1,552 research outputs found
Study of Chromium-Frit-Type Coatings for High-Temperature Protection of Molybdenum
The achievement of more compact and efficient power plants for aircraft is dependent, among other factors, on the perfection of heat-resisting materials that are superior to those in current use. Molybdenum is one of the high-melting metals (melting point, 4750 F). It is fairly abundant and also can be worked into many of the shapes required in modern power plants. To permit its widespread use at elevated temperatures, however, some means must first be found to prevent its rapid oxidation. The application of a protective coating is one method that might be used to achieve this goal. In the present work, a number of chromium-frit-type coatings were studied. These were bonded to molybdenum specimens by firing in controlled atmospheres to temperatures in the range of 2400 to 2700 F
Finger patterns produced by thermomagnetic instability in superconductors
A linear analysis of thermal diffusion and Maxwell equations is applied to
study the thermomagnetic instability in a type-II superconducting slab. It is
shown that the instability can lead to formation of spatially nonuniform
distributions of magnetic field and temperature. The distributions acquire a
finger structure with fingers perpendicular to the screening current direction.
We derive the criterion for the instability, and estimate its build-up time and
characteristic finger width. The fingering instability emerges when the
background electric field is larger than a threshold field, , and the
applied magnetic field exceeds a value . Numerical
simulations support the analytical results, and allow to follow the development
of the fingering instability beyond the linear regime. The fingering
instability may be responsible for the nucleation of dendritic flux patterns
observed in superconducting films using magneto-optical imaging.Comment: 8 pages, 6 figures, accepted to Phys. Rev. B; (new version: minor
changes
On the Equation of State of Nuclear Matter in 158A GeV Pb+Pb Collisions
Within a hydrodynamical approach we investigate the sensitivity of single
inclusive momentum spectra of hadrons in 158A GeV Pb+Pb collisions to three
different equations of state of nuclear matter. Two of the equations of state
are based on lattice QCD results and include a phase transition to a
quark-gluon plasma. The third equation of state has been extracted from the
microscopic transport code RQMD under the assumption of complete local
thermalization. All three equations of state provide reasonable fits to data
taken by the NA44 and NA49 Collaborations. The initial conditions before the
evolution of the fireballs and the space-time evolution pictures differ
dramatically for the three equations of state when the same freeze-out
temperature is used in all calculations. However, the softest of the equations
of state results in transverse mass spectra that are too steep in the central
rapidity region. We conclude that the transverse particle momenta are
determined by the effective softness of the equation of state during the
fireball expansion.Comment: 4 pages, including 4 figures and 2 tables. For a PostScript file of
the manuscript, you can also goto http://t2.lanl.gov/schlei/eprint.htm
Realistic Expanding Source Model for Invariant One-Particle Multiplicity Distributions and Two-Particle Correlations in Relativistic Heavy-Ion Collisions
We present a realistic expanding source model with nine parameters that are
necessary and sufficient to describe the main physics occuring during
hydrodynamical freezeout of the excited hadronic matter produced in
relativistic heavy-ion collisions. As a first test of the model, we compare it
to data from central Si + Au collisions at p_lab/A = 14.6 GeV/c measured in
experiment E-802 at the AGS. An overall chi-square per degree of freedom of
1.055 is achieved for a fit to 1416 data points involving invariant pi^+, pi^-,
K^+, and K^- one-particle multiplicity distributions and pi^+ and K^+
two-particle correlations. The 99-percent-confidence region of parameter space
is identified, leading to one-dimensional error estimates on the nine fitted
parameters and other calculated physical quantities. Three of the most
important results are the freezeout temperature, longitudinal proper time, and
baryon density along the symmetry axis. For these we find values of 92.9 +/-
4.4 MeV, 8.2 +/- 2.2 fm/c, and 0.0222 + 0.0096 / - 0.0069 fm^-3, respectively.Comment: 37 pages and 12 figures. RevTeX 3.0. Submitted to Physical Review C.
Complete preprint, including device-independent (dvi), PostScript, and LaTeX
versions of the text, plus PostScript files of all figures, are available at
http://t2.lanl.gov/publications/publications.html or at
ftp://t2.lanl.gov/publications/res
Graviton Cosmology in Universal Extra Dimensions
In models of universal extra dimensions, gravity and all standard model
fields propagate in the extra dimensions. Previous studies of such models have
concentrated on the Kaluza-Klein (KK) partners of standard model particles.
Here we determine the properties of the KK gravitons and explore their
cosmological implications. We find the lifetimes of decays to KK gravitons, of
relevance for the viability of KK gravitons as dark matter. We then discuss the
primordial production of KK gravitons after reheating. The existence of a tower
of KK graviton states makes such production extremely efficient: for reheat
temperature T_RH and d extra dimensions, the energy density stored in gravitons
scales as T_RH^{2+3d/2}. Overclosure and Big Bang nucleosynthesis therefore
stringently constrain T_RH in all universal extra dimension scenarios. At the
same time, there is a window of reheat temperatures low enough to avoid these
constraints and high enough to generate the desired thermal relic density for
KK WIMP and superWIMP dark matter.Comment: 19 pages, 1 figur
Perturbative QCD Analysis of the Nucleon's Pauli Form Factor F_2(Q^2)
We perform a perturbative QCD analysis of the nucleon's Pauli form factor
in the asymptotically large limit. We find that the leading
contribution to has a power behavior, consistent with the
well-known result in the literature. Its coefficient depends on the leading-
and subleading-twist light-cone wave functions of the nucleon, the latter
describing the quarks with one unit of orbital angular momentum. We also derive
at the logarithmic accurary the asymptotic scaling which describes recent Jefferson Lab data well.Comment: 4 papes, 3 figures include
Applicability of Perturbative QCD to Decays
We examine the applicability of perturbative QCD to meson decays into
mesons. We find that the perturbative QCD formalism, which includes Sudakov
effects at intermediate energy scales, is applicable to the semi-leptonic decay
, when the meson recoils fast. Following this conclusion, we
analyze the two-body non-leptonic decays and . By
comparing our predictions with experimental data, we extract the matrix element
.Comment: 18 pages in Latex, figures are available upon reques
Probing the equation of state in the AGS energy range with 3-d hydrodynamics
The effect of (i) the phase transition between a quark gluon plasma (QGP) and
a hadron gas and (ii) the number of resonance degrees of freedom in the
hadronic phase on the single inclusive distributions of 16 different types of
produced hadrons for Au+Au collisions at AGS energies is studied.
We have used an exact numerical solution of the relativistic hydrodynamical
equations without free parameters which, because of its 3-d character,
constitutes a considerable improvement over the classical Landau solution.
Using two different equations of state (eos) - one containing a phase
transition from QGP to the Hadronic Phase and two versions of a purely hadronic
eos - we find that the first one gives an overall better description of the
Au+Au experimental data at energies.
We reproduce and analyse measured meson and proton spectra and also make
predictions for anti-protons, deltas, anti-deltas and hyperons. The low m_t
enhancement in pi- spectra is explained by baryon number conservation and
strangeness equilibration.
We also find that negative kaon data are more sensitive to the eos, as well
as the K-/pi- ratio. All hyperons and deltas are sensitive to the presence of a
phase transition in the forward rapidity region. Anti-protons, Omegas and heavy
anti-baryons are sensitive in the whole rapidity range.Comment: 25 pages (.tex) and 9 figures (.ps
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