8,739 research outputs found
Constrained fitting of three-point functions
We determine matrix elements for semileptonic decay. The use of the
constrained fitting method and multiple smearings for both two- and three-point
correlators allows an improved calculation of the form factors.Comment: Talk given at Lattice2001(heavyquark), 3 pages, 4 figure
Efficient estimation of Banach parameters in semiparametric models
Consider a semiparametric model with a Euclidean parameter and an
infinite-dimensional parameter, to be called a Banach parameter. Assume: (a)
There exists an efficient estimator of the Euclidean parameter. (b) When the
value of the Euclidean parameter is known, there exists an estimator of the
Banach parameter, which depends on this value and is efficient within this
restricted model. Substituting the efficient estimator of the Euclidean
parameter for the value of this parameter in the estimator of the Banach
parameter, one obtains an efficient estimator of the Banach parameter for the
full semiparametric model with the Euclidean parameter unknown. This hereditary
property of efficiency completes estimation in semiparametric models in which
the Euclidean parameter has been estimated efficiently. Typically, estimation
of both the Euclidean and the Banach parameter is necessary in order to
describe the random phenomenon under study to a sufficient extent. Since
efficient estimators are asymptotically linear, the above substitution method
is a particular case of substituting asymptotically linear estimators of a
Euclidean parameter into estimators that are asymptotically linear themselves
and that depend on this Euclidean parameter. This more general substitution
case is studied for its own sake as well, and a hereditary property for
asymptotic linearity is proved.Comment: Published at http://dx.doi.org/10.1214/009053604000000913 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Fidelity threshold for long-range entanglement in quantum networks
We present a strategy to generate long-range entanglement in noisy quantum
networks. We consider a cubic lattice whose bonds are partially entangled mixed
states of two qubits, and where quantum operations can be applied perfectly at
the nodes. In contrast to protocols designed for one- or two-dimensional
regular lattices, we find that entanglement can be created between arbitrarily
distant qubits if the fidelity of the bonds is higher than a critical value,
independent of the system size. Therefore, we show that a constant overhead of
local resources, together with connections of finite fidelity, is sufficient to
achieve long-distance quantum communication in noisy networks.Comment: published versio
Asymptotically conical Calabi-Yau manifolds, I
This is the first part in a two-part series on complete Calabi-Yau manifolds
asymptotic to Riemannian cones at infinity. We begin by proving general
existence and uniqueness results. The uniqueness part relaxes the decay
condition needed in earlier work to ,
relying on some new ideas about harmonic functions. We then look at a few
examples: (1) Crepant resolutions of cones. This includes a new class of
Ricci-flat small resolutions associated with flag manifolds. (2) Affine
deformations of cones. One focus here is the question of the precise rate of
decay of the metric to its tangent cone. We prove that the optimal rate for the
Stenzel metric on is .Comment: 27 pages, various corrections, final versio
Structure and Phase Transitions of Alkyl Chains on Mica
We use molecular dynamics as a tool to understand the structure and phase
transitions [Osman et. al. J. Phys. Chem. B 2000, 104, 4433; 2002, 106, 653] in
alkylammonium micas. The consistent force field 91 is extended for accurate
simulation of mica and related minerals. We investigate mica sheets with 12
octadecyltrimethylammonium (C18) ions or 12 dioctadecyldimethylammonium (2C18)
ions, respectively, as single and layered structures at different temperatures
with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside
preferably above the cavities in the mica surface with an aluminum-rich
boundary. The nitrogen atoms are 380 to 390 pm distant to the superficial
silicon-aluminum plane. With increasing temperature, rearrangements of C18 ions
on the mica surface are found, while 2C18 ions remain tethered due to geometric
restraints. We present basal-plane spacings in the duplicate structures, tilt
angles of the alkyl chains, and gauche-trans ratios to analyze the chain
conformation. Also, the individual phase transitions of the two systems on
heating are explained. Where experimental data are available, the agreement is
very good. We propose a geometric parameter lamba for the saturation of the
surface with alkyl chains, which determines the preferred self-assembly
pattern, i.e., islands, intermediate, or continuous. Lambda also determines the
tilt angles in continuous layers on mica or other surfaces. The thermal
decomposition appears to be a Hofmann elimination with mica as a base-template.Comment: 45 pages with 6 tables and 5 figure
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