131 research outputs found
Scalar and vector decomposition of the nucleon self-energy in the relativistic Brueckner approach
We investigate the momentum dependence of the nucleon self-energy in nuclear
matter. We apply the relativistic Brueckner-Hartree-Fock approach and adopt the
Bonn A potential. A strong momentum dependence of the scalar and vector
self-energy components can be observed when a commonly used pseudo-vector
choice for the covariant representation of the T-matrix is applied. This
momentum dependence is dominated by the pion exchange. We discuss the problems
of this choice and its relations to on-shell ambiguities of the T-matrix
representation. Starting from a complete pseudo-vector representation of the
T-matrix, which reproduces correctly the pseudo-vector pion-exchange
contributions at the Hartree-Fock level, we observe a much weaker momentum
dependence of the self-energy. This fixes the range of the inherent uncertainty
in the determination of the scalar and vector self-energy components. Comparing
to other work, we find that extracting the self-energy components by a fit to
the single particle potential leads to even more ambiguous results.Comment: 35 pages RevTex, 7 PS figures, replaced by a revised and extended
versio
Coating of bioactive glasses with chitosan: The effects of the glass composition and coating method on the surface properties, including preliminary in vitro results
Two bioactive glasses were coated with chitosan: SCNB belongs to the SiO2-CaO-Na2O system, and SCNA has the addition of Al2O3 to enhance chemical stability. Different coating methods were compared: direct physical attachment, surface activation through tresyl chloride, and polydopamine as a linker. The samples were char-acterized through SEM-EDS, contact angle measurements, FTIR, zeta potential titrations, tape tests, in vitro precipitation of hydroxylapatite (bioactivity), and cell cultures (RAW 264.7 and UMR-106) on some selected samples. Direct physical attachment (in acetic acid) or use of polydopamine allowed complete surface coverage, while it dropped to one-quarter on both glasses by using tresyl chloride. The coating had a contact angle of about 80 degrees and it well showed typical functional groups (FTIR). The coatings on SCNA were chemically and mechan-ically stable (classified as 4-5B by the tape tests, partially maintained after soaking for 14 days), and showed an isoelectric point around 8. On SCNB, the coatings were unstable (classified as 0-3B, dissolved during soaking) but bioactivity was preserved. The coating affected the biological outcome of SCNA with M0/M1 polarization (1 day) and reduced viability of macrophages (3 days), while osteoblastic cells showed poor adhesion but maintained cell viability and osteogenic potential (3-7 days)
Influence of the in-medium pion dispersion relation in heavy ion collisions
We investigate the influence of medium corrections to the pion dispersion
relation on the pion dynamics in intermediate energy heavy ion collisions. To
do so a pion potential is extracted from the in-medium dispersion relation and
used in QMD calculations and thus we take care of both, real and imaginary part
of the pion optical potential. The potentials are determined from different
sources, i.e. from the --hole model and from phenomenological
approaches. Depending on the strength of the potential a reduction of the
anti-correlation of pion and nucleon flow in non-central collisions is observed
as well as an enhancement of the high energetic yield in transverse pion
spectra. A comparison to experiments, in particular to -spectra for the
reaction Ca+Ca at 1 GeV/nucleon and the pion in-plane flow in Ne+Pb collisions
at 800 MeV/nucleon, generally favours a weak potential.Comment: 25 pages, using REVTeX, 6 postscript figures; replaced by published
versio
Probing the nuclear equation of state by production in heavy ion collisions
The dependence of production on the nuclear equation of state is
investigated in heavy ion collisions. An increase of the excitation function of
multiplicities obtained in heavy () over light () systems
when going far below threshold which has been observed by the KaoS
Collaboration strongly favours a soft equation of state. This observation holds
despite of the influence of an in-medium kaon potential predicted by effective
chiral models which is necessary to reproduce the experimental yields.Comment: 4 pages Revtex, 4 PS figures, to appear in Phys. Rev. Let
Relativistic Brueckner-Hartree-Fock calculations with explicit intermediate negative energy states
In a relativistic Brueckner-Hartree-Fock calculation we include explicit
negative-energy states in the two-body propagator. This is achieved by using
the Gross spectator-equation, modified by medium effects. Qualitatively our
results compare well with other RBHF calculations. In some details significant
differences occur, e.g, our equation of state is stiffer and the momentum
dependence of the self-energy components is stronger than found in a reference
calculation without intermediate negative energy states.Comment: 13 pages Revtex, 5 figures included seperatel
Off shell behaviour of the in medium nucleon-nucleon cross section
The properties of nucleon-nucleon scattering inside dense nuclear matter are
investigated. We use the relativistic Brueckner-Hartree-Fock model to determine
on-shell and half off-shell in-medium transition amplitudes and cross sections.
At finite densities the on-shell cross sections are generally suppressed. This
reduction is, however, less pronounced than found in previous works. In the
case that the outgoing momenta are allowed to be off energy shell the
amplitudes show a strong variation with momentum. This description allows to
determine in-medium cross sections beyond the quasi-particle approximation
accounting thereby for the finite width which nucleons acquire in the dense
nuclear medium. For reasonable choices of the in-medium nuclear spectral width,
i.e. MeV, the resulting total cross sections are, however,
reduced by not more than about 25% compared to the on-shell values. Off-shell
effect are generally more pronounced at large nuclear matter densities.Comment: 31 pages Revtex, 12 figures, typos corrected, to appear in Phys. Rev.
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