Pseudospin symmetry in nuclei is investigated by solving the Dirac equation with Woods-Saxon scalar and vector radial potentials, and studying the correlation of the energy splittings of pseudospin partners with the nuclear potential parameters. The pseudospin interaction is related to a pseudospin-orbit term that arises in a Schroedinger-like equation for the lower component of the Dirac spinor. We show that the contribution from this term to the energy splittings of pseudospin partners is large. The near pseudospin degeneracy results from a significant cancelation among the different terms in that equation, manifesting the dynamical character of this symmetry in the nucleus. We analyze the isospin dependence of the pseudospin symmetry and find that its dynamical character is behind the different pseudospin splittings observed in neutron and proton spectra of nuclei.

nucl-th↗

arXiv2001

The Isospin Asymmetry in the Pseudo-spin Dynamical Symmetry

Pseudo-spin symmetry in nuclei is investigated considering the Dirac equation with a Lorentz structured Woods-Saxon potential. The isospin correlation of the energy splittings of pseudo-spin partners with the nuclear potential parameters is studied. We show that in an isotopic chain, the pseudo-spin symmetry is better realized for neutrons than for protons. This behavior comes from balance effects amongst the central nuclear potential parameters. In general, we found an isospin asymmetry of the nuclear pseudo-spin interaction, opposed to the nuclear spin-orbit interaction which is quasi isospin symmetric.

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arXiv2000

Neutron stars in a class of non-linear relativistic models

We introduce in this work a class of relativistic models for nuclear matter and neutron stars which exhibits a parameterization, through mathematical constants, of the non-linear meson-baryon couplings. For appropriate choices of the parameters, it recovers current QHD models found in the literature: Walecka and the Zimanyi and Moszkowski models (ZM and ZM3). For other choices of the parameters, the new models give very interesting and new physical results.We have obtained numerical values for the maximum mass and radius of a neutron star sequence, red-shift, hyperon populations, radial distribution of particles, among other relevant static properties of these stellar objects. The phenomenology of neutron stars in ZM models is presented and compared to the phenomenology obtained in other versions of the Walecka model. We have found that the ZM3 model is too soft and predicts a very small maximum neutron star mass, $\sim0.72 M_\odot$. A strong similarity between the results of ZM-like models and those with exponential couplings is noted. Sensibility of the results to the specific choice of the values for the binding energy and saturation density is pointed out. Finally, we discuss the very intense scalar condensates found in the interior of neutron stars which may lead to negative effective masses.

nucl-th↗

arXiv1999

Compton Scattering on the Deuteron in Baryon Chiral Perturbation Theory

Compton scattering on the deuteron is studied in the framework of baryon chiral perturbation theory to third order in small momenta, for photon energies of order the pion mass. The scattering amplitude is a sum of one- and two-nucleon mechanisms with no undetermined parameters. Our results are in good agreement with existing experimental data, and a prediction is made for higher-energy data being analyzed at SAL.

nucl-th↗

arXiv1999

Strange Asymmetries in the Nucleon Sea

Relativistic corrections to the strange axial form factor are evaluated within a light-cone formalism, taking into account effects due to the breaking of Lorentz covariance associated with the use of one-body currents. Similar corrections are known to also be needed for the magnetic form factor, and we discuss the importance of these for recent data on strange electromagnetic form factors from the HAPPEX Collaboration at Jefferson Lab. The strange vector form factors, the strange axial charge, and the asymmetries in strange quark distributions are shown to be consistently correlated within the chiral cloud model.

hep-ph↗

arXiv1997

Nucleon Strange Magnetic Moment and Relativistic Covariance

We calculate the corrections to the strange matrix elements of the nucleon resulting from the breaking of rotational invariance on the light-cone. In the meson cloud model, the strange magnetic moment mu_S is seen to change sign once the spurious form factors arising from this violation are subtracted. The resulting mu_S is small and slightly positive, in agreement with the trend of the recent data from the SAMPLE experiment. The value of the strange magnetic form factor is predicted to be largely Q^2 independent over the range accessible in upcoming experiments.

hep-ph↗

arXiv1996

Strangeness in the Nucleon on the Light-Cone

Strange matrix elements of the nucleon are calculated within the light-cone formulation of the meson cloud model. The $Q^2$ dependence of the strange vector and axial vector form factors is computed, and the strangeness radius and magnetic moment extracted, both of which are found to be very small and slightly negative. Within the same framework one finds a small but non-zero excess of the antistrange distribution over the strange at large $x$. Kaon loops are unlikely, however, to be the source of a large polarized strange quark distribution.

hep-ph↗

arXiv1996

Finite Nuclei in a Relativistic Mean-Field Model with Derivative Couplings

We study finite nuclei, at the mean-field level, using the Zimanyi-Moskowski model and one of its variations (the ZM3 model). We calculate energy levels and ground-state properties in nuclei where the mean-field approach is reliable. The role played by the spin-orbit potential in sorting out mean-field model descriptions is emphasized.

nucl-th↗

arXiv1996

Phase Transition in Warm Nuclear Matter with Alternative Derivative Coupling Models

An analysis is performed of the liquid-gas phase transition of nuclear matter obtained from different versions of scalar derivate coupling suggested by Zimanyi and Moszkowski (ZM) and the results are compared with those obtained from the Walecka model. We present the phase diagram for the models and one of them, the ZM3 model, has the lowest critical temperature $T_c=13.6$ MeV with the lowest critical density $ρ_c=0.037$ f$m^{-3}$ and pressure $p_c=0.157$ MeV f$m^{-3}$. These results are in accord with recent observations from energetic heavy-ion collisions, which suggest a small liquid-gas phase region.

nucl-th↗

arXiv1996

QCD phase transitions from relativistic hadron models

The models of translationally invariant infinite nuclear matter in the relativistic mean field models are very interesting and simple, since the nucleon can connect only to a constant vector and scalar meson field. Can one connect these to the complicated phase transitions of QCD ? For an affirmative answer to this question, one must consider models where the coupling constants to the scalar and vector fields must depend on density in a non-linear way, since as such the models are not explicitly chirally invariant. Once this is ensured, indeed one can derive a quark condensate indirectly from the energy density of nuclear matter which goes to zero at large density and temperature. The change to zero condensate indicates a smooth phase transition.

hep-ph↗

arXiv1996

Nuclear Matter Properties in Derivative Coupling Models Beyond Mean - Field Approximation

The structure of infinite nuclear matter is studied with two of the Zimanyi - Moszkowski (ZM) models in the framework of a relativistic approximation which takes into account Hartree terms and beyond and is compared with the results which come out of the relativistic Hartree - Fock approach in the linear Walecka model. The simple treatment applied to these models can be used in substitution to the more complicated Dirac - Brueckner - Hartree - Fock method to perform future calculations in finite nuclei.

nucl-th↗

arXiv1996

How reliable is the mean-field Nuclear Matter description for supporting Chiral Effective Lagrangians?

The link between non-linear chiral effective Lagrangians and the Walecka model description of bulk nuclear matter [1] is questioned. This fact is by itself due to the Mean Field Approximation (MFA) which in nuclear mater makes the picture of a nucleon-nucleon interaction based on scalar(vector) meson exchange, equivalent to the description of a nuclear matter based on attractive and repulsive contact interactions. We present a linear chiral model where this link between the Walecka model and an underlying to chiral symmetry realization still holds, due to MFA.

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