455 research outputs found
Non-factorizable Contributions to Decays
We investigate to what extent the experimental information on
branching fractions and CP asymmetries can be used to better understand the QCD
dynamics in these decays. For this purpose we decompose the independent isospin
amplitudes into factorizable and non-factorizable contributions. The former can
be estimated within the framework of QCD factorization for exclusive
decays. The latter vanish in the heavy-quark limit, , and are
treated as unknown hadronic parameters. We discuss at some length in which way
the non-factorizable contributions are treated in different theoretical and
phenomenological frameworks. We point out the potential differences between the
phenomenological treatment of power-corrections in the ``BBNS approach'', and
the appearance of power -suppressed operators in soft-collinear effective
theory (SCET). On that basis we define a handful of different (but generic)
scenarios where the non-factorizable part of isospin amplitudes is parametrized
in terms of three or four unknowns, which can be constrained by data. We also
give some short discussion on the implications of our analysis for decays. In particular, since non-factorizable QCD effects in
may be large, we cannot exclude sizeable non-factorizable effects, which
violate flavour symmetry, or even isospin symmetry (via long-distance
QED effects). This may help to explain certain puzzles in connection with
isospin-violating observables in decays.Comment: published version, minor correction
Vibrational signatures for low-energy intermediate-sized Si clusters
We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size
A New Non-Perturbative Approach to Quantum Theory in Curved Spacetime Using the Wigner Function
A new non-perturbative approach to quantum theory in curved spacetime and to
quantum gravity, based on a generalisation of the Wigner equation, is proposed.
Our definition for a Wigner equation differs from what have otherwise been
proposed, and does not imply any approximations. It is a completely exact
equation, fully equivalent to the Heisenberg equations of motion. The approach
makes different approximation schemes possible, e.g. it is possible to perform
a systematic calculation of the quantum effects order by order. An iterative
scheme for this is also proposed. The method is illustrated with some simple
examples and applications. A calculation of the trace of the renormalised
energy-momentum tensor is done, and the conformal anomaly is thereby related to
non-conservation of a current in d=2 dimensions and a relationship between a
vector and an axial-vector current in d=4 dimensions.
The corresponding ``hydrodynamic equations'' governing the evolution of
macroscopic quantities are derived by taking appropriate moments. The emphasis
is put on the spin-1/2 case, but it is shown how to extend to arbitrary spins.
Gravity is treated first in the Palatini formalism, which is not very
tractable, and then more successfully in the Ashtekar formalism, where the
constraints lead to infinite order differential equations for the Wigner
functions.Comment: LaTeX2e (uses amssymb), 36 page
Risk factors for high anti-HHV-8 antibody titers (≥1:51,200) in black, HIV-1 negative South African cancer patients: a case control study
Background: Infection with human herpesvirus 8 (HHV-8) is the necessary causal agent in the
development of Kaposi's sarcoma (KS). Infection with HIV-1, male gender and older age all increase
risk for KS. However, the geographic distribution of HHV-8 and KS both prior to the HIV/AIDS
epidemic and with HIV/AIDS suggest the presence of an additional co-factor in the development of
KS.
Methods: Between January 1994 and October 1997, we interviewed 2576 black in-patients with
cancer in Johannesburg and Soweto, South Africa. Blood was tested for antibodies against HIV-1
and HHV-8 and the study was restricted to 2191 HIV-1 negative patients. Antibodies against the
latent nuclear antigen of HHV-8 encoded by orf73 were detected with an indirect
immunofluorescence assay. We examined the relationship between high anti-HHV-8 antibody
titers (≥1:51,200) and sociodemographic and behavioral factors using unconditional logistic
regression models. Variables that were significant at p = 0.10 were included in multivariate analysis.
