27 research outputs found
Delta degrees of freedom in antisymmetrized molecular dynamics and (p,p') reactions in the delta region
Delta degrees of freedom are introduced into antisymmetrized molecular
dynamics (AMD). This is done by increasing the number of basic states in the
AMD wave function, introducing a Skyrme-type delta-nucleon potential, and
including reactions in the collision description.
As a test of the delta dynamics, the extended AMD is applied to (p,p)
recations at MeV for a C target. It is found that the
ratio and the absolute values for delta peak and quasielastic peak (QEP) in the
C(p,p) reaction are reproduced for angles \Theta_{\rm lab} \agt
40^\circ, pointing to a correct treatment of the delta dynamics in the
extended AMD. For forward angles the QEP is overestimated. The results of the
AMD calculations are compared to one-step Monte Carlo (OSMC) calculations and a
detailed analysis of multi-step and delta potential effects is given. As
important side results we present a way to apply a Gallilei invariant theory
for (N,N) reactions up to MeV which ensures
approximate Lorentz invariance and we discuss how to fix the width parameter
of the single particle momentum distribution for outgoing nucleons in the
AMD calculation.Comment: 28 pages, revtex, 12 figures included, figures are also available
upon request as postscript files from the authors (e-mail:
[email protected]), submitted to Phys. Rev.
Role of isospin dependent mean field in pion production in heavy ion reactions
The importance of a isospin dependent nuclear mean field (IDMF) in regard to
the pion production mechanism is studied for the reaction at 1
GeV/nucleon using the Quantum Molecular Dynamics (QMD) model. In particular,
the effect of the IDMF on pion spectra and the charged pion ratio are analyzed.
It is found that the inclusion of a IDMF considerably suppresses the low
pions, thus, leading to a better agreement with the data on pion spectra.
Moreover, the rapidity distribution of the charged pion ratio appears to be
sensitive to the isospin dependence of the nuclear mean field.Comment: 16 pages, using RevTex, 6 PS-Figure
Medium Effects on Binary Collisions with the Delta Resonance
To facilitate the relativistic heavy-ion calculations based on transport
equations, the binary collisions involving a resonance in either the
entrance channel or the exit channel are investigated within a Hamiltonian
formulation of interactions. An averaging procedure is developed to
define a quasi-particle and to express the experimentally measured
cross section in terms of an effective cross section. In contrast to previous works, the main feature of
the present approach is that the mass and the momentum of the produced
's are calculated dynamically from the bare vertex interaction of the model Hamiltonian and are constrained by the
unitarity condition. The procedure is then extended to define the effective
cross sections for the experimentally inaccessible and reactions. The predicted cross
sections are significantly different from what are commonly assumed in
relativistic heavy-ion calculations. The potential in nuclear matter
has been calculated by using a Bruckner-Hartree-Fock approximation. By
including the mean-field effects on the propagation, the effective
cross sections of the , and reactions in nuclear matter are
predicted. It is demonstrated that the density dependence is most dramatic in
the energy region close to the pion production threshold.Comment: 20 pages, RevTe
Formation of 24Mg* in the Splitting of 28Si Nuclei by 1-GeV Protons
The 28Si(p, p' gamma)24Mg reaction has been studied at the ITEP accelerator
by the hadron-gamma coincidence method for a proton energy of 1 GeV. Two
reaction products are detected: a 1368.6-keV gamma-ray photon accompanying the
transition of the 24Mg* nucleus from the first excited state to the ground
state and a proton p' whose momentum is measured in a magnetic spectrometer.
The measured distribution in the energy lost by the proton in interaction is
attributed to five processes: the direct knockout of a nuclear alpha cluster,
the knockout of four nucleons with a total charge number of 2, the formation of
the DeltaSi isobaric nucleus, the formation of the Delta isobar in the
interaction of the incident proton with a nuclear nucleon, and the production
of a pi meson, which is at rest in the nuclear reference frame. The last
process likely corresponds to the reaction of the formation of a deeply bound
pion state in the 28P nucleus. Such states were previously observed only on
heavy nuclei. The cross sections for the listed processes have been estimated.Comment: 14 pages, 3 figures submitted to JETP Letter
Effects of Compression and Collective Expansion on Particle Emission from Central Heavy-Ion Reactions
Conditions under which compression occurs and collective expansion develops
in energetic reactions of heavy nuclei, are analyzed, together with their
effects on emitted light baryons and pions. Within transport simulations, it is
shown that shock fronts perpendicular to beam axis form in head-on reactions.
The fronts separate hot compressed matter from normal. As impact parameter
increases, the angle of inclination of the fronts relative to beam axis
decreases, and in-between the fronts a weak tangential discontinuity develops.
Hot matter exposed to the vacuum in directions perpendicular to shock motion
(and parallel to fronts), starts to expand sideways, early within reactions.
Expansion in the direction of shock motion follows after the shocks propagate
through nuclei, but due to the delay does not acquire same strength. Expansion
affects angular distributions, mean-energy components, shapes of spectra and
mean energies of different particles emitted into any one direction, and
further particle yields. Both the expansion and a collective motion associated
with the weak discontinuity, affect the magnitude of sideward flow within
reaction plane. Differences in mean particle energy components in and out of
the reaction plane in semicentral collisions, depend sensitively on the
relative magnitude of shock speed in normal matter and speed of sound in hot
matter.Comment: 71 pages, 33 figures (available on request), report MSUCL-94
Photon Rates for Heavy-Ion Collisions from Hidden Local Symmetry
We study photon production from the hidden local symmetry approach that
includes pions, rho and a1 mesons and compute the corresponding photon emission
rates from a hadronic gas in thermal equilibrium. Together with experimental
radiative decay widths of the background, these rates are used in a
relativistic transport model to calculate single photon spectra in heavy-ion
collisions at SPS energies. We then employ this effective theory to test three
scenarios for the chiral phase transition in high-temperature nuclear matter
including decreasing vector meson masses. Although all calculations respect the
upper bound set by the WA80 Collaboration, we find the scenarios could be
distinguished with more detailed data.Comment: 12 pages, 12 Postscript figures; discussion of thermal equilibrium
rates expanded, minor corrections to text and graph
Nucleon-nucleon elastic scattering analysis to 2.5 GeV
A partial-wave analysis of NN elastic scattering data has been completed.
This analysis covers an expanded energy range, from threshold to a laboratory
kinetic energy of 2.5 GeV, in order to include recent elastic pp scattering
data from the EDDA collaboration. The results of both single-energy and
energy-dependent analyses are described.Comment: 23 pages of text. Postscript files for the figures are available from
ftp://clsaid.phys.vt.edu/pub/said/n