arXiv subjects

H. H. Wolter

Publications and source records attributed to H. H. Wolter.

At least 55 records · Page 3Linked to original sources

arXiv2004

Isospin effects in intermediate energy heavy ion collisions

We investigate the density dependence of the symmetry energy in a relativistic description by decomposing the iso-vector mean field into contributions with different Lorentz properties. We find important effects of the iso-vector, scalar $δ$ channel on the density behavior of the symmetry energy. Finite nuclei studies show only moderate effects originating from the virtual $δ$ meson. In heavy ion collisions from Fermi to relativistic energies up to $1-2 AGeV$ one finds important contributions on the dynamics arising from the different treatment of the microscopic Lorentz structure of the symmetry energy. We discuss a variety of possible signals which could set constraints on the still unknown density dependence of the symmetry energy, when experimental data will be available. Examples of such observables are isospin collective flow, threshold production of pions and kaons, isospin equilibration and stopping in asymmetric systems like $Au+Au$, $Sn+Sn$ and $Ru(Zr)+Zr(Ru)$.

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arXiv2003

On the Lorentz structure of the symmetry energy

We investigate in detail the density dependence of the symmetry energy in a relativistic description by decomposing the iso-vector mean field into contributions with different Lorentz covariant properties. We find important effects of the iso-vector, scalar channel (i.e. $δ$-meson like) on the high density behavior of the symmetry energy. Applications to static properties of finite nuclei and to dynamic situations of heavy ion collisions are explored and related to each other. The nuclear structure studies show only moderate effects originating from the virtual $δ$ meson. At variance, in heavy ion collisions one finds important contributions on the reaction dynamics arising from the different Lorentz structure of the high density symmetry energy when a scalar iso-vector $δ$ field is introduced. Particularly interesting is the related neutron/proton effective mass splitting for nucleon transport effects and for resonance and particle production around the threshold. We show that the $δ$-like channel turns out to be essential for the production of pions, when comparing with experimental data, in particular for high momentum selections.

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arXiv2003

Relativistic effects in the search for high density symmetry energy

Intermediate energy heavy ion collisions open the unique possibility to explore the Equation of State ($EOS$) of nuclear matter far from saturation, in particular the density dependence of the symmetry energy. Within a relativistic transport model it is shown that the isovector-scalar $δ$-meson, which affects the high density behavior of the symmetry energy density, influences the dynamics of heavy ion collisions in terms of isospin collective flows. The effect is largely enhanced by a relativistic mechanism related to the covariant nature of the fields contributing to the isovector channel. Results for reactions induced by $^{132}Sn$ radioactive beams are presented. The elliptic flows of nucleons and light isobars appear to be quite sensitive to microscopic structure of the symmetry term, in particular for particles with large transverse momenta, since they represent an earlier emission from a compressed source. Thus future, more exclusive, experiments with relativistic radioactive beams should be able to set stringent constraints on the density dependence of the symmetry energy far from ground state nuclear matter.

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arXiv2002

Non-Equilibrium and Collective Flow Effects in Relativistic Heavy Ion Collisions

We discuss the problems involved in extracting the nuclear equation-of-state from heavy-ion collisions. We demonstrate that the equation of state becomes effectively softer in non-equilibrium and this effect is observable in terms of collective flow effects. Thus, non-equilibrium effects must be included in transport descriptions on the level of the effective mean fields. A comparison with transverse momentum, rapidity, and centrality selected flow data show the reliability and limitations of the underlying interaction which was derived from microscopic Dirac-Brueckner (DB) results.

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arXiv2002

Relativistic Mean Field Model with Generalized Derivative Nucleon-Meson Couplings

The quantum hadrodynamics (QHD) model with minimal nucleon-meson couplings is generalized by introducing couplings of mesons to derivatives of the nucleon field in the Lagrangian density. This approach allows an effective description of a state-dependent in-medium interaction in the mean-field approximation. Various parametrizations for the generalized couplings are developed and applied to infinite nuclear matter. In this approach, scalar and vector self-energies depend on both density and momentum similarly as in the Dirac-Brueckner theory. The Schrödiger-equivalent optical potential is much less repulsive at high nucleon energies as compared to standard relativistic mean field models and thus agrees better with experimental findings. The derivative couplings in the extended model have significant effects on properties of symmetric nuclear matter and neutron matter.

