420 research outputs found
Influence of the pion-nucleon interaction on the collective pion flow in heavy ion reactions
We investigate the influence of the real part of the in-medium pion optical
potential on the pion dynamics in intermediate energy heavy ion reactions at 1
GeV/A. For different models, i.e. a phenomenological model and the
--hole model, a pionic potential is extracted from the dispersion
relation and used in Quantum Molecular Dynamics calculations. In addition with
the inelastic scattering processes we thus take care of both, real and
imaginary part of the pion optical potential. A strong influence of the real
pionic potential on the pion in-plane flow is observed. In general such a
potential has the tendency to reduce the anticorrelation of pion and nucleon
flow in non-central collisions.Comment: 12 pages Latex, 4 PS-figure
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
Correlations and the relativistic structure of the nucleon self-energy
A key point of Dirac Brueckner Hartree Fock calculations for nuclear matter
is to decompose the self energy of the nucleons into Lorentz scalar and vector
components. A new method is introduced for this decomposition. It is based on
the dependence of the single-particle energy on the small component in the
Dirac spinors used to calculate the matrix elements of the underlying NN
interaction. The resulting Dirac components of the self-energy depend on the
momentum of the nucleons. At densities around and below the nuclear matter
saturation density this momentum dependence is dominated by the non-locality of
the Brueckner G matrix. At higher densities these correlation effects are
suppressed and the momentum dependence due to the Fock exchange terms is
getting more important. Differences between symmetric nuclear matter and
neutron matter are discussed. Various versions of the Bonn potential are
considered.Comment: 18 pages LaTeX, including 6 figure
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
Origin of subthreshold K^+ production in heavy ion collisions
We investigate the origin of subthreshold production in heavy ion
collisions at intermediate energies. In particular we study the influence of
the pion induced creation processes. We find that this channel shows a
strong dependence on the size of the system, i.e., the number of participating
nucleons as well as on the incident energy of the reaction. In an energy region
between 1--2 GeV/nucleon the pion induced processes essentially contribute to
the total yield and can even become dominant in reactions with a large number
of participating nucleons. Thus we are able to reproduce recent measurements of
the KaoS Collaboration for 1 GeV/nucleon Au on Au reactions adopting a
realistic momentum dependent nuclear mean field.Comment: 6 pages Latex using RevTex, revised version accepted for publication
in Phys. Rev.
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
Density Dependent Hadron Field Theory
A fully covariant approach to a density dependent hadron field theory is
presented. The relation between in--medium NN interactions and
field--theoretical meson--nucleon vertices is discussed. The medium dependence
of nuclear interactions is described by a functional dependence of the
meson--nucleon vertices on the baryon field operators. As a consequence, the
Euler--Lagrange equations lead to baryon rearrangement self--energies which are
not obtained when only a parametric dependence of the vertices on the density
is assumed. It is shown that the approach is energy--momentum conserving and
thermodynamically consistent. Solutions of the field equations are studied in
the mean--field approximation. Descriptions of the medium dependence in terms
of the baryon scalar and vector density are investigated. Applications to
infinite nuclear matter and finite nuclei are discussed. Density dependent
coupling constants obtained from Dirac--Brueckner calculations with the Bonn
NN-potentials are used. Results from Hartree calculations for energy spectra,
binding energies and charge density distributions of , and
are presented. Comparisons to data strongly support the importance
of rearrangement in a relativistic density dependent field theory. Most
striking is the simultanuous improvement of charge radii, charge densities and
binding energies. The results indicate the appearance of a new "Coester line"
in the nuclear matter equation of state.Comment: 48 LateX pages, 12 Figures, figures and full paper are available as
postscript files by anonymous ftp at ftp://theorie.physik.uni-giessen.de/dd
Semi-supervised learning towards automated segmentation of PET images with limited annotations: Application to lymphoma patients
The time-consuming task of manual segmentation challenges routine systematic
quantification of disease burden. Convolutional neural networks (CNNs) hold
significant promise to reliably identify locations and boundaries of tumors
from PET scans. We aimed to leverage the need for annotated data via
semi-supervised approaches, with application to PET images of diffuse large
B-cell lymphoma (DLBCL) and primary mediastinal large B-cell lymphoma (PMBCL).
We analyzed 18F-FDG PET images of 292 patients with PMBCL (n=104) and DLBCL
(n=188) (n=232 for training and validation, and n=60 for external testing). We
employed FCM and MS losses for training a 3D U-Net with different levels of
supervision: i) fully supervised methods with labeled FCM (LFCM) as well as
Unified focal and Dice loss functions, ii) unsupervised methods with Robust FCM
(RFCM) and Mumford-Shah (MS) loss functions, and iii) Semi-supervised methods
based on FCM (RFCM+LFCM), as well as MS loss in combination with supervised
Dice loss (MS+Dice). Unified loss function yielded higher Dice score (mean +/-
standard deviation (SD)) (0.73 +/- 0.03; 95% CI, 0.67-0.8) compared to Dice
loss (p-value<0.01). Semi-supervised (RFCM+alpha*LFCM) with alpha=0.3 showed
the best performance, with a Dice score of 0.69 +/- 0.03 (95% CI, 0.45-0.77)
outperforming (MS+alpha*Dice) for any supervision level (any alpha) (p<0.01).
The best performer among (MS+alpha*Dice) semi-supervised approaches with
alpha=0.2 showed a Dice score of 0.60 +/- 0.08 (95% CI, 0.44-0.76) compared to
another supervision level in this semi-supervised approach (p<0.01).
Semi-supervised learning via FCM loss (RFCM+alpha*LFCM) showed improved
performance compared to supervised approaches. Considering the time-consuming
nature of expert manual delineations and intra-observer variabilities,
semi-supervised approaches have significant potential for automated
segmentation workflows
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