281 research outputs found
Asperisporium and Pantospora (Mycosphaerellaceae): epitypifications and phylogenetic placement
The species-rich family Mycosphaerellaceae contains considerable morphological diversity and includes numerous anamorphic genera, many of which are economically important plant pathogens. Recent revisions and phylogenetic research have resulted in taxonomic instability. Ameliorating this problem requires phylogenetic placement of type species of key genera. We present an examination of the type species of the anamorphic Asperisporium and Pantospora. Cultures isolated from recent port interceptions were studied and described, and morphological studies were made of historical and new herbarium specimens. DNA sequence data from the ITS region and nLSU were generated from these type species, analysed phylogenetically, placed into an evolutionary context within Mycosphaerellaceae, and compared to existing phylogenies. Epitype specimens associated with living cultures and DNA sequence data are designated herein. Asperisporium caricae, the type of Asperisporium and cause of a leaf and fruit spot disease of papaya, is closely related to several species of Passalora including P. brachycarpa. The status of Asperisporium as a potential generic synonym of Passalora remains unclear. The monotypic genus Pantospora, typified by the synnematous Pantospora guazumae, is not included in Pseudocercospora sensu stricto or sensu lato. Rather, it represents a distinct lineage in the Mycosphaerellaceae in an unresolved position near Mycosphaerella microsora
Anomalous dimension of non-singlet quark currents at O(1/Nf^2) in QCD
We compute the O(1/Nf^2) corrections to the flavour non-singlet quark
bilinear currents in QCD in arbitrary spacetime dimensions. Hence, the
anomalous dimension of the QED current is
deduced at four loops in the MSbar scheme up to one unknown parameter.Comment: 8 latex page
Permeability of compacting porous lavas
The highly transient nature of outgassing commonly observed at volcanoes is in part controlled
by the permeability of lava domes and shallow conduits. Lava domes generally consist of a porous outer
carapace surrounding a denser lava core with internal shear zones of variable porosity. Here we examine
densification using uniaxial compression experiments on variably crystalline and porous rhyolitic dome lavas
from the Taupo Volcanic Zone. Experiments were conducted at 900°C and an applied stress of 3MPa to 60%
strain, while monitoring acoustic emissions to track cracking. The evolution of the porous network was
assessed via X-ray computed tomography, He-pycnometry, and relative gas permeability. High starting
connected porosities led to low apparent viscosities and high strain rates, initially accompanied by abundant
acoustic emissions. As compaction ensued, the lavas evolved; apparent viscosity increased and strain rate
decreased due to strain hardening of the suspensions. Permeability fluctuations resulted from the interplay
between viscous flow and brittle failure. Where phenocrysts were abundant, cracks had limited spatial
extent, and pore closure decreased axial and radial permeability proportionally, maintaining the initial
anisotropy. In crystal-poor lavas, axial cracks had a more profound effect, and permeability anisotropy
switched to favor axial flow. Irrespective of porosity, both crystalline samples compacted to a threshold
minimum porosity of 17–19%, whereas the crystal-poor sample did not achieve its compaction limit. This
indicates that unconfined loading of porous dome lavas does not necessarily form an impermeable plug
and may be hindered, in part by the presence of crystals
Big Corrections from a Little Higgs
We calculate the tree-level expressions for the electroweak precision
observables in the SU(5)/SO(5) littlest Higgs model. The source for these
corrections are the exchange of heavy gauge bosons, explicit corrections due to
non-linear sigma-model dynamics and a triplet Higgs VEV. Weak isospin violating
contributions are present because there is no custodial SU(2) global symmetry.
The bulk of these weak isospin violating corrections arise from heavy gauge
boson exchange while a smaller contribution comes from the triplet Higgs VEV. A
global fit is performed to the experimental data and we find that throughout
the parameter space the symmetry breaking scale is bounded by f > 4 TeV at 95%
C.L. Stronger bounds on f are found for generic choices of the high energy
gauge couplings. We find that even in the best case scenario one would need
fine tuning of less than a percent to get a Higgs mass as light as 200 GeV.Comment: 20 pages, 5 figures included, typos fixed, comments on the effects of
extra vector-like heavy fermions adde
A Gaussian Theory of Superfluid--Bose-Glass Phase Transition
We show that gaussian quantum fluctuations, even if infinitesimal, are
sufficient to destroy the superfluidity of a disordered boson system in 1D and
2D. The critical disorder is thus finite no matter how small the repulsion is
between particles. Within the gaussian approximation, we study the nature of
the elementary excitations, including their density of states and mobility edge
transition. We give the gaussian exponent at criticality in 1D and show
that its ratio to of the pure system is universal.Comment: Revtex 3.0, 11 pages (4 figures will be sent through airmail upon
request
Infrared Features of the Landau Gauge QCD
The infrared features of Landau gauge QCD are studied by the lattice
simulation of and . We
adopt two definitions of the gauge field; 1) linear 2) and
measured the gluon propagator and ghost propagator. Infrared singularity of the
gluon propagator is less than that of tree level result but the gluon
propagator at 0 momentum remains finite. The infrared singularity of ghost
propagator is stronger than the tree level. The QCD running coupling measured
by using the gluon propagator and the ghost propagator has a maximum
at around and decreases as approaches 0.