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arXiv2002

Isotope yields from central 112,124Sn+112,124Sn collisions, dynamical emission?

Isotopic yields for light particles and intermediate mass fragments have been measured for 112Sn+112Sn, 112Sn+124Sn, 124Sn+112Sn and 124Sn+124Sn central collisions at E/A=50 MeV and compared with predictions of stochastic mean field calculations. These calculations predict a sensitivity of the isotopic distributions to the density dependence of the asymmetry term of the nuclear equation of state. However, the secondary decay of the excited fragments modifies significantly the primary isotopic distributions and these modifications are rather sensitive to theoretical uncertainties in the excitation energies of the hot fragments. The predicted final isotope distributions are narrower than the experimental data and the sensitivity of the predicted yields to the density dependence of the asymmetry term is reduced.

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arXiv2002

Equal-time hierarchies for transport descriptions of fermionic fields

A transport theory which is not restricted to the gradient and quasi-particle approximations is presented which is formulated in terms of the energy moments, or equivalently the equal-time derivatives of the one-particle Green functions. A set of infinite hierarchies of kinetic and constraint equations for equal-time quantities for the spectral and the kinetic part of the one-particle Green function are derived. The hierarchies for the spectral function truncate automatically as in the mean field approximation. The possibility of a systematic truncation of the hierarchies for the kinetic part is discussed. The effects the quantum corrections are illustrated in a simple one-dimensional model.

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arXiv2001

Testing Dirac-Brueckner models in collective flow of heavy-ion collisions

We investigate differential in-plane and out-of-plane flow observables in heavy ion reactions at intermediate energies from $0.2÷2$ AGeV within the framework of relativistic BUU transport calculations. The mean field is based on microscopic Dirac-Brueckner-Hartree-Fock (DBHF) calculations. We apply two different sets of DBHF predictions, those of ter Haar and Malfliet and more recent ones from the Tübingen group, which are similar in general but differ in details. The latter DBHF calculations exclude spurious contributions from the negative energy sector to the mean field which results in a slightly softer equation of state and a less repulsive momentum dependence of the nucleon-nucleus potential at high densities and high momenta. For the application to heavy ion collisions in both cases non-equilibrium features of the phase space are taken into account on the level of the effective interaction. The systematic comparison to experimental data favours the less repulsive and softer model. Relative to non-relativistic approaches one obtains larger values of the effective nucleon mass. This produces a sufficient amount of repulsion to describe the differential flow data reasonably well.

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arXiv2001

Isospin effects in Nuclear Fragmentation

We investigate properties of the symmetry term in the equation-of-state (EOS) of nuclear matter (NM) from the analysis of simulations of fragmentation events in intermediate energy heavy ion collisions. For charge asymmetric systems a qualitative new feature in the liquid-gas phase transition is predicted: the onset of chemical instabilities with a mixture of isoscalar and isovector components. This leads to a separation into a higher density (``liquid'') symmetric and a low density (``gas'') neutron-rich phase, the so-called neutron distillation effect. We analyse the simulations with respect to the time evolution of the isospin dynamics, as well as with respect to the distribution and asymmetry of the final primary fragments. Qualitatively different effects arise in central collisions, with bulk fragmentation, and peripheral collisions with neck-fragmentation. The neck fragments produced in this type of process appear systematically more neutron-rich from a dynamical nucleon migration effect which is very sensitive to the symmetry term in regions just below normal density. In general the isospin dynamics plays an important role in all the steps of the reaction, from prompt nucleon emission to the sequential decay of the primary fragments. A fully microscopic description of the reaction dynamics including stochastic elements to treat fluctuations realistically is absolutely necessary in order to extract precise information on the fragmentation and the nuclear equation of state. We have performed simulations for fragment production events in $n$-rich ($^{124}Sn$) and $n$-poor ($^{112}Sn$) symmetric colliding systems.

