674 research outputs found
Equation of state for the two component Van der Waals gas with relativistic excluded volumes
A canonical partition function for the two-component excluded volume model is derived, leading to two di erent van der Waals approximations. The one is known as the Lorentz-Berthelot mixture and the other has been proposed recently. Both models are analysed in the canonical and grand canonical ensemble. In comparison with the one-component van der Waals excluded volume model the suppression of particle densities is reduced in these two-component formulations, but in two essentially di erent ways. Presently used multi-component models have no such reduction. They are shown to be not correct when used for components with di erent hard-core radii. For high temperatures the excluded volume interaction is refined by accounting for the Lorentz contraction of the spherical excluded volumes, which leads to a distinct enhancement of lighter particles. The resulting e ects on pion yield ratios are studied for AGS and SPS data
Transport model analysis of particle correlations in relativistic heavy ion collisions at femtometer scales
The pion source as seen through HBT correlations at RHIC energies is
investigated within the UrQMD approach. We find that the calculated transverse
momentum, centrality, and system size dependence of the Pratt-HBT radii
and are reasonably well in line with experimental data. The predicted
values in central heavy ion collisions are larger as compared to
experimental data. The corresponding quantity of the
pion emission source is somewhat larger than experimental estimates.Comment: 12 pages, 5 figures, to be published in PR
Hydrodynamic Models for Heavy-Ion Collisions, and beyond
A generic property of a first-order phase transition in equilibrium, and in
the limit of large entropy per unit of conserved charge, is the smallness of
the isentropic speed of sound in the ``mixed phase''. A specific prediction is
that this should lead to a non-isotropic momentum distribution of nucleons in
the reaction plane (for energies around 40 AGeV in our model calculation). On
the other hand, we show that from present effective theories for low-energy QCD
one does not expect the thermal transition rate between various states of the
effective potential to be much larger than the expansion rate, questioning the
applicability of the idealized Maxwell/Gibbs construction. Experimental data
could soon provide essential information on the dynamics of the phase
transition.Comment: 10 Pages, 4 Figures. Presented at 241st WE-Heraeus Seminar: Symposium
on Fundamental Issues in Elementary Matter: In Honor and Memory of Michael
Danos, Bad Honnef, Germany, 25-29 Sep 200
Anisotropic flow at RHIC: How unique is the number-of-constituent-quark scaling?
The transverse momentum dependence of the anisotropic flow for ,
, nucleon, , and is studied for Au+Au collisions at
GeV within two independent string-hadron transport
approaches (RQMD and UrQMD). Although both models reach only 60% of the
absolute magnitude of the measured , they both predict the particle type
dependence of , as observed by the RHIC experiments: exhibits a
hadron-mass hierarchy (HMH) in the low region and a
number-of-constituent-quark (NCQ) dependence in the intermediate region.
The failure of the hadronic models to reproduce the absolute magnitude of the
observed indicates that transport calculations of heavy ion collisions at
RHIC must incorporate interactions among quarks and gluons in the early, hot
and dense phase. The presence of an NCQ scaling in the string-hadron model
results suggests that the particle-type dependencies observed in heavy-ion
collisions at intermediate might be related to the hadronic cross
sections in vacuum rather than to the hadronization process itself.Comment: 10 pages, 5 figures; A new author (H. Petersen) is added; A new
figure (fig.1) on time evolution of elliptic flow and number of collisions is
added; Version accepted for publication in J. Phys.
Reliability and Reproducibility of Hadamard Encoded Pseudo-Continuous Arterial Spin Labeling in Healthy Elderly
The perfusion parameters cerebral blood flow (CBF) and arterial transit time (ATT) measured with arterial spin labeling (ASL) magnetic resonance imaging (MRI) provide valuable essentials to assess the integrity of cerebral tissue. Brain perfusion changes, due to aging, an intervention, or neurodegenerative diseases for example, could be investigated in longitudinal ASL studies with reliable ASL sequences. Generally, pseudo-continuous ASL (pCASL) is preferred because of its larger signal-to-noise ratio (SNR) compared to pulsed ASL (PASL) techniques. Available pCASL versions differ regarding their feature details. To date only little is known about the reliability and reproducibility of CBF and ATT measures obtained with the innovative Hadamard encoded pCASL variant, especially if applied on participants in old age. Therefore, we investigated an in-house developed Hadamard encoded pCASL sequence on a group of healthy elderly at two different 3 Tesla Siemens MRI systems (Skyra and mMR Biograph) and evaluated CBF and ATT reliability and reproducibility for several regions-of-interests (ROI). Calculated within-subject coefficients of variation (wsCV) demonstrated an excellent reliability of perfusion measures, whereas ATT appeared to be even more reliable than CBF [e.g., wsCV(CBF) = 2.9% vs. wsCV(ATT) = 2.3% for a gray matter (GM) ROI on Skyra system]. Additionally, a substantial agreement of perfusion values acquired on both MRI systems with an inter-session interval of 78 ± 17.6 days was shown by high corresponding intra-class correlation (ICC) coefficients [e.g., ICC(CBF) = 0.704 and ICC(ATT) = 0.754 for a GM ROI]. The usability of this novel Hadamard encoded pCASL sequence might improve future follow-up perfusion studies of the aging and/or diseased brain
Large enhancement from freeze out
Freeze out of particles across three dimensional space-time hypersurface is
discussed in a simple kinetic model. The final momentum distribution of emitted
particles, for freeze out surfaces with space-like normal, shows a
non-exponential transverse momentum spectrum. The slope parameter of the
distribution increases with increasing , in agreement with recently
measured SPS pion and spectra.Comment: 8 pages, 1 figure. Accepted for publication in Physics Letters
Molecular identification of Nectriaceae in infections of apple replant disease affected roots collected by Harris Uni-Core punching or laser microdissection
Apple replant disease (ARD) negatively affects growth and yield of apple plants worldwide. Fungi belonging to the Nectriaceae have often been isolated from roots grown in replant soils and thus are proposed among others as one biotic cause of the disease complex. Microscopic analyses of ARD-affected roots revealed characteristic symptoms associated with fungal infection sites. Here, two extraction methods of such tissue sites were applied to directly identify an unknown fungus that forms typical cauliflower-like structures in diseased root cortex cells. Punching small tissue samples of about 0.5 mm3 volume with the Harris Uni-Core is a quick and easy method to harvest symptomatic material. Secondly, a laser microdissection (LMD) protocol for apple roots was established. This technique allows the extraction of defined cell or tissue fractions from thin cryo-sections. Tissue harvesting was followed by the identification of fungi via PCR amplification of two gene fragments and Sanger sequencing. For Harris samples, Chelex was used for DNA stabilization, while LMD samples were directly submitted to PCR. In Harris samples, mainly the Nectriaceae species Dactylonectria torresensis, Ilyonectria robusta and Rugonectria rugulosa were identified. In addition to these, in LMD samples Cylindrocladiella sp. and Ilyonectria europaea were detected. Thus, the intracellular CF structures contained different species of Nectriaceae in the ARD-affected cortex cells. These results contribute considerably to the etiology of the ARD. Both protocols offer the possibility to identify fungi from selected symptomatic small root sections by molecular tools avoiding isolation and subsequent axenic pure cultures of single fungal isolates
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