26 research outputs found
Kaon squeeze-out in heavy ion reactions
The squeeze-out phenomenon of and mesons, i.e. the azimuthal
asymmetry of and mesons emitted at midrapidity in heavy ion
reactions, is investigated for beam energies of 1-2 A.GeV. It is found that the
squeeze-out signal is strongly affected by in-medium potentials of these
mesons. The repulsive -nucleus potential gives rise to a pronounced
out-of-plane emission of 's at midrapidity. With the potential we
reproduce well the experimental data of the azimuthal distribution. It is
found that the attractive -nucleus potential cancels to a large extent the
influence of rescattering and reabsorption of the mesons on the
projectile and target residuals (i.e. shadowing). This results in an
azimuthally isotropic emission of the midrapidity mesons with transverse
momentum up to 0.8 GeV/c. Since it is well accepted that the shadowing alone
would lead to a significant out-of-plane preference of particle emission, in
particular at high transverse momenta, the disappearance of the out-of-plane
preference for the mesons can serve as an unambiguous signal of the
attractive potential. We also apply a covariant formalism of the kaon
dynamics to the squeeze-out phenomenon. Discrepancies between the theory and
the experiments and possible solutions are discussed.Comment: 24 pages Latex using Elsevier style, 7 PS figures, accepted for
publication in Euro. Phys. Jour.
Nuclear stopping and flow in heavy ion collisions and the in-medium NN cross section
We present transport calculations for heavy ion reactions in which the mean
field and the in-medium nucleon-nucleon cross section are consistently based on
the same effective interaction, i.e. the in-medium T-matrix from microscopic
Dirac-Brueckner calculations. Doing so, the stopping in central reactions in
terms of the recently proposed observable and the correlation to
the behavior of the directed flow is investigated. The relation to the nuclear
shear viscosity is discussed.Comment: 9 pages, 4 figure
Asymmetric nuclear matter : a variational approach
We discuss here a self-consistent method to calculate the properties of the
cold asymmetric nuclear matter. In this model, the nuclear matter is dressed
with s-wave pion pairs and the nucleon-nucleon (N-N) interaction is mediated by
these pion pairs, and mesons. The parameters of these
interactions are calculated self-consistently to obtain the saturation
properties like equilibrium binding energy, pressure, compressibility and
symmetry energy. The computed equation of state is then used in the Tolman-
Oppenheimer-Volkoff (TOV) equation to study the mass and radius of a neutron
star in the pure neutron matter limit.Comment: 10 pages, 5 figures and 1 tabl
Asymmetric Colliding Nuclear Matter Approach in Heavy Ion Collisions
The early stage of a heavy ion collision is governed by local non-equilibrium
momentum distributions which have been approximated by colliding nuclear matter
configurations, i.e. by two Lorentz elongated Fermi ellipsoids. This approach
has been extended from the previous assumption of symmetric systems to
asymmetric 2-Fermi sphere configurations, i.e. to different densities. This
provides a smoother transition from the limiting situation of two
interpenetrating currents to an equilibrated system. The model is applied to
the dynamical situations of heavy ion collisions at intermediate energies
within the framework of relativistic transport (RBUU) calculations. We find
that the extended colliding nuclear matter approach is more appropriate to
describe collective reaction dynamics in terms of flow observables, in
particular, for the elliptic flow at low energies.Comment: 21 pages, 8 figures, accepted for publication in Nuclear Physics
Covariant representations of the relativistic Brueckner T-matrix and the nuclear matter problem
We investigate nuclear matter properties in the relativistic Brueckner
approach. The in-medium on-shell T-matrix is represented covariantly by five
Lorentz invariant amplitudes from which we deduce directly the nucleon
self-energy. We discuss the ambiguities of this approach and the failure of
previously used covariant representations in reproducing the nucleon
self-energies on the Hartree-Fock level. To enforce correct Hartree-Fock
results we develop a subtraction scheme which treats the bare nucleon-nucleon
potential exactly in accordance to the different types of meson exchanges. For
the remaining ladder kernel, which contains the higher order correlations, we
employ then two different covariant representations in order to study the
uncertainty inherent in the approach. The nuclear matter bulk properties are
only slightly sensitive on the explicit representation used for the kernel.