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arXiv2000

Asymmetry Effects on Nuclear Fragmentation

We show the possibility of extracting important information on the symmetry term of the Equation of State ($EOS$) directly from multifragmentation reactions using stable isotopes with different charge asymmetries. We study n-rich and n-poor $Sn + Sn$ collisions at $50AMeV$ using a new stochastic transport approach with all isospin effects suitably accounted for. For central collisions a chemical component in the spinodal instabilities is clearly seen. This effect is reduced in the neck fragmentation observed for semiperipheral collisions, pointing to a different nature of the instability. In spite of the low asymmetry tested with stable isotopes the results are showing an interesting and promising dependence on the stiffness of the symmetry term, with an indication towards an increase of the repulsion above normal density.

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arXiv2000

What can we learn from Nuclear Matter Instabilities?

We discuss the features of instabilities in binary systems, in particular, for asymmetric nuclear matter. We show its relevance for the interpretation of results obtained in experiments and in "ab initio" simulations of the reaction between $^{124}Sn+^{124}Sn$ at 50AMeV.}

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arXiv2000

Can we determine the nuclear equation of state from heavy ion collisions?

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arXiv2000

Thermodynamical description of heavy ion collisions

We analyze the thermodynamical state of nuclear matter in transport descriptions of heavy ion reactions. We determine thermodynamical variables from an analysis of local momentum space distributions and compare to blast model parameters from an analysis of fragment energy spectra. These descriptions are applied to spectator and fireball matter in semi-central and central Au+Au collisions at SIS-energies, respectively.

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arXiv2000

The Past and Future of Coulomb Dissociation in Hadron- and Astrophysics

Breakup reactions are generally quite complicated, they involve nuclear and electromagnetic forces including interference effects. Coulomb dissociation is an especially simple and important mechanism since the perturbation due to the electric field of the nucleus is exactly known. Therefore firm conclusions can be drawn from such measurements. Electromagnetic matrixelements, radiative capture cross-sections and astrophysical S-factors can be extracted from experiments. We describe the basic theory, give analytical results for higher order effects in the dissociation of neutron halo nuclei and briefly review the experimental results obtained up to now. Some new applications of Coulomb dissociation for nuclear astrophysics and nuclear structure physics are discussed.

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arXiv2000

Fragment Formation and Phase Transitions in Heavy Ion Collisions

We investigate the thermodynamical properties of nuclear matter in heavy ion collisions, in particular with respect to questions of thermodynamical instability, phase transitions and fragmentation. For this we analyze results of relativistic transport calculations of Au+Au collisons at intermediate energies, separately for spectator and participant matter. On one hand, we determine local thermodynamical variables from the analysis of the local momentum distribution; on the other, we analyze fragment energy spectra in a blast model scenario. We find that the spectator represents an instable, equilibrized fragmenting source, while in the participant no such common source can be identified. Our results compare well with experimental determinations of temperatures and flow velocities.

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arXiv2000

Spectator and participant decay in heavy ion collisions

We analyze the thermodynamical state of nuclear matter in transport calculations of heavy--ion reactions. In particular we determine temperatures and radial flow parameters from an analysis of fragment energy spectra and compare to local microscopic temperatures obtained from an analysis of local momentum space distributions. The analysis shows that the spectator reaches an equilibrated freeze-out configuration which undergoes simultaneous fragmentation. The fragments from the participant region, on the other hand, do not seem to come from a common fragmenting source, but rather heavier fragments seem to be formed earlier.

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arXiv2000

Mechanisms for Direct Breakup Reactions

We review some simple mechanisms of breakup in nuclear reactions. We mention the spectator breakup, which is described in the post-form DWBA. The relation to other formulations is also indicated. An especially important mechanism is Coulomb dissociation. It is a distinct advantage that the perturbation due to the electric field of the nucleus is exactly known. Therefore firm conclusions can be drawn from such measurements. Some new applications of Coulomb dissociation for nuclear astrophysics are discussed.

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arXiv1999

Thermodynamical characterization of heavy ion reactions

An detailed study of the thermodynamical state of nuclear matter in transport calculations of heavy--ion reactions is presented. In particular we determine temperatures from an analysis of the local momentum space distribution on one hand, and from a fit to fragment energy spectra in terms of a blast model with radial flow and temperature on the other. We apply this to spectator and participant matter. In spectator we find regions of spinodal instability with temperatures and densities which are consistent with experiments. In the participant we find different temperatures for different fragment masses, indicating that the fragments are not emitted from a source in thermal and chemical equilibrium.

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