Results: Of the 2191 HIV-1 negative patients who did not have Kaposi's sarcoma, 854 (39.0%)
were positive for antibodies against HHV-8 according to the immunofluorescent assay. Among
those seropositive for HHV-8, 530 (62.1%) had low titers (1:200), 227 (26.6%) had medium titers
(1:51,200) and 97 (11.4%) had highest titers (1:204,800). Among the 2191 HIV-1 negative patients,
the prevalence of high anti-HHV-8 antibody titers (≥1:51,200) was independently associated with
increasing age (ptrend = 0.04), having a marital status of separated or divorced (p = 0.003), using
wood, coal or charcoal as fuel for cooking 20 years ago instead of electricity (p = 0.02) and
consuming traditional maize beer more than one time a week (p = 0.02; p-trend for increasing
consumption = 0.05) although this may be due to chance given the large number of predictors
considered in this analysis.
Conclusions: Among HIV-negative subjects, patients with high anti-HHV-8 antibody titers are
characterized by older age. Other associations that may be factors in the development of high anti-
HHV-8 titers include exposure to poverty or a low socioeconomic status environment and
consumption of traditional maize beer. The relationship between these variables and high anti-
HHV-8 titers requires further, prospective study
Social preferences, accountability, and wage bargaining
We assess the extent of preferences for employment in a collective wage bargaining situation with heterogeneous workers. We vary the size of the union and introduce a treatment mechanism transforming the voting game into an individual allocation task. Our results show that highly productive workers do not take employment of low productive workers into account when making wage proposals, regardless of whether insiders determine the wage or all workers. The level of pro-social preferences is small in the voting game, while it increases as the game is transformed into an individual allocation task. We interpret this as an accountability effect
The Spitzer Spectroscopic Survey of S-type Stars
S-type AGB stars are thought to be in the transitional phase between M-type
and C-type AGB stars. Because of their peculiar chemical composition, one may
expect a strong influence of the stellar C/O ratio on the molecular chemistry
and the mineralogy of the circumstellar dust. In this paper, we present a large
sample of 87 intrinsic galactic S-type AGB stars, observed at infrared
wavelengths with the Spitzer Space Telescope, and supplemented with
ground-based optical data. On the one hand, we derive the stellar parameters
from the optical spectroscopy and photometry, using a grid of model
atmospheres. On the other, we decompose the infrared spectra to quantify the
flux-contributions from the different dust species. Finally, we compare the
independently determined stellar parameters and dust properties. For the stars
without significant dust emission, we detect a strict relation between the
presence of SiS absorption in the Spitzer spectra and the C/O ratio of the
stellar atmosphere. These absorption bands can thus be used as an additional
diagnostic for the C/O ratio. For stars with significant dust emission, we
define three groups, based on the relative contribution of certain dust species
to the infrared flux. We find a strong link between group-membership and C/O
ratio. We show that these groups can be explained by assuming that the
dust-condensation can be cut short before silicates are produced, while the
remaining free atoms and molecules can then form the observed magnesium
sulfides or the carriers of the unidentified 13 and 20 micron features.
Finally, we present the detection of emission features attributed to molecules
and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons
and magnesium sulfide grains. We show that we often detect magnesium sulfides
together with molecular SiS and we propose that it is formed by a reaction of
SiS molecules with Mg.Comment: Accepted for publication in A&
The Discovery Potential of a Super B Factory
The Proceedings of the 2003 SLAC Workshops on flavor physics with a high
luminosity asymmetric e+e- collider. The sensitivity of flavor physics to
physics beyond the Standard Model is addressed in detail, in the context of the
improvement of experimental measurements and theoretical calculations.Comment: 476 pages. Printed copies may be obtained by request to
[email protected] . arXiv admin note: v2 appears to be identical to v
The Problem of Non-Renewable Energy Resources in the Production of Physical Capital
This paper studies the possibilities of technical progress to deal with the growth limit problem imposed by the usage of non-renewable energy resources, when physical capital production is relatively more energy-intensive than consumption. In particular, this work presents the conditions under which energy-saving technologies can sustain long-run growth, although energy is produced by means of non-renewable energy resources. The mechanism behind that is energy efficiency
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