1,767 research outputs found
Statistical Ensembles with Fluctuating Extensive Quantities
We suggest an extension of the standard concept of statistical ensembles.
Namely, we introduce a class of ensembles with extensive quantities fluctuating
according to an externally given distribution. As an example the influence of
energy fluctuations on multiplicity fluctuations in limited segments of
momentum space for a classical ultra-relativistic gas is considered.Comment: 4 pages, 2 figure
Viscosity in the excluded volume hadron gas model
The shear viscosity in the van der Waals excluded volume
hadron-resonance gas model is considered. For the shear viscosity the result of
the non-relativistic gas of hard-core particles is extended to the mixture of
particles with different masses, but equal values of hard-core radius r. The
relativistic corrections to hadron average momenta in thermal equilibrium are
also taken into account. The ratio of the viscosity to the entropy
density s is studied. It monotonously decreases along the chemical freeze-out
line in nucleus-nucleus collisions with increasing collision energy. As a
function of hard-core radius r, a broad minimum of the ratio near fm is found at high collision energies. For the
charge-neutral system at MeV, a minimum of the ratio is reached for fm. To justify a hydrodynamic approach to
nucleus-nucleus collisions within the hadron phase the restriction from below,
fm, on the hard-core hadron radius should be fulfilled in the
excluded volume hadron-resonance gas.Comment: 12 pages, 3 figure
Fluctuations and correlations from microscopic transport theory
The multiplicity fluctuations in A+A collisions at SPS and RHIC energies are studied within the HSD transport approach. We find a dominant role of the fluctuations in the nucleon participant number for the final fluctuations. In order to extract physical fluctuations one should decrease the fluctuations in the participants number. This can be done considering very central collisions. The system size dependence of the multiplicity fluctuations in central A+A collisions at the SPS energy range – obtained in the HSD and UrQMD transport models – is presented. The results can be used as a ‘background’ for experimental measurements of fluctuations as a signal of the critical point. Event-by-event fluctuations of the K/p , K/p and p/p ratios in A+A collisions are also studied. Event-by-event fluctuations of the kaon to pion number ratio in nucleus-nucleus collisions are studied for SPS and RHIC energies. We find that the HSD model can qualitatively reproduce the measured excitation function for the K/p ratio fluctuations in central Au+Au (or Pb+Pb) collisions from low SPS up to top RHIC energies. The forward-backward correlation coefficient measured by the STAR Collaboration in Au+Au collisions at RHIC is also studied. We discuss the effects of initial collision geometry and centrality bin definition on correlations in nucleus-nucleus collisions. We argue that a study of the dependence of correlations on the centrality bin definition as well as the bin size may distinguish between these ‘trivial’ correlations and correlations arising from ‘new physics’. 5th International Workshop on Critical Point and Onset of Deconfinement - CPOD 2009, June 08 - 12 2009 Brookhaven National Laboratory, Long Island, New York, US
Particle number fluctuations in nuclear collisions within excluded volume hadron gas model
The multiplicity fluctuations are studied in the van der Waals excluded
volume hadron-resonance gas model. The calculations are done in the grand
canonical ensemble within the Boltzmann statistics approximation. The scaled
variances for positive, negative and all charged hadrons are calculated along
the chemical freeze-out line of nucleus-nucleus collisions at different
collision energies. The multiplicity fluctuations are found to be suppressed in
the van der Waals gas. The numerical calculations are presented for two values
of hard-core hadron radius, fm and 0.5 fm, as well as for the upper
limit of the excluded volume suppression effects.Comment: 19 pages, 4 figure
Forward-backward correlations in nucleus-nucleus collisions: baseline contributions from geometrical fluctuations
We discuss the effects of initial collision geometry and centrality bin
definition on correlation and fluctuation observables in nucleus-nucleus
collisions. We focus on the forward-backward correlation coefficient recently
measured by the STAR Collaboration in Au+Au collisions at RHIC. Our study is
carried out within two models: the Glauber Monte Carlo code with a `toy'
wounded nucleon model and the hadron-string dynamics (HSD) transport approach.
