1,556 research outputs found
The Rashba Hamiltonian and electron transport
The Rashba Hamiltonian describes the splitting of the conduction band as a
result of spin-orbit coupling in the presence of an external field and is
commonly used to model the electronic structure of confined narrow-gap
semiconductors. Due to the mixing of spin states some care has to be exercised
in the calculation of transport properties. We derive the velocity operator for
the Rashba-split conduction band and demonstrate that the transmission of an
interface between a ferromagnet and a Rashba-split semiconductor does not
depend on the magnetization direction, in contrast with previous assertions in
the literature.Comment: one tex file, two figures; paper to appear in this form in PRB (RC
Testing factorization in B -> D(*)X decays
In QCD the amplitude for B0 -> D(*)+pi- factorizes in the large Nc limit or
in the large energy limit Q >> Lambda_QCD where Q = {m_b, m_c, m_b-m_c}. Data
also suggests factorization in exclusive processes B-> D* pi+ pi- pi- pi0 and
B-> D* omega pi-, however by themselves neither large Nc nor large Q can
account for this. Noting that the condition for large energy release in B0-> D+
pi- is enforced by the SV limit, m_b, m_c >> m_b-m_c >> Lambda, we propose that
the combined large Nc and SV limits justify factorization in B -> D(*) X. This
combined limit is tested with the inclusive decay spectrum measured by CLEO. We
also give exact large Nc relations among isospin amplitudes for B -> D(*)X and
B -> D(*) D-bar(*)X, which can be used to test factorization through exclusive
or inclusive measurements. Predictions for the modes B-> D(*) pi pi, B-> D(*)K
K-bar and B-> D(*) D-bar(*) K are discussed using available data.Comment: 15 pages, 3 included .eps figures, minor change
Nonfactorization and the decays and
In six chosen scenarios for the dependence of the form factors involved
in transition, we have determined the allowed domain
of and from the experimentally measured
ratios and in a scheme that uses the value of the phenomenological parameter and includes nonfactorized
contribution. We find that the experimentally measured values of and
from semileptonic decays of favor solutions which have significant
nonfactorized contribution, and, in particular, favors solutions in
scenarios where is either flat or decreasing with .Comment: 15 pages, Latex, four figure (available on request)
Subleading Shape Functions in Inclusive B Decays
The contributions of subleading shape functions to inclusive decay
distributions of B mesons are derived from a systematic two-step matching of
QCD current correlators onto soft-collinear and heavy-quark effective theory.
At tree-level, the results can be expressed in terms of forward matrix elements
of bi-local light-cone operators. Four-quark operators, which arise at O(g^2),
are included. Their effects can be absorbed entirely into a redefinition of
other shape functions. Our results are in disagreement with some previous
studies of subleading shape-function effects in the literature. A numerical
analysis of B->X_u+l+nu decay distributions suggests that power corrections are
small, with the possible exception of the endpoint region of the charged-lepton
energy spectrum.Comment: 22 pages, 2 figures; several typos corrected; version published in
JHE
Time-of-arrival distribution for arbitrary potentials and Wigner's time-energy uncertainty relation
A realization of the concept of "crossing state" invoked, but not
implemented, by Wigner, allows to advance in two important aspects of the time
of arrival in quantum mechanics: (i) For free motion, we find that the
limitations described by Aharonov et al. in Phys. Rev. A 57, 4130 (1998) for
the time-of-arrival uncertainty at low energies for certain mesurement models
are in fact already present in the intrinsic time-of-arrival distribution of
Kijowski; (ii) We have also found a covariant generalization of this
distribution for arbitrary potentials and positions.Comment: 4 pages, revtex, 2 eps figures include
The Multimodal Brain Tumor Image Segmentation Benchmark (BRATS)
In this paper we report the set-up and results of the Multimodal Brain Tumor Image Segmentation Benchmark (BRATS) organized in conjunction with the MICCAI 2012 and 2013 conferences. Twenty state-of-the-art tumor segmentation algorithms were applied to a set of 65 multi-contrast MR scans of low-and high-grade glioma patients-manually annotated by up to four raters-and to 65 comparable scans generated using tumor image simulation software. Quantitative evaluations revealed considerable disagreement between the human raters in segmenting various tumor sub-regions (Dice scores in the range 74%-85%), illustrating the difficulty of this task. We found that different algorithms worked best for different sub-regions (reaching performance comparable to human inter-rater variability), but that no single algorithm ranked in the top for all sub-regions simultaneously. Fusing several good algorithms using a hierarchical majority vote yielded segmentations that consistently ranked above all individual algorithms, indicating remaining opportunities for further methodological improvements. The BRATS image data and manual annotations continue to be publicly available through an online evaluation system as an ongoing benchmarking resource
Nonfactorization and Color-Suppressed Decays
Using value of the parameter but including a modest
nonfactorized amplitude, we show that it is possible to understand all data,
including polarization, for color-suppressed
decays in all commonly used models of form factors. We show that for decay one can define an effective , which is process-dependent and,
in general, complex; but it is not possible to define an effective for
decay. We also explain why nonfactorized amplitudes do not
play a significant role in color-favored B decays.Comment: 13 pages, Latex, one figure (not included
Nonmesonic weak decay spectra of He
To comprehend the recent Brookhaven National Laboratory experiment E788 on
He, we have outlined a simple theoretical framework, based on the
independent-particle shell model, for the one-nucleon-induced nonmesonic weak
decay spectra. Basically, the shapes of all the spectra are tailored by the
kinematics of the corresponding phase space, depending very weakly on the
dynamics, which is gauged here by the one-meson-exchange-potential. In spite of
the straightforwardness of the approach a good agreement with data is acheived.
This might be an indication that the final-state-interactions and the
two-nucleon induced processes are not very important in the decay of this
hypernucleus. We have also found that the exchange potential with soft
vertex-form-factor cutoffs GeV, GeV), is able to account simultaneously for the available experimental
data related to and for H, He,
and He.Comment: 12 pages, 4 figures, 1 table, submitted for publication; v2: major
revision, 18 pages, one author added, table, figures and bibliography change
Final-state interaction and s-quark helicity conservation in B -> J/psi K*
The Section of charm quark spin conservation is deleted since it involves
more dynamical assumptions than previously stated. A few comments are added in
view of new experimental results.Comment: To replace the earlier version of hep-ph/0106354. Minor additions and
one deletion with no change in the main argument nor the conclusio
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