11,774 research outputs found
Comment on "Light-front Schwinger model at finite temperature"
In a recent paper by A. Das and X. Zhou [Phys. Rev. D 68, 065017 (2003)] it
is claimed that explicit evaluation of the thermal photon self-energy in the
Schwinger model gives off-shell thermal Green functions that are different in
light-front and conventional quantizations. We show that the claimed difference
originates from an erroneous simplification of the fermion propagator used in
the light-front calculation.Comment: 8 pages, revtex4, added section refuting the massless limit proposed
in hep-th/031102
Mirror Symmetry, Mirror Map and Applications to Complete Intersection Calabi-Yau Spaces
We extend the discussion of mirror symmetry, Picard-Fuchs equations,
instanton-corrected Yukawa couplings, and the topological one-loop partition
function to the case of complete intersections with higher-dimensional moduli
spaces. We will develop a new method of obtaining the instanton-corrected
Yukawa couplings through a close study of the solutions of the Picard-Fuchs
equations. This leads to closed formulas for the prepotential for the K\"ahler
moduli fields induced from the ambient space for all complete intersections in
non singular weighted projective spaces. As examples we treat part of the
moduli space of the phenomenologically interesting three-generation models that
are found in this class. We also apply our method to solve the simplest model
in which a topology change was observed and discuss examples of complete
intersections in singular ambient spaces.Comment: 50 page
Finite difference modeling of rotor flows including wake effects
Rotary wing finite difference methods are investigated. The main concern is the specification of boundary conditions to properly account for the effect of the wake on the blade. Examples are given of an approach where wake effects are introduced by specifying an equivalent angle of attack. An alternate approach is also given where discrete vortices are introduced into the finite difference grid. The resulting computations of hovering and high advance ratio cases compare well with experiment. Some consideration is also given to the modeling of low to moderate advance ratio flows
Consistency and convergence rate of phylogenetic inference via regularization
It is common in phylogenetics to have some, perhaps partial, information
about the overall evolutionary tree of a group of organisms and wish to find an
evolutionary tree of a specific gene for those organisms. There may not be
enough information in the gene sequences alone to accurately reconstruct the
correct "gene tree." Although the gene tree may deviate from the "species tree"
due to a variety of genetic processes, in the absence of evidence to the
contrary it is parsimonious to assume that they agree. A common statistical
approach in these situations is to develop a likelihood penalty to incorporate
such additional information. Recent studies using simulation and empirical data
suggest that a likelihood penalty quantifying concordance with a species tree
can significantly improve the accuracy of gene tree reconstruction compared to
using sequence data alone. However, the consistency of such an approach has not
yet been established, nor have convergence rates been bounded. Because
phylogenetics is a non-standard inference problem, the standard theory does not
apply. In this paper, we propose a penalized maximum likelihood estimator for
gene tree reconstruction, where the penalty is the square of the
Billera-Holmes-Vogtmann geodesic distance from the gene tree to the species
tree. We prove that this method is consistent, and derive its convergence rate
for estimating the discrete gene tree structure and continuous edge lengths
(representing the amount of evolution that has occurred on that branch)
simultaneously. We find that the regularized estimator is "adaptive fast
converging," meaning that it can reconstruct all edges of length greater than
any given threshold from gene sequences of polynomial length. Our method does
not require the species tree to be known exactly; in fact, our asymptotic
theory holds for any such guide tree.Comment: 34 pages, 5 figures. To appear on The Annals of Statistic
On the convergence of the maximum likelihood estimator for the transition rate under a 2-state symmetric model
Maximum likelihood estimators are used extensively to estimate unknown
parameters of stochastic trait evolution models on phylogenetic trees. Although
the MLE has been proven to converge to the true value in the independent-sample
case, we cannot appeal to this result because trait values of different species
are correlated due to shared evolutionary history. In this paper, we consider a
-state symmetric model for a single binary trait and investigate the
theoretical properties of the MLE for the transition rate in the large-tree
limit. Here, the large-tree limit is a theoretical scenario where the number of
taxa increases to infinity and we can observe the trait values for all species.
Specifically, we prove that the MLE converges to the true value under some
regularity conditions. These conditions ensure that the tree shape is not too
irregular, and holds for many practical scenarios such as trees with bounded
edges, trees generated from the Yule (pure birth) process, and trees generated
from the coalescent point process. Our result also provides an upper bound for
the distance between the MLE and the true value
Tunable temperature induced magnetization jump in a GdVO3 single crystal
We report a novel feature of the temperature induced magnetization jump
observed along the a-axis of the GdVO3 single crystal at temperature TM = 0.8
K. Below TM, the compound shows no coercivity and remanent magnetization
indicating a homogenous antiferromagnetic structure. However, we will
demonstrate that the magnetic state below TM is indeed history dependent and it
shows up in different jumps in the magnetization only when warming the sample
through TM. Such a magnetic memory effect is highly unusual and suggesting
different domain arrangements in the supposedly homogenous antiferromagnetic
phase of the compound.Comment: 17 pages, 8 Figure
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