22,248 research outputs found
Effects of memory on the shapes of simple outbreak trees
Genomic tools, including phylogenetic trees derived from sequence data, are increasingly used to understand outbreaks of infectious diseases. One challenge is to link phylogenetic trees to patterns of transmission. Particularly in bacteria that cause chronic infections, this inference is affected by variable infectious periods and infectivity over time. It is known that non-exponential infectious periods can have substantial effects on pathogens’ transmission dynamics. Here we ask how this non-Markovian nature of an outbreak process affects the branching trees describing that process, with particular focus on tree shapes. We simulate Crump-Mode-Jagers branching processes and compare different patterns of infectivity over time. We find that memory (non-Markovian-ness) in the process can have a pronounced effect on the shapes of the outbreak’s branching pattern. However, memory also has a pronounced effect on the sizes of the trees, even when the duration of the simulation is fixed. When the sizes of the trees are constrained to a constant value, memory in our processes has little direct effect on tree shapes, but can bias inference of the birth rate from trees. We compare simulated branching trees to phylogenetic trees from an outbreak of tuberculosis in Canada, and discuss the relevance of memory to this dataset
Heavy Meson Masses in the \epsilon-Regime of HM\chi PT
The pseudoscalar and vector heavy meson masses are calculated in the
\epsilon-regime of Heavy Meson Chiral Perturbation Theory to order \epsilon^4.
The results of this calculation will allow the determination of low-energy
coefficients (LECs) directly from Lattice QCD calculations of the heavy mesons
masses for lattices that satisfy the \epsilon-regime criteria. In particular,
the LECs that parametrize the NLO volume dependance of the heavy meson masses
are necessary for evaluating the light pseudoscalar meson (\pi, K, \eta) and
heavy meson ({D^0, D^+, D^+_s}, {B^-,\bar{B}^0,\bar{B}^0_s}) scattering phase
shifts.Comment: 16 pages, 6 figure
Bounds on Heavy-to-Heavy Mesonic Form Factors
We provide upper and lower bounds on the form factors for B -> D, D^* by
utilizing inclusive heavy quark effective theory sum rules. These bounds are
calculated to leading order in Lambda_QCD/m_Q and alpha_s. The O(alpha_s^2
beta_0) corrections to the bounds at zero recoil are also presented. We compare
our bounds with some of the form factor models used in the literature. All the
models we investigated failed to fall within the bounds for the combination of
form factors (omega^2 - 1)/(4 omega)|omega h_{A2}+h_{A3}|^2.Comment: 27 pages, 10 figure
Model-Independent Semileptonic Form Factors Using Dispersion Relations
We present a method for parametrizing heavy meson semileptonic form factors
using dispersion relations, and from it produce a two-parameter description of
the B -> B elastic form factor. We use heavy quark symmetry to relate this
function to B -> D* l nu form factors, and extract
|V_cb|=0.0355^{+0.0029}_{-0.0025} from experimental data with a least squares
fit. Our method eliminates model-dependent uncertainties inherent in choosing a
parametrization for the extrapolation of the differential decay rate to
threshold.Comment: uses lanlmac(harvmac) and epsf, 12 pages, 1 eps figure included (Talk
by BG at the 6-th International Symposium on Heavy Flavour Physics, Pisa,
Italy, 6--10 June, 1995
New Constraints on Dispersive Form Factor Parameterizations from the Timelike Region
We generalize a recent model-independent form factor parameterization derived
from rigorous dispersion relations to include constraints from data in the
timelike region. These constraints dictate the convergence properties of the
parameterization and appear as sum rules on the parameters. We further develop
a new parameterization that takes into account finiteness and asymptotic
conditions on the form factor, and use it to fit to the elastic \pi
electromagnetic form factor. We find that the existing world sample of timelike
data gives only loose bounds on the form factor in the spacelike region, but
explain how the acquisition of additional timelike data or fits to other form
factors are expected to give much better results. The same parameterization is
seen to fit spacelike data extremely well.Comment: 24 pages, latex (revtex), 3 eps figure
Dynamic method to distinguish between left- and right-handed chiral molecules
We study quantum systems with broken symmetry that can be modelled as cyclic
three-level atoms with coexisting one- and two-photon transitions. They can be
selectively optically excited to any state. As an example, we show that left-
and right-handed chiral molecules starting in the same initial states can
evolve into different final states by a purely dynamic transfer process. That
means, left- and right-handed molecules can be distinguished purely
dynamically.Comment: 4 pages, submitted to Phys. Rev.
Spatial and Temporal Hadron Correlators below and above the Chiral Phase Transition
Hadronic correlation functions at finite temperature in QCD, with four
flavours of dynamical quarks, have been analyzed both above and below the
chiral symmetry restoration temperature. We have used both point and extended
sources for spatial as well as temporal correlators. The effect of periodic
temporal boundary conditions for the valence quarks on the spatial meson
correlators has also been investigated. All our results are consistent with the
existence of individual quarks at high temperatures. A measurement of the
residual interaction between the quarks is presented.Comment: 19 pages HLRZ 54/93, BI-TP 93/76, TIFR/TH/94-1
Wide-bandwidth, tunable, multiple-pulse-width optical delays using slow light in cesium vapor
We demonstrate an all-optical delay line in hot cesium vapor that tunably
delays 275 ps input pulses up to 6.8 ns and 740 input ps pulses up to 59 ns
(group index of approximately 200) with little pulse distortion. The delay is
made tunable with a fast reconfiguration time (hundreds of ns) by optically
pumping out of the atomic ground states.Comment: 4 pages, 6 figure
Application of ERTS-A data to agricultural practices in the Mississippi Delta region
There are no author-identified significant results in this report
On-chip spectroscopy with thermally-tuned high-Q photonic crystal cavities
Spectroscopic methods are a sensitive way to determine the chemical
composition of potentially hazardous materials. Here, we demonstrate that
thermally-tuned high-Q photonic crystal cavities can be used as a compact
high-resolution on-chip spectrometer. We have used such a chip-scale
spectrometer to measure the absorption spectra of both acetylene and hydrogen
cyanide in the 1550 nm spectral band, and show that we can discriminate between
the two chemical species even though the two materials have spectral features
in the same spectral region. Our results pave the way for the development of
chip-size chemical sensors that can detect toxic substances
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