Măgurele, Romania
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Cozma M.D.,IFIN HH | Leifels Y.,Helmholtz Center for Heavy Ion Research | Trautmann W.,Helmholtz Center for Heavy Ion Research | Li Q.,Huzhou Teachers College | Russotto P.,National Institute of Nuclear Physics, Italy
Physical Review C - Nuclear Physics | Year: 2013

Neutron-proton elliptic flow difference and ratio have been shown to be promising observables in the attempt to constrain the density dependence of the symmetry energy above the saturation point from heavy-ion collision data. Their dependence on model parameters such as microscopic nucleon-nucleon cross sections, compressibility of nuclear matter, optical potential, and symmetry energy parametrization is thoroughly studied. By using a parametrization of the symmetry energy derived from the momentum-dependent Gogny force in conjunction with the Tübingen quantum molecular dynamics (QMD) model and comparing the results with the experimental FOPI-LAND data for 197Au +197Au collisions at 400 MeV/nucleon, a moderately stiff (Lsym=122±57 MeV and K sym=229±363 MeV) symmetry energy is extracted, a result that agrees with that of a similar study that employed the UrQMD transport model and a power-law parametrization of the symmetry energy. This contrasts with diverging results extracted from the FOPI π-/π+ ratio available in the literature. © 2013 American Physical Society.

Gulminelli F.,National Engineering School of Caen | Raduta Ad.R.,IFIN HH | Oertel M.,University Paris Diderot | Margueron J.,University Paris - Sud
Physical Review C - Nuclear Physics | Year: 2013

The phase diagram of a system constituted of neutrons, protons, Λ hyperons, and electrons is evaluated in the mean-field approximation in the complete three-dimensional space given by the baryon, lepton, and strange charge. It is shown that the phase diagram at subsaturation densities is strongly affected by the electromagnetic interaction, while it is almost independent of the electric charge at suprasaturation density. As a consequence, stellar matter under the condition of strangeness equilibrium is expected to experience a first as well as a second-order strangeness-driven phase transition at high density, while the liquid-gas phase transition is expected to be quenched. An RPA calculation indicates that the presence of this critical point might have sizable implications for the neutrino propagation in core-collapse supernovae. © 2013 American Physical Society.

Gulminelli F.,French National Center for Scientific Research | Gulminelli F.,National Engineering School of Caen | Raduta A.R.,IFIN HH | Oertel M.,University Paris Diderot
Physical Review C - Nuclear Physics | Year: 2012

The phase diagram of a system constituted of neutrons and Λ hyperons in thermal equilibrium is evaluated in the mean-field approximation. It is shown that this simple system exhibits a complex phase diagram with first- and second-order phase transitions. Due to the generic presence of attractive and repulsive couplings, the existence of phase transitions involving strangeness appears independent of the specific interaction model. In addition we will show under which conditions a phase transition towards strange matter at high density exists, which is expected to persist even within a complete treatment including all the different strange and nonstrange baryon states. The impact of this transition on the composition of matter in the inner core of neutron stars is discussed. © 2012 American Physical Society.

Oertel M.,University Paris Diderot | Providencia C.,University of Coimbra | Gulminelli F.,University of Caen Lower Normandy | Raduta Ad.R.,IFIN HH
Journal of Physics G: Nuclear and Particle Physics | Year: 2015

Since the discovery of neutron stars with masses around 2 M the composition of matter in the central part of these massive stars has been intensively discussed. Within this paper we will (re)investigate the question of the appearance of hyperons. To that end we will perform an extensive parameter study within relativistic mean field models. We will show that it is possible to obtain high mass neutron stars with (i) a substantial amount of hyperons, (ii) radii of 12-13 km for the canonical mass of 1.4 M, and (iii) a spinodal instability at the onset of hyperons. The results depend strongly on the interaction in the hyperon-hyperon channels, on which only very little information is available from terrestrial experiments up to now. © 2015 IOP Publishing Ltd.

