241 research outputs found
Application of a laser induced fluorescence model to the numerical simulation of detonation waves in hydrogen-oxygen-diluent mixtures
A laser-induced-fluorescence model has been implemented and used to post-process detonation wave numerical simulation results to allow a direct comparison with previous experimental visualizations of detonations in hydrogen–oxygen–diluent mixtures. The model is first applied to steady one-dimensional simulation results obtained with detailed chemistry. The effects on the fluorescence intensity of the model parameters are examined to explore the dominant processes. The dominant interference process in the experiments carried out to date is the absorption of incident laser light by the high concentration of OH in and behind the reaction zone. The model is then applied to unsteady two-dimensional simulation results obtained with reduced chemical schemes to obtain synthetic PLIF image. The results demonstrate good qualitative agreement between the experimental and calculated laser-induced-fluorescence intensities. The model limitations and the experimental uncertainties are discussed together with a critical evaluation of the modeling approach
Phase Transitions in a Forest-Fire Model
We investigate a forest-fire model with the density of empty sites as control
parameter. The model exhibits three phases, separated by one first-order phase
transition and one 'mixed' phase transition which shows critical behavior on
only one side and hysteresis. The critical behavior is found to be that of the
self-organized critical forest-fire model [B. Drossel and F. Schwabl, Phys.
Rev. Lett. 69, 1629 (1992)], whereas in the adjacent phase one finds the spiral
waves of the Bak et al. forest-fire model [P. Bak, K. Chen and C. Tang, Phys.
Lett. A 147, 297 (1990)]. In the third phase one observes clustering of trees
with the fire burning at the edges of the clusters. The relation between the
density distribution in the spiral state and the percolation threshold is
explained and the implications for stationary states with spiral waves in
arbitrary excitable systems are discussed. Furthermore, we comment on the
possibility of mapping self-organized critical systems onto 'ordinary' critical
systems.Comment: 30 pages RevTeX, 9 PostScript figures (Figs. 1,2,4 are of reduced
quality), to appear in Phys. Rev.
Observation of a baryon resonance with positive strangeness in K+ collisions with Xe nuclei
The status of our investigation of low-energy Xe collisions in the Xenon
bubble chamber DIANA is reported. In the charge-exchange reaction the spectrum of effective mass shows a resonant enhancement
with MeV/c and ^24.4\sigma$. The mass and width of the
observed resonance are consistent with expectations for the lightest member of
the anti-decuplet of exotic pentaquark baryons, as predicted in the framework
of the chiral soliton model.Comment: 9 pages, 4 figure
Further evidence for formation of a narrow baryon resonance with positive strangeness in K+ collisions with Xe nuclei
We have continued our investigation of the charge-exchange reaction K^+ Xe
--> K^0 p Xe' in the bubble chamber DIANA. In agreement with our previous
results based on part of the present statistics, formation of a narrow p K^0
resonance with mass of 1537+-2 MeV/c^2 is observed in the elementary transition
K^+ n --> K^0 p on a neutron bound in the Xenon nucleus. Visible width of the
peak is consistent with being entirely due to instrumental resolution and
allows to place an upper limit on its intrinsic width: \Gamma < 9 MeV/c^2. A
more precise estimate of the resonance intrinsic width, \Gamma = 0.36+-0.11
MeV/c^2, is obtained from the ratio between the numbers of resonant and
non-resonant charge-exchange events. The signal is observed in a restricted
interval of incident K^+ momentum, that is consistent with smearing of a narrow
p K^0 resonance by Fermi motion of the target neutron. Statistical significance
of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators
S/sqrt{B}, S/sqrt{S+B}, and S/sqrt{S+2B}, respectively. This observation
confirms and reinforces our earlier results, and offers strong evidence for
formation of a pentaquark baryon with positive strangeness in the
charge-exchange reaction K^+ n --> K^0 p on a bound neutron.Comment: 13 pages, 8 figures, some chenges in text and references, more
precise estimate of Theta(1540) to add, submitted to Phys.Atom.Nucl(Yad.Fiz.
Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio to Q^2 = 8.5 GeV^2
Among the most fundamental observables of nucleon structure, electromagnetic
form factors are a crucial benchmark for modern calculations describing the
strong interaction dynamics of the nucleon's quark constituents; indeed, recent
proton data have attracted intense theoretical interest. In this letter, we
report new measurements of the proton electromagnetic form factor ratio using
the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5
GeV2. By extending the range of Q2 for which GEp is accurately determined by
more than 50%, these measurements will provide significant constraints on
models of nucleon structure in the non-perturbative regime
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