25,192 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
Two Flavour QCD Phase Transition
Results on the phase transition in QCD with two flavours of light staggered
fermions from an ongoing simulation are presented. We find the restoration of
the chiral SU(2) x SU(2) symmetry, but not of the axial U_A(1) symmetry.Comment: LaTeX2e, amstex package. 4 pages, 2 figures. Talk presented at the
10th International Conference on Problems of Quantum Field Theory, Crime
Interaction of a Modulated Electron Beam with a Plasma
The results of a theoretical and experimental investigation of the high-frequency interaction of an electron beam with a plasma are reported. An electron beam, modulated at a microwave frequency, passes through a uniform region of a mercury arc discharge after which it is demodulated. Exponentially growing wave amplification along the electron beam was experimentally observed for the first time at a microwave frequency equal to the plasma frequency. Approximate theories of the effects of 1) plasma-electron collision frequencies, 2) plasma-electron thermal velocities and 3) finite beam diameter, are given. In a second experiment the interaction between a modulated electron beam and a slow electrostatic wave on a plasma column has been studied. A strong interaction occurs when the velocity of the electron beam is approximately equal to the velocity of the wave and the interaction is essentially the same as that which occurs in traveling-wave amplifiers, except that here the plasma colum replaces the usual helical slow-wave circuit. The theory predicting rates of growth is presented and compared with the experimental results
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
Post Big Bang Processing of the Primordial Elements
We explore the Gnedin-Ostriker suggestion that a post-Big-Bang
photodissociation process may modify the primordial abundances of the light
elements. We consider several specific models and discuss the general features
that are necessary (but not necessarily sufficient) to make the model work. We
find that with any significant processing, the final D and He abundances,
which are independent of their initial standard big bang nucleosynthesis (SBBN)
values, rise quickly to a level several orders of magnitude above the
observationally inferred primordial values. Solutions for specific models show
that the only initial abundances that can be photoprocessed into agreement with
observations are those that undergo virtually no processing and are already in
agreement with observation. Thus it is unlikely that this model can work for
any non-trivial case unless an artificial density and/or photon distribution is
invoked.Comment: 12 page Latex file (AASTEX style). Tarred, gzipped, and uuencoded
postscript files of seven figures. Also available (with ps file of paper) at
ftp://www-physics.mps.ohio-state.edu/pub/nucex/phot
Detecting D-Wave Pairing and Collective Modes in Fermionic Condensates with Bragg Scattering
We show how the appearance of d-wave pairing in fermionic condensates
manifests itself in inelastic light scattering. Specifically, we calculate the
Bragg scattering intensity from the dynamic structure factor and the spin
susceptibility, which can be inferred from spin flip Raman transitions. This
information provides a precise tool with which we can identify nontrivial
correlations in the state of the system beyond the information contained in the
density profile imaging alone. Due to the lack of Coulomb effects in neutral
superfluids, this is also an opportunity to observe the Anderson-Bogoliubov
collective mode
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
Properties of bright squeezed vacuum at increasing brightness
A bright squeezed vacuum (BSV) is a nonclassical macroscopic state of light, which is generated through high-gain parametric down-conversion or four-wave mixing. Although the BSV is an important tool in quantum optics and has a lot of applications, its theoretical description is still not complete. In particular, the existing description in terms of Schmidt modes with gain-independent shapes fails to explain the spectral broadening observed in the experiment as the mean number of photons increases. Meanwhile, the semiclassical description accounting for the broadening does not allow us to decouple the intermodal photon-number correlations. In this work, we present a new generalized theoretical approach to describe the spatial properties of a multimode BSV. In the multimode case, one has to take into account the complicated interplay between all involved modes: each plane-wave mode interacts with all other modes, which complicates the problem significantly. The developed approach is based on exchanging the (k, t ) and (ω, z) representations and solving a system of integrodifferential equations. Our approach predicts correctly the dynamics of the Schmidt modes and the broadening of the angular distribution with the increase in the BSV mean photon number due to a stronger pumping. Moreover, the model correctly describes various properties of a widely used experimental configuration with two crystals and an air gap between them, namely, an SU(1,1) interferometer. In particular, it predicts the narrowing of the intensity distribution, the reduction and shift of the side lobes, and the decline in the interference visibility as the mean photon number increases due to stronger pumping. The presented experimental results confirm the validity of the new approach. The model can be easily extended to the case of the frequency spectrum, frequency Schmidt modes, and other experimental configurations
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