299 research outputs found
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Analysis of acoustic emission during the melting of embedded indium particles in an aluminum matrix: a study of plastic strain accommodation during phase transformation
Acoustic emission is used here to study melting and solidification of
embedded indium particles in the size range of 0.2 to 3 um in diameter and to
show that dislocation generation occurs in the aluminum matrix to accommodate a
2.5% volume change. The volume averaged acoustic energy produced by indium
particle melting is similar to that reported for bainite formation upon
continuous cooling. A mechanism of prismatic loop generation is proposed to
accommodate the volume change and an upper limit to the geometrically necessary
increase in dislocation density is calculated as 4.1 x 10^9 cm^-2 for the
Al-17In alloy. Thermomechanical processing is also used to change the size and
distribution of the indium particles within the aluminum matrix. Dislocation
generation with accompanied acoustic emission occurs when the melting indium
particles are associated with grain boundaries or upon solidification where the
solid-liquid interfaces act as free surfaces to facilitate dislocation
generation. Acoustic emission is not observed for indium particles that require
super heating and exhibit elevated melting temperatures. The acoustic emission
work corroborates previously proposed relaxation mechanisms from prior internal
friction studies and that the superheat observed for melting of these
micron-sized particles is a result of matrix constraint.Comment: Presented at "Atomistic Effects in Migrating Interphase Interfaces -
Recent Progress and Future Study" TMS 201
Is weak temperature dependence of electron dephasing possible?
The first-principle theory of electron dephasing by disorder-induced two
state fluctuators is developed. There exist two mechanisms of dephasing. First,
dephasing occurs due to direct transitions between the defect levels caused by
inelastic electron-defect scattering. The second mechanism is due to violation
of the time reversal symmetry caused by time-dependent fluctuations of the
scattering potential. These fluctuations originate from an interaction between
the dynamic defects and conduction electrons forming a thermal bath. The first
contribution to the dephasing rate saturates as temperature decreases. The
second contribution does not saturate, although its temperature dependence is
rather weak, . The quantitative estimates based on the
experimental data show that these mechanisms considered can explain the weak
temperature dependence of the dephasing rate in some temperature interval.
However, below some temperature dependent on the model of dynamic defects the
dephasing rate tends rapidly to zero. The relation to earlier studies of the
dephasing caused by the dynamical defects is discussed.Comment: 14 pages, 6 figures, submitted to PR
Low-temperature co-sintering for fabrication of zirconia/ceria bi-layer electrolyte via tape casting using a Fe2O3 sintering aid
Bilayer electrolytes have potential in solid oxide cells to improve ionic conduction whilst blocking electronic
conduction. GDC/YSZ bilayer electrolyte processinghas provenproblematic due to thermochemical
instability at high sintering temperatures. We first match the shrinkage profile of the two bulk materials
using a Fe2O3 sintering additive. Additions of 5 mol% of Fe2O3 in the GDC layer and 2 mol% of Fe2O3 in
the YSZ layer prevents delamination during co-sintering. The addition of Fe2O3 promotes densification,
enabling achievement of a dense bilayer at a reduced sintering temperature of 1300 ◦C; ∼150 ◦C below
conventional sintering temperatures. Elemental analysis showed the compositional distribution curves
across the bilayer interface to be asymmetric when Fe2O3 is employed. The Fe2O3 increases the total
conductivity of the bilayer electrolyte by an order of magnitude; this is explained by the effect of Fe2O3
on reducing the resistive solid solution interlayer at YSZ/GDC interface from ∼15 to ∼5 m
Measurement of the p-pbar -> Wgamma + X cross section at sqrt(s) = 1.96 TeV and WWgamma anomalous coupling limits
The WWgamma triple gauge boson coupling parameters are studied using p-pbar
-> l nu gamma + X (l = e,mu) events at sqrt(s) = 1.96 TeV. The data were
