216 research outputs found
Next-to-leading-order temperature corrections to correlators in QCD
Corrections of order to vector and axial current correlators in QCD at
a finite temperature are obtained using dispersion relations for the
amplitudes of deep inelastic scattering on pions. Their relation with the
operator product expansion is presented. An interpretation of the results in
terms of -dependent meson masses is given: masses of and start
to move with temperature in order .Comment: 13 pages, no figures, CERN-TH.7215/94, BUTP-94/
Thermally stable composite system Al2O3-Ce 0.75Zr0.25O2 for automotive three-way catalysts
Present-day three-way catalysts operate in contact with exhaust gases whose temperature is as high as >1000 C, so the problem of developing thermally stable catalytic compositions is still topical. A series of Al2O 3-Ce0.75Zr0.25O2 composites containing 0, 10, 25, and 50 wt % Al2O3 has been synthesized by direct precipitation. The as-prepared composites and those calcined in air at 1000 and 1100 C have been characterized by BET, X-ray diffraction, transmission electron microscopy, and temperature-programmed reduction methods. The composites aged at 1050 C in a 2% O2 + 10% H2O + 88% N2 atmosphere have been used to prepare monolith catalysts, and the oxygen storage capacity (OSC) of the latter has been measured using a gas analysis setup. As the proportion of Al2O 3 in the composite is raised, the mixing uniformity and degree of dispersion of Ce x Zr1-x O2-δ particles increase, their chemical composition becomes homogeneous, and the amount of cerium involved in oxidation and reduction increases. The composite containing 50 wt % Al2O3 is a mixture of Ce x Zr 1-x O2-δ and Al2O3 crystallites, whose size is practically unaffected by calcination. The (Pt/Al2O3 + Al2O3-Ce 0.75Zr0.25O2) based on this composite has the highest OSC and is the most active. For this reason, full-scale testing of this catalyst is recommended. © 2013 Pleiades Publishing, Ltd
Hot Nucleons in Chiral Soliton Models
Chiral lagrangians as effective field theories of QCD are most suitable for
the study of nucleons in a hot pion gas because they contain pions and also
baryons as solitons of the same action. The semiclassical treatment of the
soliton solutions must be augmented by pionic fluctuations which requires
renormalisation to 1-loop, and finite temperatures do not introduce new
ultraviolet divergencies and may easily be considered. Alternatively, a
renormalisation scheme based on the renormalisation group equation at finite
temperature comprises and extends the rigorous results of chiral perturbation
theory and renders the low energy constants temperature-dependent which allows
the construction of temperature-dependent solitons below the critical
temperature. The temperature-dependence of the baryon energy and the
pion-nucleon coupling is studied. There is no simple scaling law for the
temperature-dependence of these quantities.Comment: 17 pages (RevTeX), 5 figure
Skyrmion Multi-Walls
Skyrmion walls are topologically-nontrivial solutions of the Skyrme system
which are periodic in two spatial directions. We report numerical
investigations which show that solutions representing parallel multi-walls
exist. The most stable configuration is that of the square -wall, which in
the limit becomes the cubically-symmetric Skyrme crystal. There is
also a solution resembling parallel hexagonal walls, but this is less stable.Comment: 7 pages, 1 figur
X-ray Emission from Wind Blown Bubbles. III. ASCA SIS Observations of NGC6888
We present ASCA SIS observations of the wind-blown bubble NGC6888. Owing to
the higher sensitivity of the SIS for higher energy photons compared to the
ROSAT PSPC, we are able to detect a T ~ 8x10^6 K plasma component in addition
to the T ~ 1.3x10^6 K component previously detected in PSPC observations. No
significant temperature variations are detected within NGC6888. Garcia-Segura &
Mac Low's (1995) analytical models of WR bubbles constrained by the observed
size, expansion velocity, and mass of the nebular shell under-predict the
stellar wind luminosity, and cannot reproduce simultaneously the observed X-ray
luminosity, spectrum, surface brightness profile, and SIS count rate of
NGC6888's bubble interior. The agreement between observations and expectations
from models can be improved if one or more of the following ad hoc assumptions
are made: (1) the stellar wind luminosity was weaker in the past, (2) the
bubble is at a special evolutionary stage and the nebular shell has recently
been decelerated to 1/2 of its previous expansion velocity, and (3) the heat
conduction between the hot interior and the cool nebular shell is suppressed.
Chandra and XMM-Newton observations with high spatial resolution and high
sensitivity are needed to determine accurately the physical conditions
NGC6888's interior hot gas for critical comparisons with bubble models.Comment: 24 pages, 6 figures; accepted for Astrophysical Journal, Nov 1, 2005
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Computation of the winding number diffusion rate due to the cosmological sphaleron
A detailed quantitative analysis of the transition process mediated by a
sphaleron type non-Abelian gauge field configuration in a static Einstein
universe is carried out. By examining spectra of the fluctuation operators and
applying the zeta function regularization scheme, a closed analytical
expression for the transition rate at the one-loop level is derived. This is a
unique example of an exact solution for a sphaleron model in spacetime
dimensions.Comment: Some style corrections suggested by the referee are introduced
(mainly in Sec.II), one reference added. To appear in Phys.Rev.D 29 pages,
LaTeX, 3 Postscript figures, uses epsf.st
Chiral-symmetry restoration in the linear sigma model at nonzero temperature and baryon density
We study the chiral phase transition in the linear sigma model with 2 quark
flavors and colors. One-loop calculations predict a first-order phase
transition at both and . We also discuss the phase diagram
and make a comparison with a thermal parametrization of existing heavy-ion
experimental data.Comment: 12 pages, 6 ps-figures, LaTe
Radioactive decays at limits of nuclear stability
The last decades brought an impressive progress in synthesizing and studying
properties of nuclides located very far from the beta stability line. Among the
most fundamental properties of such exotic nuclides, usually established first,
is the half-life, possible radioactive decay modes, and their relative
probabilities. When approaching limits of nuclear stability, new decay modes
set in. First, beta decays become accompanied by emission of nucleons from
highly excited states of daughter nuclei. Second, when the nucleon separation
energy becomes negative, nucleons start to be emitted from the ground state.
Here, we present a review of the decay modes occurring close to the limits of
stability. The experimental methods used to produce, identify and detect new
species and their radiation are discussed. The current theoretical
understanding of these decay processes is overviewed. The theoretical
description of the most recently discovered and most complex radioactive
process - the two-proton radioactivity - is discussed in more detail.Comment: Review, 68 pages, 39 figure
Fermion sea along the sphaleron barrier
In this revised version we have improved the treatment of the top and bottom
quark mass. This leads to slight changes of the numerical results, especially
of those presented in Fig.4. The discussion of the numerical procedure and
accuracy has been extended.Comment: 39 pages (LaTex) plus 5 figures (uuencoded postscript files);
RUB-TPII-62/93, to appear in Phys.Rev.
The Effect of Bound Dineutrons upon BBN
We have examined the effects of a bound dineutron, n2, upon big bang
nucleosynthesis (BBN) as a function of its binding energy B_n2. We find a
weakly bound dineutron has little impact but as B_n2 increases its presence
begins to alter the flow of free nucleons to helium-4. Due to this disruption,
and in the absence of changes to other binding energies or fundamental
constants, BBN sets a reliable upper limit of B_n2 <~ 2.5 MeV in order to
maintain the agreement with the observations of the primordial helium-4 mass
fraction and D/H abundance
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