513 research outputs found
Thermodynamics of Delta resonances
The thermodynamic potential of a system of pions and nucleons is computed
including the piN interactions in the P33 channel. A consistent treatment of
the width of the resonance in this channel, the Delta(1232) resonance, is
explored in detail. In the low-density limit we recover the leading term of the
virial expansion for the thermodynamic potential. An instructive diagrammatic
interpretation of the contributions to the total baryon number is presented.
Furthermore, we examine within a fireball model the consequences for the pion
spectra in heavy-ion collisions at intermediate energies, including the effect
of collective flow. A consistent treatment of the Delta width leads to a
substantial enhancement of the pion yield at low momenta.Comment: 12 pages, 3 Postscript figures, LaTeX, elsart, epsfig, minor changes,
references added, to be published in Physics Letters
Determination of the Equation of State of Dense Matter
Nuclear collisions can compress nuclear matter to densities achieved within
neutron stars and within core-collapse supernovae. These dense states of matter
exist momentarily before expanding. We analyzed the flow of matter to extract
pressures in excess of 10^34 pascals, the highest recorded under
laboratory-controlled conditions. Using these analyses, we rule out strongly
repulsive nuclear equations of state from relativistic mean field theory and
weakly repulsive equations of state with phase transitions at densities less
than three times that of stable nuclei, but not equations of state softened at
higher densities because of a transformation to quark matter.Comment: 26 pages, 6 figures; final versio
Particle production in quantum transport theories
The particle production in the intermediate energy heavy ion collisions is
discussed in the framework of the nonequilibrium Green's functions formalism.
The evolution equations of the Green's functions for fermions allows for the
discussion of the off-shell fermion propagator and of the large momentum
component in the initial state. For the case of a homogeneous system numerical
calculations of the meson production rate are performed and compared with the
semiclassical production rate.Comment: 45 pages, figures included, uses FEYNMAN macro
Constraints on the density dependence of the symmetry energy
Collisions involving 112Sn and 124Sn nuclei have been simulated with the
improved Quantum Molecular Dynamics transport model. The results of the
calculations reproduce isospin diffusion data from two different observables
and the ratios of neutron and proton spectra. By comparing these data to
calculations performed over a range of symmetry energies at saturation density
and different representations of the density dependence of the symmetry energy,
constraints on the density dependence of the symmetry energy at sub-normal
density are obtained. Results from present work are compared to constraints put
forward in other recent analysis.Comment: 8 pages, 4 figures,accepted for publication in Phy. Rev. Let
Nuclear Flow in Consistent Boltzmann Algorithm Models
We investigate the stochastic Direct Simulation Monte Carlo method (DSMC) for
numerically solving the collision-term in heavy-ion transport theories of the
Boltzmann-Uehling-Uhlenbeck (BUU) type. The first major modification we
consider is changes in the collision rates due to excluded volume and
shadowing/screening effects (Enskog theory). The second effect studied by us is
the inclusion of an additional advection term. These modifications ensure a
non-vanishing second virial and change the equation of state for the scattering
process from that of an ideal gas to that of a hard-sphere gas. We analyse the
effect of these modifications on the calculated value of directed nuclear
collective flow in heavy ion collisions, and find that the flow slightly
increases.Comment: 12 pages, REVTeX, figures available in PostScript from the authors
upon reques
Optimized Discretization of Sources Imaged in Heavy-Ion Reactions
We develop the new method of optimized discretization for imaging the
relative source from two particle correlation functions. In this method, the
source resolution depends on the relative particle separation and is adjusted
to available data and their errors. We test the method by restoring assumed pp
sources and then apply the method to pp and IMF data. In reactions below 100
MeV/nucleon, significant portions of the sources extend to large distances (r >
20 fm). The results from the imaging show the inadequacy of common Gaussian
source-parametrizations. We establish a simple relation between the height of
the pp correlation function and the source value at short distances, and
between the height and the proton freeze-out phase-space density.Comment: 36 pages (inc. 9 figures), RevTeX, uses epsf.sty. Submitted to Phys.
Rev.
Transport Model Simulations of Projectile Fragmentation Reactions at 140 MeV/nucleon
The collisions in four different reaction systems using Ca and
Ni isotope beams and a Be target have been simulated using the Heavy
Ion Phase Space Exploration and the Antisymmetrized Molecular Dynamics models.
The present study mainly focuses on the model predictions for the excitation
energies of the hot fragments and the cross sections of the final fragments
produced in these reactions. The effects of various factors influencing the
final fragment cross sections, such as the choice of the statistical decay code
and its parameters have been explored. The predicted fragment cross sections
are compared to the projectile fragmentation cross sections measured with the
A1900 mass separator. At MeV, reaction dynamics can significantly
modify the detection efficiencies for the fragments and make them different
from the efficiencies applied to the measured data reported in the previous
work. The effects of efficiency corrections on the validation of event
generator codes are discussed in the context of the two models.Comment: 28 pages, 13 figure
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