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L. Mornas

Publications and source records attributed to L. Mornas.

15 recordsLinked to original sources
arXiv2004

Neutrino scattering rates in the presence of hyperons from a Skyrme model in the RPA approximation

The contribution of Lambda hyperons to neutrino scattering rates is calculated in the random phase approximation in a model where the interaction is described by a Skyrme potential. Finite temperature and neutrino trapping are taken into account in view of applications to the deleptonization stage of protoneutron star cooling. The hyperons can remove the problem of ferromagnetic instability common to (nearly) all Skyrme parametrizations of the nucleon-nucleon interaction. As a consequence, there is not any longer a pole at the transition in the neutrino-baryon cross section. However there still remains an enhancement in this region. In the absence of ferromagnetism the mean free path in npLambda matter is reduced compared to its value in np matter as consequence of the presence of this additional degree of freedom. At high density the results are very sensitive to the choice of the Lambda-Lambda interaction.

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arXiv2004

Neutron stars in a Skyrme model with hyperons

Available Skyrme parametrizations with hyperons are examined from the point of view of their suitability for applications to neutron stars. It is shown that the hyperons can attenuate or even remove the problem of ferromagnetic instability common to (nearly) all Skyrme parametrizations of the nucleon-nucleon interaction. At high density the results are very sensitive to the choice of the Lambda-Lambda interaction. The selected parameter sets are then used to obtain the resulting properties of both cold neutron stars and hot protoneutron stars. The general features known from other models are recovered.

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arXiv2002

Hyperon ordering in neutron star matter

We explore the possible formation of ordered phases in neutron star matter. In the framework of a quantum hadrodynamics model where neutrons, protons and Lambda hyperons interact via the exchange of mesons, we compare the energy of the usually assumed uniform, liquid phase, to that of a configuration in which di-lambda pairs immersed in an uniform nucleon fluid are localized on the nodes of a regular lattice. The confining potential is calculated self-consistently as resulting from the combined action of the nucleon fluid and the other hyperons, under the condition of beta equilibrium. We are able to obtain stable ordered phases for some reasonable sets of values of the model parameters. This could have important consequences on the structure and cooling of neutron stars.

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arXiv2001

Propagation of neutral mesons in asymmetric nuclear matter

We calculate dispersion relations and propagators for the $σ$, $ω$, $ρ$ and $δ$ mesons in relativistic nuclear matter with a proton-neutron asymmetry. In addition to the $σ$-$ω$ and $δ$-$ρ$ mixings already present in symmetric matter, mixing occur between all components of the $σ$-$ω$-$δ$-$ρ$ system when the proton fraction differs from 1/2.

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arXiv2001

Dispersion relations of mesons in symmetric nuclear matter

We calculate dispersion relations and propagators for the $σ$, $ω$, $π$, $ρ$, $δ$, $η$ and $a_1$ mesons in relativistic, dense, hot, symmetric nuclear matter. In addition to the usual mixing of the $σ-ω$ system, we obtain mixing of the $δ$ with the longitudinal $ρ$ mode and of the tranverse $ρ$ with the transverse $a_1$ mode. Finally, the component of the $a_1$ polarization along the transferred momentum modifies the in-medium pion propagator in a way similar to the Migdal contact interaction, but with the opposite sign. The spurious pion condensate as well as the additional contribution from the $a_1$ meson are removed by a contact term. We compare two ways of implementing contact term subtraction.

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arXiv2001

Neutrino-nucleon scattering rate in proto neutron star matter

We present a calculation of the neutrino-nucleon scattering cross section which takes into account the nuclear correlations in the relativistic random phase approximation. Our approach is based on a quantum hadrodynamics model with exchange of $σ$, $ω$, $π$, $ρ$ and $δ$ mesons. In view of applications to neutrino transport in the final stages of supernova explosion and protoneutron star cooling, we study the evolution of the neutrino mean free path as a function of density, proton-neutron asymmetry and temperature. Special attention was paid to the issues of renormalization of the Dirac sea, residual interactions in the tensor channel and meson mixing. It is found that RPA corrections, with respect to the mean field approximation, amount to only 10% to 15% at high density.

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arXiv2001

Formation of an ordered phase in neutron star matter

In this work, we explore the possible formation of ordered phases in hadronic matter, related to the presence of hyperons at high densities. We analyze a microscopic mechanism which can lead to the crystallization of the hyperonic sector by the confinement of the hyperons on the nodes of a lattice. For this purpose, we introduce a simplified model of the hadronic plasma, in which the nuclear interaction between protons, neutrons and hyperons is mediated by meson fields. We find that, for some reasonable sets of values of the model parameters, such ordered phases are energetically favoured as density increases beyond a threshold value.

