357 research outputs found
Collective Excitations and Ground State Correlations
A generalized RPA formalism is presented which treats pp and ph correlations
on an equal footing. The effect of these correlations on the single-particle
Green function is discussed and it is demonstrated that a self-consistent
treatment of the single-particle Green function is required to obtain stable
solutions. A simple approximation scheme is presented which incorporates for
this self-consistency requirement and conserves the number of particles.
Results of numerical calculations are given for O using a G-matrix
interaction derived from a realistic One-Boson-Exchange potential.Comment: 16 Pages + 2 Figures (included at the end as uuencoded ps-files),
TU-18089
Long-Range Correlations and the Momentum Distribution in Nuclei
The influence of correlations on the momentum distribution of nucleons in
nuclei is evaluated starting from a realistic nucleon-nucleon interaction. The
calculations are performed directly for the finite nucleus \,^{16}O making
use of the Green's function approach. The emphasis is focused on the
correlations induced by the excitation modes at low energies described within a
model-space of shell-model configurations including states up to the sdg shell.
Our analysis demonstrates that these long-range correlations do not produce any
significant enhancement of the momentum distribution at high missing momenta
and low missing energies. This is in agreement with high resolution
experiments for this nucleus. We also try to simulate the corresponding effects
in large nuclei by quenching the energy-spacing between single-particle orbits.
This yields a sizable enhancement of the spectral function at large momenta and
small energy. Such behavior could explain the deviation of the momentum
distribution from the mean field prediction, which has been observed in
experiments on heavy nuclei like Pb
Long-Range Correlations in Closed Shell Nuclei
The effects of correlations on the bulk properties of nuclei are investigated
in large model spaces including up to 21 single-particle orbits. The evaluation
of the single-particle Green function is made feasible by means of the BAGEL
approximation. The spectral function for single-nucleon pick-up and removal is
investigated for the nuclei and . Special attention is paid
to the effects produced by correlations on the calculated ground state
properties of closed shell nuclei. It is observed that correlations beyond the
Brueckner Hartree Fock approximation tend to improve the results obtained using
realistic nucleon nucleon interactions.Comment: 23 pages 4 figures not included, Report Tu-93-081
Momentum and Energy Distributions of Nucleons in Finite Nuclei due to Short-Range Correlations
The influence of short-range correlations on the momentum and energy
distribution of nucleons in nuclei is evaluated assuming a realistic
meson-exchange potential for the nucleon-nucleon interaction. Using the
Green-function approach the calculations are performed directly for the finite
nucleus O avoiding the local density approximation and its reference to
studies of infinite nuclear matter. The nucleon-nucleon correlations induced by
the short-range and tensor components of the interaction yield an enhancement
of the momentum distribution at high momenta as compared to the Hartree-Fock
description. These high-momentum components should be observed mainly in
nucleon knockout reactions like leaving the final nucleus in a state
of high excitation energy. Our analysis also demonstrates that non-negligible
contributions to the momentum distribution should be found in partial waves
which are unoccupied in the simple shell-model. The treatment of correlations
beyond the Brueckner-Hartree-Fock approximation also yields an improvement for
the calculated ground-state properties.Comment: 12 pages RevTeX, 7 figures postscript files appende
Correlations and the Cross Section of Exclusive () Reactions for O
The reduced cross section for exclusive () reactions has been studied
in DWIA for the example of the nucleus O using a spectral function
containing effects of correlations. The spectral function is evaluated directly
for the finite nucleus starting from a realistic nucleon-nucleon interaction
within the framework of the Green's function approach. The emphasis is focused
on the correlations induced by excitation modes at low energies described
within a model-space of shell-model configurations including states up to the
shell. Cross sections for the -wave quasi-hole transitions at low
missing energies are presented and compared with the most recent experimental
data. In the case of the so-called perpendicular kinematics the reduced cross
section derived in DWIA shows an enhancement at high missing momenta as
compared to the PWIA result. Furthermore the cross sections for the - and
-wave quasi-hole transitions are presented and compared to available data at
low missing momenta. Also in these cases, which cannot be described in a model
without correlations, a good agreement with the experiment is obtained.Comment: 12 pages, LaTeX, 4 figures include
Two-Body Correlations in Nuclear Systems
Correlations in the nuclear wave-function beyond the mean-field or
Hartree-Fock approximation are very important to describe basic properties of
nuclear structure. Various approaches to account for such correlations are
described and compared to each other. This includes the hole-line expansion,
the coupled cluster or ``exponential S'' approach, the self-consistent
evaluation of Greens functions, variational approaches using correlated basis
functions and recent developments employing quantum Monte-Carlo techniques.