We show that strong correlations can arise due to averaging over events in one
centrality bin. We, furthermore, argue that a study of the dependence of
correlations on the centrality bin definition as well as the bin size may
distinguish between these `trivial' correlations and correlations arising from
`new physics'.Comment: 12 pages, 6 figure
Enhanced Anandamide Plasma Levels in Patients with Complex Regional Pain Syndrome following Traumatic Injury: A Preliminary Report
The complex regional pain syndrome (CRPS) is a disabling neuropathic pain condition that may develop following injuries of the extremities. The pathogenesis of this syndrome is not clear; however, it includes complex interactions between the nervous and the immune system resulting in chronic inflammation, pain and trophic changes. This interaction may be mediated by chronic stress which is thought to activate the endogenous cannabinoid (endocannabinoid) system (ECS). We conducted an open, prospective, comparative clinical study to determine plasma level of the endocannabinoid anandamide by high-performance liquid chromatography and a tandem mass spectrometry system in 10 patients with CRPS type I versus 10 age- and sex-matched healthy controls. As compared to healthy controls, CRPS patients showed significantly higher plasma concentrations of anandamide. These results indicate that the peripheral ECS is activated in CRPS. Further studies are warranted to evaluate the role of the ECS in the limitation of inflammation and pain. Copyright (C) 2009 S. Karger AG, Base
Multiplicity fluctuations in relativistic nuclear collisions
Multiplicity distributions of hadrons produced in central nucleus-nucleus
collisions are studied within the hadron-resonance gas model in the large
volume limit. In the canonical ensemble conservation of three charges (baryon
number, electric charge, and strangeness) is enforced. In addition, in the
micro-canonical ensemble energy conservation is included. An analytical method
is used to account for resonance decays. Multiplicity distributions and scaled
variances for negatively charged hadrons are presented along the chemical
freeze-out line of central Pb+Pb (Au+Au) collisions from SIS to LHC energies.
Predictions obtained within different statistical ensembles are compared with
preliminary NA49 experimental results on central Pb+Pb collisions in the SPS
energy range. The measured fluctuations are significantly narrower than a
Poisson reference distribution, and clearly favor expectations for the
micro-canonical ensemble.Comment: 6 pages, 3 figure
Multiplicity fluctuations in relativistic nuclear collisions: statistical model versus experimental data
The multiplicity distributions of hadrons produced in central nucleus-nucleus
collisions are studied within the hadron-resonance gas model in the large
volume limit. The microscopic correlator method is used to enforce conservation
of three charges - baryon number, electric charge, and strangeness - in the
canonical ensemble. In addition, in the micro-canonical ensemble energy
conservation is included. An analytical method is used to account for resonance
decays. The multiplicity distributions and the scaled variances for negatively,
positively, and all charged hadrons are calculated along the chemical
freeze-out line of central Pb+Pb (Au+Au) collisions from SIS to LHC energies.
Predictions obtained within different statistical ensembles are compared with
the preliminary NA49 experimental results on central Pb+Pb collisions in the
SPS energy range. The measured fluctuations are significantly narrower than the
Poisson ones and clearly favor expectations for the micro-canonical ensemble.
Thus this is a first observation of the recently predicted suppression of the
multiplicity fluctuations in relativistic gases in the thermodynamical limit
due to conservation laws.Comment: 27 pages, 9 figures, corrected reference
Molecular genetic and crystal structural analysis of 1-(4-hydroxyphenyl)-ethanol dehydrogenase from 'Aromatoleum aromaticum' EbN1.
he dehydrogenation of 1-(4-hydroxyphenyl)-ethanol to 4-hydroxyacetophenone represents the second reaction step during anaerobic degradation of p-ethylphenol in the denitrifying bacterium ‘Aromatoleum aromaticum' EbN1. Previous proteogenomic studies identified two different proteins (ChnA and EbA309) as possible candidates for catalyzing this reaction [Wöhlbrand et al: J Bacteriol 2008;190:5699-5709]. Physiological-molecular characterization of newly generated unmarked in-frame deletion and complementation mutants allowed defining ChnA (renamed here as Hped) as the enzyme responsible for 1-(4-hydroxyphenyl)-ethanol oxidation. Hped [1-(4-hydroxyphenyl)-ethanol dehydrogenase] belongs to the ‘classical' family within the short-chain alcohol dehydrogenase/reductase (SDR) superfamily. Hped was overproduced in Escherichia coli, purified and crystallized. The X-ray structures of the apo- and NAD+-soaked form were resolved at 1.5 and 1.1 Å, respectively, and revealed Hped as a typical homotetrameric SDR. Modeling of the substrate 4-hydroxyacetophenone (reductive direction of Hped) into the active site revealed the structural determinants of the strict (R)-specificity of Hped (Phe187), contrasting the (S)-specificity of previously reported 1-phenylethanol dehydrogenase (Ped; Tyr93) from strain EbN1 [Höffken et al: Biochemistry 2006;45:82-93]
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