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arXiv2001

Hyperonic crystallization in hadronic matter

Published in Hadrons, Nuclei and Applications, World Scientific, Singapore, Proc.of the Conference Bologna2000. Structure of the Nucleus at the Dawn of the Century, G. Bonsignori, M. Bruno, A. Ventura, D. Vretenar Editors, pag. 319.

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arXiv2001

Nucleon-nucleon effective potential in dense matter including rho-meson exchange

We obtain the RPA summed one-meson exchange potential between nucleons in symmetric nuclear matter at zero temperature, from a model which includes $ρ$, $σ$, $ω$ and $π$ mesons. The behavior of rho mesons inside the medium is first discussed using different schemes to extract a finite contribution from the vacuum polarization. These schemes give qualitatively different results for the in-medium rho mass. The results are discussed in connection with the non-renormalizability of the model. We next study the modified potential as density increases. In the intermediate distance range, it is qualitatively modified by matter and vacuum effects. In the long-distance range ($r>2$ fm), one observes the presence of oscillations, which are not present in free-space. Features on this distance range are insensitive to the renormalization scheme.

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arXiv1999

Relativistic transport equations with generalized mass shell constraints

We reexamine the derivation of relativistic transport equations for fermions when conserving the most general spinor structure of the interaction and Green function. Such an extension of the formalism is needed when dealing with {\it e.g.} spin-polarized nuclear matter or non-parity conserving interactions. It is shown that some earlier derivations can lead to an incomplete description of the evolution of the system even in the case of parity-conserving, spin-saturated systems. The concepts of kinetic equation and mass shell condition have to be extended, in particular both of them acquire a non trivial spinor structure which describe a rich polarization dynamics.

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arXiv1998

Enhancement of nucleon-nucleon cross section in sidewards flowing nuclear matter through zero sound mode excitation

We investigate the modification induced on the nucleon-nucleon cross section by screening of the interaction inside nuclear matter. The interaction is described by the relativistic one boson exchange of $σ$, $ω$ and $π$ mesons. Medium effects enter as loop corrections to the meson propagators and are characterized not only by density, but also by temperature and velocity with respect to the center of mass of the collision. The cross section displays peaks for some specific values of the velocity, corresponding to the excitation of zero-sound modes of the longitudinal $ω$ field. The enhancement factor amounts to about 1.5 under reasonable conditions. It increases with density and is reduced at finite temperature. These findings may have verifiable consequences on the angular dependence of the measurable secondary particle distributions.

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arXiv1997

Screening effects on the elastic nucleon-nucleon cross section in relativistic nuclear matter

We investigate the screening effects on the nucleon-nucleon elastic cross section inside nuclear matter at zero and finite temperature. The N-N interaction is described phenomenologically via meson exchanges ($σ$, $π$ and $ω$) in the framework of a relativistic lagrangian model. The expressions for the in-medium meson propagators, which take into account the effects of matter polarization and renormalized vacuum polarization, are used in the calculation of the various meson contributions to the total cross section in the one-boson exchange approximation. The final expressions allow a satisfactory fit of the Coulomb substracted proton-proton cross section data in vacuum. At finite density or temperature the cross section is reduced as compared to the vacuum values and compatible with the existing experimental constraints. At high density the $p$-$p$ differential cross section gets increasingly forward and backward peaked. The $σ$-$ω$ mixing plays a key role in the understanding of the results.

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arXiv1994

Coherent Bremsstrahlung Emission in the Early Stage of Relativistic Heavy Ion Collisions

We present a model of coherent Bremsstrahlung emission from the compression phase of relativistic heavy ion collisions based on a hydrodynamical description of the collision process. The decceleration of the matter through a shock front, calculated in 1-D relativistic dissipative hydrodynamics, is at the origin of a copious Bremsstrahlung emission. The interferences between the Bremsstrahlung emitted by two receeding shock fronts result in deviations from the usual $1/ω$ law, with in particular a strong suppression of low energy photons in symmetric collisions. We show how it might be possible to obtain information on the equation of state, possible transition to a quark gluon plasma and dynamical properties of the matter from the study of the interference pattern. We show that the optimal region for the observation of coherent photons is in SIS - AGS energy range.

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arXiv1994

On the Role of Dissipation in the Early Stages of Relativistic Heavy Ion Collisions

The influence of the dissipative terms on the conditions of formation and the characteristic parameters of shock waves in relativistic nuclear collisions is investigated for three types of equation of state (non linear QHD-1, resonance gas and lattice QCD). Energy and velocity profiles are obtained in a one-dimensional model; the duration of the shock phase and width of the shock front are calculated. It is shown that the presence of a phase transition results in a strong enhancement of the width of the shock front, which results in an increase of transparency. This effect, combined with the fact that the nuclei have a finite size, prevents the energy density to rise to its maximum value (full stopping) as would be predicted by a non dissipative shock model.

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