1,105 research outputs found
Strange particle production at RHIC in a single-freeze-out model
Strange particle ratios and pT-spectra are calculated in a thermal model with
single freeze-out, previously used successfully to describe non-strange
particle production at RHIC. The model and the recently released data for phi,
Lambda, anti-Lambda, and K*(892) are in very satisfactory agreement, showing
that the thermal approach can be used to describe the strangeness production at
RHIC.Comment: We have added the comparison of the model predictions to the newly
released Lambda and K*(892) pT-spectra from STA
Baryon phase-space density in heavy-ion collisions
The baryon phase-space density at mid-rapidity from central heavy-ion
collisions is estimated from proton spectra with interferometry and deuteron
coalescence measurements. It is found that the mid-rapidity phase-space density
of baryons is significantly lower at the SPS than the AGS, while those of total
particles (pion + baryon) are comparable. Thermal and chemical equilibrium
model calculations tend to over-estimate the phase-space densities at both
energies.Comment: 5 pages, 2 tables, no figure. RevTeX style. Accepted for publication
in Phys. Rev. C Rapid Communicatio
Chiral phase boundary of QCD at finite temperature
We analyze the approach to chiral symmetry breaking in QCD at finite
temperature, using the functional renormalization group. We compute the running
gauge coupling in QCD for all temperatures and scales within a simple truncated
renormalization flow. At finite temperature, the coupling is governed by a
fixed point of the 3-dimensional theory for scales smaller than the
corresponding temperature. Chiral symmetry breaking is approached if the
running coupling drives the quark sector to criticality. We quantitatively
determine the phase boundary in the plane of temperature and number of flavors
and find good agreement with lattice results. As a generic and testable
prediction, we observe that our underlying IR fixed-point scenario leaves its
imprint in the shape of the phase boundary near the critical flavor number:
here, the scaling of the critical temperature is determined by the
zero-temperature IR critical exponent of the running coupling.Comment: 39 pages, 8 figure
Impedance adaptation for optimal robot–environment interaction
In this paper, impedance adaptation is investigated for robots interacting with unknown environments. Impedance control is employed for the physical interaction between robots and environments, subject to unknown and uncertain environments dynamics. The unknown environments are described as linear systems with unknown dynamics, based on which the desired impedance model is obtained. A cost function that measures the tracking error and interaction force is defined, and the critical impedance parameters are found to minimize it. Without requiring the information of the environments dynamics, the proposed impedance adaptation is feasible in a large number of applications where robots physically interact with unknown environments. The validity of the proposed method is verified through simulation studies
Neutral Pions and Eta Mesons as Probes of the Hadronic Fireball in Nucleus-Nucleus Collisions around 1A GeV
Chemical and thermal freeze-out of the hadronic fireball formed in symmetric
collisions of light, intermediate-mass, and heavy nuclei at beam energies
between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated,
isospin-symmetric ideal hadron gas with grand-canonical baryon-number
conservation. For each collision system the baryochemical potential mu_B and
the chemical freeze-out temperature T_c are deduced from the inclusive neutral
pion and eta yields which are augmented by interpolated data on deuteron
production. With increasing beam energy mu_B drops from 800 MeV to 650 MeV,
while T_c rises from 55 MeV to 90 MeV. For given beam energy mu_B grows with
system size, whereas T_c remains constant. The centrality dependence of the
freeze-out parameters is weak as exemplified by the system Au+Au at 0.8A GeV.
For the highest beam energies the fraction of nucleons excited to resonance
states reaches freeze-out values of nearly 15 %, suggesting resonance densities
close to normal nuclear density at maximum compression. In contrast to the
particle yields, which convey the status at chemical freeze-out, the shapes of
the related transverse-mass spectra do reflect thermal freeze-out. The observed
thermal freeze-out temperatures T_th are equal to or slightly lower than T_c,
indicative of nearly simultaneous chemical and thermal freeze-out.Comment: 42 pages, 12 figure
Quantum size effects in Pb islands on Cu(111): Electronic-structure calculations
The appearance of "magic" heights of Pb islands grown on Cu(111) is studied
by self-consistent electronic structure calculations. The Cu(111) substrate is
modeled with a one-dimensional pseudopotential reproducing the essential
features, i.e. the band gap and the work function, of the Cu band structure in
the [111] direction. Pb islands are presented as stabilized jellium overlayers.
The experimental eigenenergies of the quantum well states confined in the Pb
overlayer are well reproduced. The total energy oscillates as a continuous
function of the overlayer thickness reflecting the electronic shell structure.
The energies for completed Pb monolayers show a modulated oscillatory pattern
reminiscent of the super-shell structure of clusters and nanowires. The energy
minima correlate remarkably well with the measured most probable heights of Pb
islands. The proper modeling of the substrate is crucial to set the
quantitative agreement.Comment: 4 pages, 4 figures. Submitte
Asymptotes in SU(2) Recoupling Theory: Wigner Matrices, Symbols, and Character Localization
In this paper we employ a novel technique combining the Euler Maclaurin
formula with the saddle point approximation method to obtain the asymptotic
behavior (in the limit of large representation index ) of generic Wigner
matrix elements . We use this result to derive asymptotic
formulae for the character of an SU(2) group element and for
Wigner's symbol. Surprisingly, given that we perform five successive
layers of approximations, the asymptotic formula we obtain for is
in fact exact. This result provides a non trivial example of a
Duistermaat-Heckman like localization property for discrete sums.Comment: 36 pages, 3 figure
and couplings in QCD
We calculate the and couplings using QCD sum rules on the
light-cone. In this approach, the large-distance dynamics is incorporated in a
set of pion wave functions. We take into account two-particle and
three-particle wave functions of twist 2, 3 and 4. The resulting values of the
coupling constants are and .
From this we predict the partial width \Gamma (D^{*+} \ra D^0 \pi^+ )=32 \pm
5~ keV . We also discuss the soft-pion limit of the sum rules which is
equivalent to the external axial field approach employed in earlier
calculations. Furthermore, using and the pole
dominance model for the B \ra \pi and D\ra \pi semileptonic form factors
is compared with the direct calculation of these form factors in the same
framework of light-cone sum rules.Comment: 27 pages (LATEX) +3 figures enclosed as .uu file MPI-PhT/94-62 ,
CEBAF-TH-94-22, LMU 15/9
Stability of mode-locked kinks in the ac driven and damped sine-Gordon lattice
Kink dynamics in the underdamped and strongly discrete sine-Gordon lattice
that is driven by the oscillating force is studied. The investigation is
focused mostly on the properties of the mode-locked states in the {\it
overband} case, when the driving frequency lies above the linear band. With the
help of Floquet theory it is demonstrated that the destabilizing of the
mode-locked state happens either through the Hopf bifurcation or through the
tangential bifurcation. It is also observed that in the overband case the
standing mode-locked kink state maintains its stability for the bias amplitudes
that are by the order of magnitude larger than the amplitudes in the
low-frequency case.Comment: To appear in Springer Series on Wave Phenomena, special volume
devoted to the LENCOS'12 conference; 6 figure
Hexagonal dielectric resonators and microcrystal lasers
We study long-lived resonances (lowest-loss modes) in hexagonally shaped
dielectric resonators in order to gain insight into the physics of a class of
microcrystal lasers. Numerical results on resonance positions and lifetimes,
near-field intensity patterns, far-field emission patterns, and effects of
rounding of corners are presented. Most features are explained by a
semiclassical approximation based on pseudointegrable ray dynamics and boundary
waves. The semiclassical model is also relevant for other microlasers of
polygonal geometry.Comment: 12 pages, 17 figures (3 with reduced quality
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