collected with the DO detector from an integrated luminosity of 162 pb^{-1}
delivered by the Fermilab Tevatron Collider. The cross section times branching
fraction for p-pbar -> W(gamma) + X -> l nu gamma + X with E_T^{gamma} > 8 GeV
and Delta R_{l gamma} > 0.7 is 14.8 +/- 1.6 (stat) +/- 1.0 (syst) +/- 1.0 (lum)
pb. The one-dimensional 95% confidence level limits on anomalous couplings are
-0.88 < Delta kappa_{gamma} < 0.96 and -0.20 < lambda_{gamma} < 0.20.Comment: Submitted to Phys. Rev. D Rapid Communication
Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt{s} = 1.96 TeV using Kinematic Characteristics of Lepton + Jets Events
We present a measurement of the top quark pair ttbar production cross section
in ppbar collisions at a center-of-mass energy of 1.96 TeV using 230 pb**{-1}
of data collected by the DO detector at the Fermilab Tevatron Collider. We
select events with one charged lepton (electron or muon), large missing
transverse energy, and at least four jets, and extract the ttbar content of the
sample based on the kinematic characteristics of the events. For a top quark
mass of 175 GeV, we measure sigma(ttbar) = 6.7 {+1.4-1.3} (stat) {+1.6- 1.1}
(syst) +/-0.4 (lumi) pb, in good agreement with the standard model prediction.Comment: submitted to Phys.Rev.Let
Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt(s)=1.96 TeV using Lepton + Jets Events with Lifetime b-tagging
We present a measurement of the top quark pair () production cross
section () in collisions at TeV
using 230 pb of data collected by the D0 experiment at the Fermilab
Tevatron Collider. We select events with one charged lepton (electron or muon),
missing transverse energy, and jets in the final state. We employ
lifetime-based b-jet identification techniques to further enhance the
purity of the selected sample. For a top quark mass of 175 GeV, we
measure pb, in
agreement with the standard model expectation.Comment: 7 pages, 2 figures, 3 tables Submitted to Phys.Rev.Let
Search for W' bosons decaying to an electron and a neutrino with the D0 detector
This Letter describes the search for a new heavy charged gauge boson W'
decaying into an electron and a neutrino. The data were collected with the D0
detector at the Fermilab Tevatron proton-antiproton Collider at a
center-of-mass energy of 1.96 TeV, and correspond to an integrated luminosity
of about 1 inverse femtobarn. Lacking any significant excess in the data in
comparison with known processes, an upper limit is set on the production cross
section times branching fraction, and a W' boson with mass below 1.00 TeV can
be excluded at the 95% C.L., assuming standard-model-like couplings to
fermions. This result significantly improves upon previous limits, and is the
most stringent to date.Comment: submitted to Phys. Rev. Let
Measurement of the ratios of the Z/G* + >= n jet production cross sections to the total inclusive Z/G* cross section in ppbar collisions at sqrt(s) = 1.96 TeV
We present a study of events with Z bosons and jets produced at the Fermilab
Tevatron Collider in ppbar collisions at a center of mass energy of 1.96 TeV.
The data sample consists of nearly 14,000 Z/G* -> e+e- candidates corresponding
to the integrated luminosity of 0.4 fb-1 collected using the D0 detector.
Ratios of the Z/G* + >= n jet cross sections to the total inclusive Z/G* cross
section have been measured for n = 1 to 4 jet events. Our measurements are
found to be in good agreement with a next-to-leading order QCD calculation and
with a tree-level QCD prediction with parton shower simulation and
hadronization.Comment: 7 pages, 2 figures, slightly modified, submitted to Phys. Lett.
Measurement of the Isolated Photon Cross Section in p-pbar Collisions at sqrt{s}=1.96 TeV
The cross section for the inclusive production of isolated photons has been
measured in p anti-p collisions at sqrt{s}=1.96 TeV with the D0 detector at the
Fermilab Tevatron Collider. The photons span transverse momenta 23 to 300 GeV
and have pseudorapidity |eta|<0.9. The cross section is compared with the
results from two next-to-leading order perturbative QCD calculations. The
theoretical predictions agree with the measurement within uncertainties.Comment: 7 pages, 5 figures, submitted to Phys.Lett.
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