However, we obtain new Coester lines for the various Bonn potentials which are
shifted towards the empirical region of saturation. In addition the nuclear
equation-of-state turns out to be significantly softer in the new approach.Comment: 39 pages Latex using Elsevier style, 16 PS figure
Aspects of particle production in isospin asymmetric matter
The production/absorption rate of particles in compressed and heated
asymmetric matter is studied using a Relativistic Mean Field (RMF) transport
model with an isospin dependent collision term. Just from energy conservation
in the elementary production/absorption processes we expect to see a strong
dependence of the yields on the basic Lorentz structure of the isovector
effective interaction, due to isospin effects on the scalar and vector
self-energies of the hadrons. This will be particularly evident for the ratio
of the rates of particles produced with different charges: results are shown
for \pi(+)/\pi(-), K(+)/K(0) yields. In order to simplify the analysis we
perform RMF cascade simulations in a box with periodic boundary conditions. In
this way we can better pin down all such fine relativistic effects in particle
production, that could likely show up even in realistic heavy ion collisions.
In particular, K(+,0) production is expected to be directly related to the high
density behaviour of the symmetry energy, since kaons are produced very early
during the high density stage of the collision and their mean free path is
rather large. We show that the K(+)/K(0) ratio reflects important isospin
contributions on the production rates just because of the large sensitivity
around the threshold. The results are very promising for the possibility of a
direct link between particle production data in exotic Heavy Ion Collisions and
the isospin dependent part of the Equation of State at high baryon densities.Comment: 26 pages, 8 figures; Nucl.Phys. A, accepte
Consequences of kinetic non-equilibrium for the nuclear equation-of-state in heavy ion collision
Highly compressed nuclear matter created in relativistic heavy collisions is
to large extent governed by local non-equilibrium. As an idealized scenario
colliding nuclear matter configurations are studied within both, relativistic
mean field theory and using more realistic in-medium interactions based on the
Dirac-Brueckner T-matrix. The equation of state in anisotropic matter is
thereby governed by two competing effects: The enlarged phase space volume in
colliding matter tends to soften the internal potential energy of the
subsystems whereas the relative motion of the two currents leads to a strong
additional repulsion in the system. An effective EOS constructed for
anisotropic momentum configurations shows a significant net softening compared
to ground state nuclear matter. This effect is found to be to large extend
independent on the particular choice of the nuclear interaction. A critical
discussion of standard transport approaches with respect to the considered
non-equilibrium effects is given.Comment: 41 pages, 13 figures, to appear in Nucl. Phys.
Lambda collective flow in heavy ion reactions
Collective flow of Lambda hyperons in heavy ion reactions at SIS energies is
investigated. It is found that a mean field constructed on the basis
of the quark model leads to a good description of the experimental data of the
in-plane transverse flow of 's. The attractive mean field can also
give rise to an additional "virtual" radial flow directed inwards,
which is reflected by a "concave" structure of the transverse mass spectrum of
the hyperons emitted at midrapidity. The radial flow is
found to exhibit a strong mass dependence: The flow is visible in the Ni+Ni
system, but is strongly reduced in the system of Au on Au.Comment: 18 pages LeTex, using Elsevier style, 6 PS-figures, accepted for
publication in Nuclear Physics
Fragment Formation in Central Heavy Ion Collisions at Relativistic Energies
We perform a systematic study of the fragmentation path of excited nuclear
matter in central heavy ion collisions at the intermediate energy of . The theoretical calculations are based on a Relativistic
Boltzmann-Uehling-Uhlenbeck () transport equation including stochastic
effects. A Relativistic Mean Field () approach is used, based on a
non-linear Lagrangian, with coupling constants tuned to reproduce the high
density results of calculations with correlations.
At variance with the case at Fermi energies, a new fast clusterization
mechanism is revealed in the early compression stage of the reaction dynamics.
Fragments appear directly produced from phase-space fluctuations due to
two-body correlations. In-medium effects of the elastic nucleon-nucleon cross
sections on the fragmentation dynamics are particularly discussed. The
subsequent evolution of the primordial clusters is treated using a simple
phenomenological phase space coalescence algorithm.
The reliability of the approach, formation and recognition, is investigated
in detail by comparing fragment momentum space distributions {\it and
simultaneously} their yields with recent experimental data of the
collaboration by varying the system size of the colliding system, i.e. its
compressional energy (pressure, radial flow). We find an excellent agreement
between theory and experiment in almost all the cases and, on the other hand,
some limitations of the simple coalescence model. Furthermore, the temporal
evolution of the fragment structure is explored with a clear evidence of an
earlier formation of the heavier clusters, that will appear as interesting
of the high density phase of the nuclear Equation of State ().Comment: 21 pages, 8 figures, Latex Elsart Style, minor corrections in p.7,
two refs. added, Nucl.Phys.A, accepte