Details of these correlations are explored and their sensitivity to the
underlying nucleon-nucleon interaction. Special attention is paid to the
attempts to investigate these correlations in exclusive nucleon knock-out
experiments induced by electron scattering. Another important issue of nuclear
structure physics is the role of relativistic effects as contained in
phenomenological mean field models. The sensitivity of various nuclear
structure observables on these relativistic features are investigated. The
report includes the discussion of nuclear matter as well as finite nuclei.Comment: Review, 104 pages including figure
Acute kidney injury and tools for risk-stratification in 456 patients with hantavirus-induced nephropathia epidemica
BACKGROUND Puumala virus (PUUV) is the most common species of hantavirus in Central Europe. Nephropathia epidemica (NE), caused by PUUV, is characterized by acute kidney injury (AKI) and thrombocytopenia. The major goals of this study were to provide a clear clinical phenotyping of AKI in patients with NE and to develop an easy prediction rule to identify patients, who are at lower risk to develop severe AKI. METHODS A cross-sectional prospective survey of 456 adult patients with serologically confirmed NE was performed. Data were collected from medical records and prospectively at follow-up visit. Severe AKI was defined by standard criteria according to the RIFLE (Risk, Injury, Failure, Loss, End-stage kidney disease) classification. Fuller statistical models were developed and validated to estimate the probability for severe AKI. RESULTS During acute NE, 88% of the patients had AKI according to the RILFE criteria during acute NE. A risk index score for severe AKI was derived by using three independent risk factors in patients with normal kidney function at time of diagnosis: thrombocytopenia [two points; odds ratios (OR): 3.77; 95% confidence intervals (CI): 1.82, 8.03], elevated C-reactive protein levels (one point; OR: 3.02; 95% CI: 1.42, 6.58) and proteinuria (one point; OR: 3.92; 95% CI: 1.33, 13.35). On the basis of a point score of one or two, the probability of severe AKI was 0.18 and 0.28 with an area under the curve of 0.71. CONCLUSION This clinical prediction rule provides a novel and diagnostically accurate strategy for the potential prevention and improved management of kidney complications in patients with NE and, ultimately, for a possible decrease in unnecessary hospitalization in a high number of patient
Comprehensive Analysis of MGMT Promoter Methylation: Correlation with MGMT Expression and Clinical Response in GBM
O6-methylguanine DNA-methyltransferase (MGMT) promoter methylation has been identified as a potential prognostic marker for glioblastoma patients. The relationship between the exact site of promoter methylation and its effect on gene silencing, and the patient's subsequent response to therapy, is still being defined. The aim of this study was to comprehensively characterize cytosine-guanine (CpG) dinucleotide methylation across the entire MGMT promoter and to correlate individual CpG site methylation patterns to mRNA expression, protein expression, and progression-free survival. To best identify the specific MGMT promoter region most predictive of gene silencing and response to therapy, we determined the methylation status of all 97 CpG sites in the MGMT promoter in tumor samples from 70 GBM patients using quantitative bisulfite sequencing. We next identified the CpG site specific and regional methylation patterns most predictive of gene silencing and improved progression-free survival. Using this data, we propose a new classification scheme utilizing methylation data from across the entire promoter and show that an analysis based on this approach, which we call 3R classification, is predictive of progression-free survival (HR  = 5.23, 95% CI [2.089–13.097], p<0.0001). To adapt this approach to the clinical setting, we used a methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) test based on the 3R classification and show that this test is both feasible in the clinical setting and predictive of progression free survival (HR  = 3.076, 95% CI [1.301–7.27], p = 0.007). We discuss the potential advantages of a test based on this promoter-wide analysis and compare it to the commonly used methylation-specific PCR test. Further prospective validation of these two methods in a large independent patient cohort will be needed to confirm the added value of promoter wide analysis of MGMT methylation in the clinical setting
Searches for Neutrinos from Gamma-Ray Bursts Using the IceCube Neutrino Observatory
Gamma-ray bursts (GRBs) are considered as promising sources of ultra-high-energy cosmic rays (UHECRs) due to their large power output. Observing a neutrino flux from GRBs would offer evidence that GRBs are hadronic accelerators of UHECRs. Previous IceCube analyses, which primarily focused on neutrinos arriving in temporal coincidence with the prompt gamma-rays, found no significant neutrino excess. The four analyses presented in this paper extend the region of interest to 14 days before and after the prompt phase, including generic extended time windows and targeted precursor searches. GRBs were selected between 2011 May and 2018 October to align with the data set of candidate muon-neutrino events observed by IceCube. No evidence of correlation between neutrino events and GRBs was found in these analyses. Limits are set to constrain the contribution of the cosmic GRB population to the diffuse astrophysical neutrino flux observed by IceCube. Prompt neutrino emission from GRBs is limited to ≲1% of the observed diffuse neutrino flux, and emission on timescales up to 104 s is constrained to 24% of the total diffuse flux.Peer Reviewe
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