Gulminelli F.,University of Caen Lower Normandy | Raduta A.R.,IFIN HH
Physical Review C - Nuclear Physics | Year: 2015

The standard variational derivation of stellar-matter structure in the Wigner-Seitz approximation is generalized to the finite-temperature situation where a wide distribution of different nuclear species can coexist in the same density and proton fraction condition, possibly out of β equilibrium. The same theoretical formalism is shown to describe on one side the single-nucleus approximation (SNA), currently used in most core-collapse supernova simulations and on the other side the nuclear statistical equilibrium (NSE) approach, routinely employed in r- and p-process explosive nucleosynthesis problems. In particular, we show that in-medium effects have to be accounted for in NSE to have a theoretical consistency between the zero-temperature and the finite-temperature modeling. The bulk part of these in-medium effects is analytically calculated in the local density approximation and shown to be different from a Van der Waals excluded-volume term. This unified formalism allows controlling quantitatively the deviations from the SNA in the different thermodynamic conditions, as well as having a NSE model which is reliable at any arbitrarily low value of the temperature, with potential applications for neutron-star cooling and accretion problems. We present different illustrative results with several mass models and effective interactions, showing the importance of accounting for the nuclear species distribution even at temperatures lower than 1 MeV. © 2015 American Physical Society.

Cozma M.D.,IFIN HH
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2011

We employ an isospin dependent version of the QMD transport model to study the influence of the isospin dependent part of the nuclear matter equation of state and in-medium nucleon-nucleon cross-sections on the dynamics of heavy-ion collisions at intermediate energies. We find that the extraction of useful information on the isospin-dependent part of the equation of state of nuclear matter from proton or neutron elliptic flows is obstructed by their sensitivity to model parameters and in-medium values of nucleon-nucleon cross-sections. Opposite to that, neutron-proton elliptic flow difference shows little dependence on those variables while its dependence on the isospin asymmetric EoS is enhanced, making it more suitable for a model independent constraining of the high-density behaviour of asy-EoS. Comparison with existing experimental FOPI-LAND neutron-hydrogen data can be used to set an upper limit to the softness of asy-EoS. Successful constraining of the asy-EoS via neutron-proton elliptic flow difference will require experimental data of higher accuracy than presently available. © 2011 Elsevier B.V.

The statistical mechanics of one- and two-dimensional Ginzburg-Landau systems is evaluated analytically, via the transfer matrix method, using an expression of the ground state energy of the quartic anharmonic oscillator in an external field. In the two-dimensional case, the critical temperature of the order-disorder phase transition is expressed as a Lambert function of the inverse inter-chain coupling constant. © 2010 2011 Taylor & Francis.

Barsan V.,IFIN HH
Philosophical Magazine | Year: 2014

The eigenvalue equations for the energy of bound states of a particle in a square well are solved, and the exact solutions are obtained, as power series. Accurate analytical approximate solutions are also given. The application of these results in the physics of quantum wells are discussed, especially for ultra-thin metallic films, but also in the case of resonant cavities, heterojunction lasers, revivals and super-revivals. © 2013 Taylor & Francis.

Cozma M.D.,IFIN HH
EPJ Web of Conferences | Year: 2016

An upgraded version of the isospin dependent Tubingen QMD transport model, which allows the conservation of the total energy, is presented. This is achieved by including in the energy-balance equations of the density, isospin asymmetry and momentum dependent inmedium baryon potential energies. It leads to an effective modification of particle production thresholds with respect to the vacuum ones. Compatible constraints for the symmetry energy stiffness from π-/π+ multiplicity ratio and elliptic flow experimental data of Au+Au collisions at 400 MeV/nucleon can be extracted in this case. However, an important dependence of the π-/π+ observable on the strength of the isovector part of the Δ(1232) isobar potential is also demonstrated. The present lack of information on this quantity prevents a precise extraction of the value for the symmetry energy stiffness employing the mentioned observable alone. © Owned by the authors, published by EDP Sciences-SIF, 2016.

Cozma M.D.,IFIN HH
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2016

The charged pion multiplicity ratio in intermediate energy central heavy-ion collisions has been proposed as a suitable observable to constrain the high density dependence of the isovector part of the equation of state. A comparison of various transport model predictions with existing experimental data has led, however, to contradictory results. Using an upgraded version of the Tübingen QMD transport model, which allows the conservation of energy at a local or global level by accounting for the potential energy of hadrons in two-body collisions and leading thus to particle production threshold shifts, we demonstrate that compatible constraints for the symmetry energy stiffness can be extracted from pion multiplicity and elliptic flow observables. However, pion multiplicities and ratios are proven to be highly sensitive to the yet unknown isovector part of the in-medium δ(1232) potential which hinders, at present, the extraction of meaningful information on the high density dependence of the symmetry energy. A solution to this problem together with the inclusion of contributions presently neglected, such as in-medium pion potentials and retardation effects, are needed for a final verdict on this topic. © 2015.

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