The data are analyzed in use of formula of the principle of minimal
sensitivity(PMS), the effective charge method and the contour-improved
perturbation method, which suggest necessity of the resummation of perturbation
series in the infrared region together with existence of the infrared fixed
point. Kugo-Ojima parameter saturates at about -0.8 in contrast to the
theoretically expected value -1.Comment: RevTex4, 9 pages, 10 eps figures, Typos corrected. To be published in
Phys. Rev. D(2004
Disordered Boson Systems: A Perturbative Study
A hard-core disordered boson system is mapped onto a quantum spin 1/2
XY-model with transverse random fields. It is then generalized to a system of
spins with an arbitrary magnitude S and studied through a 1/S expansion. The
first order 1/S expansion corresponds to a spin-wave theory. The effect of weak
disorder is studied perturbatively within such a first order 1/S scheme. We
compute the reduction of the speed of sound and the life time of the Bloch
phonons in the regime of weak disorder. Generalizations of the present study to
the strong disordered regime are discussed.Comment: 27 pages, revte
LERW as an example of off-critical SLEs
Two dimensional loop erased random walk (LERW) is a random curve, whose
continuum limit is known to be a Schramm-Loewner evolution (SLE) with parameter
kappa=2. In this article we study ``off-critical loop erased random walks'',
loop erasures of random walks penalized by their number of steps. On one hand
we are able to identify counterparts for some LERW observables in terms of
symplectic fermions (c=-2), thus making further steps towards a field theoretic
description of LERWs. On the other hand, we show that it is possible to
understand the Loewner driving function of the continuum limit of off-critical
LERWs, thus providing an example of application of SLE-like techniques to
models near their critical point. Such a description is bound to be quite
complicated because outside the critical point one has a finite correlation
length and therefore no conformal invariance. However, the example here shows
the question need not be intractable. We will present the results with emphasis
on general features that can be expected to be true in other off-critical
models.Comment: 45 pages, 2 figure
Four loop wave function renormalization in the non-abelian Thirring model
We compute the anomalous dimension of the fermion field with N_f flavours in
the fundamental representation of a general Lie colour group in the non-abelian
Thirring model at four loops. The implications on the renormalization of the
two point Green's function through the loss of multiplicative renormalizability
of the model in dimensional regularization due to the appearance of evanescent
four fermi operators are considered at length. We observe the appearance of one
new colour group Casimir, d_F^{abcd} d_F^{abcd}, in the final four loop result
and discuss its consequences for the relation of the Knizhnik-Zamolodchikov
critical exponents in the Wess Zumino Witten Novikov model to the non-abelian
Thirring model. Renormalization scheme changes are also considered to ensure
that the underlying Fierz symmetry broken by dimensional regularization is
restored.Comment: 25 latex pages with 9 postscript figure
Semiparametric theory and empirical processes in causal inference
In this paper we review important aspects of semiparametric theory and
empirical processes that arise in causal inference problems. We begin with a
brief introduction to the general problem of causal inference, and go on to
discuss estimation and inference for causal effects under semiparametric
models, which allow parts of the data-generating process to be unrestricted if
they are not of particular interest (i.e., nuisance functions). These models
are very useful in causal problems because the outcome process is often complex
and difficult to model, and there may only be information available about the
treatment process (at best). Semiparametric theory gives a framework for
benchmarking efficiency and constructing estimators in such settings. In the
second part of the paper we discuss empirical process theory, which provides
powerful tools for understanding the asymptotic behavior of semiparametric
estimators that depend on flexible nonparametric estimators of nuisance
functions. These tools are crucial for incorporating machine learning and other
modern methods into causal inference analyses. We conclude by examining related
extensions and future directions for work in semiparametric causal inference
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