2,493 research outputs found
Avoided crossings in mesoscopic systems: electron propagation on a non-uniform magnetic cylinder
We consider an electron constrained to move on a surface with revolution
symmetry in the presence of a constant magnetic field parallel to the
surface axis. Depending on and the surface geometry the transverse part of
the spectrum typically exhibits many crossings which change to avoided
crossings if a weak symmetry breaking interaction is introduced. We study the
effect of such perturbations on the quantum propagation. This problem admits a
natural reformulation to which tools from molecular dynamics can be applied. In
turn, this leads to the study of a perturbation theory for the time dependent
Born-Oppenheimer approximation
Effective degrees of freedom and gluon condensation in the high temperature deconfined phase
The Equation of State and the properties of matter in the high temperature
deconfined phase are analyzed by a quasiparticle approach for . In
order to fix the parameters of our model we employ the lattice QCD data of
energy density and pressure. First we consider the pure SU(3) gluon plasma and
it turns out that such a system can be described in terms of a gluon condensate
and of gluonic quasiparticles whose effective number of degrees of freedom and
mass decrease with increasing temperature. Then we analyze QCD with finite
quark masses. In this case the numerical lattice data for energy density and
pressure can be fitted assuming that the system consists of a mixture of gluon
quasiparticles, fermion quasiparticles, boson correlated pairs (corresponding
to in-medium mesonic states) and gluon condensate. We find that the effective
number of boson degrees of freedom and the in-medium fermion masses decrease
with increasing temperature. At only the correlated pairs
corresponding to the mesonic nonet survive and they completely disappear at . The temperature dependence of the velocity of sound of the
various quasiparticles, the effects of the breaking of conformal invariance and
the thermodynamic consistency are discussed in detail.Comment: 18 pages, 9 figure
Stabilizing Hadron Resonance Gas Models against Future Discoveries
We examine the stability of hadron resonance gas models by extending them to
take care of undiscovered resonances through the Hagedorn formula. We find that
the influence of unknown resonances on thermodynamics is large but bounded.
Hadron resonance gases are internally consistent up to a temperature higher
than the cross over temperature in QCD; but by examining quark number
susceptibilities we find that their region of applicability seems to end even
below the QCD cross over. We model the decays of resonances and investigate the
ratios of particle yields in heavy-ion collisions. We find that observables
such as hydrodynamics and hadron yield ratios change little upon extending the
model. As a result, heavy-ion collisions at RHIC and LHC are insensitive to a
possible exponential rise in the hadronic density of states, thus increasing
the stability of the predictions of hadron resonance gas models
Transport dynamics of a high-power-density matrix-type hydrogen-oxygen fuel cell
Experimental transport dynamics tests were made on a space power fuel cell of current design. Various operating transients were introduced and transport-related response data were recorded with fluidic humidity sensing instruments. Also, sampled data techniques were developed for measuring the cathode-side electrolyte concentration during transient operation
Experimental evaluation of a breadboard heat and product-water removal system for a space-power fuel cell designed with static water removal and evaporative cooling
A test program was conducted to evaluate the design of a heat and product-water removal system to be used with fuel cell having static water removal and evaporative cooling. The program, which was conducted on a breadboard version of the system, provided a general assessment of the design in terms of operational integrity and transient stability. This assessment showed that, on the whole, the concept appears to be inherently sound but that in refining this design, several facets will require additional study. These involve interactions between pressure regulators in the pumping loop that occur when they are not correctly matched and the question of whether an ejector is necessary in the system
Radion Stabilization by Stringy Effects in General Relativity
We consider the effects of a gas of closed strings (treated quantum
mechanically) on a background where one dimension is compactified on a circle.
After we address the effects of a time dependent background on aspects of the
string spectrum that concern us, we derive the energy-momentum tensor for a
string gas and investigate the resulting space-time dynamics. We show that a
variety of trajectories are possible for the radius of the compactified
dimension, depending on the nature of the string gas, including a demonstration
within the context of General Relativity (i.e. without a dilaton) of a solution
where the radius of the extra dimension oscillates about the self-dual radius,
without invoking matter that violates the various energy conditions. In
particular, we find that in the case where the string gas is in thermal
equilibrium, the radius of the compactified dimension dynamically stabilizes at
the self-dual radius, after which a period of usual Friedmann-Robertson-Walker
cosmology of the three uncompactified dimensions can set in. We show that our
radion stabilization mechanism requires a stringy realization of inflation as
scalar field driven inflation invalidates our mechanism. We also show that our
stabilization mechanism is consistent with observational bounds.Comment: New section on the phenomonological implications of and bound on the
model has been adde
Particle Ratios as a Probe of the QCD Critical Temperature
We show how the measured particle ratios can be used to provide non-trivial
information about the critical temperature of the QCD phase transition. This is
obtained by including the effects of highly massive Hagedorn resonances on
statistical models, which are used to describe hadronic yields. The inclusion
of Hagedorn states creates a dependence of the thermal fits on the Hagedorn
temperature, , which is assumed to be equal to , and leads to an
overall improvement of thermal fits. We find that for Au+Au collisions at RHIC
at GeV the best square fit measure, , occurs at
MeV and produces a chemical freeze-out temperature of 172.6 MeV
and a baryon chemical potential of 39.7 MeV.Comment: 6 pages, 4 figure
Chemical equilibration due to heavy Hagedorn states
A scenario of heavy resonances, called massive Hagedorn states, is proposed
which exhibits a fast ( fm/c) chemical equilibration of (strange)
baryons and anti-baryons at the QCD critical temperature . For
relativistic heavy ion collisions this scenario predicts that hadronization is
followed by a brief expansion phase during which the equilibration rate is
higher than the expansion rate, so that baryons and antibaryons reach chemical
equilibrium before chemical freeze-out occurs.Comment: 9 pages, 2 figures. Invited talk given at 8th International
Conference on Strangeness in Quark Matter (SQM2004), Cape Town, South Africa,
15-20 September 200
A statistical model analysis of fluctuations in heavy ion collisions
We briefly describe two statistical hadronization models, based respectively
on the presence and absence of light quark chemical equilibrium, used to
analyze particle yields in heavy ion collisions. We then try to distinguish
between these models using fluctuations data. We find that while the
non-equilibrium model provides an acceptable description of fluctuations at top
SPS and RHIC energies, both models considerably under-estimate fluctuations at
low SPS energies.Comment: References updated Poster in QM2006 conference, Shangha
Particle production in p-p collisions and prediction for LHC energy
We analyze recent data on particle production yields obtained in p-p
collisions at SPS and RHIC energies within the statistical model. We apply the
model formulated in the canonical ensemble and focus on strange particle
production. We introduce different methods to account for strangeness
suppression effects and discuss their phenomenological verification. We show
that at RHIC the midrapidity data on strange and multistrange particle
multiplicity can be successfully described by the canonical statistical model
with and without an extra suppression effects. On the other hand, SPS data
integrated over the full phase-space require an additional strangeness
suppression factor that is beyond the conventional canonical model. This factor
is quantified by the strangeness saturation parameter or strangeness
correlation volume. Extrapolating all relevant thermal parameters from SPS and
RHIC to LHC energy we present predictions of the statistical model for particle
yields in p-p collisions at sqrt(s) = 14TeV. We discuss the role and the
influence of a strangeness correlation volume on particle production in p-p
collisions at LHC.Comment: 6 pages, 6 figures, 4 tables, submitted to Phys. Rev.
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