27,420 research outputs found
Factorization and Unitarity in Superstring Theory
The overall coefficient of the two-loop 4-particle amplitude in superstring
theory is determined by making use of the factorization and unitarity. To
accomplish this we computed in detail all the relevant tree and one-loop
amplitudes involved and determined their overall coefficients in a consistent
way.Comment: LaTex file, 19 pages, 4 figures; v2, minor corrections and figures
corrected; v3, minor corrections with the English, to be published in JHE
Rare decays and in \the topcolor-assisted technicolor model
We examine the rare decays and in the
framework of the topcolor-assisted technicolor () model. The contributions
of the new particles predicted by this model to these rare decay processes are
evaluated. We find that the values of their branching ratios are larger than
the standard model predictions by one order of magnitude in wide range of the
parameter space. The longitudinal polarization asymmetry of leptons in can approach \ord(10^{-2}). The forward-backward asymmetry of leptons
in is not large enough to be measured in future experiments. We
also give some discussions about the branching ratios and the asymmetry
observables related to these rare decay processes in the littlest Higgs model
with T-parity.Comment: 29 pages, 9 figure, corrected typos, the version to appear in PR
Factorization of the Two Loop Four-Particle Amplitude in Superstring Theory Revisited
We study in detail the factorization of the newly obtained two-loop
four-particle amplitude in superstring theory. In particular some missing
factors from the scalar correlators are obtained correctly, in comparing with a
previous study of the factorization in two-loop superstring theory. Some
details for the calculation of the factorization of the kinematic factor are
also presented.Comment: 11 pages, 1 figure; v2, minor corrections and references update
Excitation of nonlinear ion acoustic waves in CH plasmas
Excitation of nonlinear ion acoustic wave (IAW) by an external electric field
is demonstrated by Vlasov simulation. The frequency calculated by the
dispersion relation with no damping is verified much closer to the resonance
frequency of the small-amplitude nonlinear IAW than that calculated by the
linear dispersion relation. When the wave number increases,
the linear Landau damping of the fast mode (its phase velocity is greater than
any ion's thermal velocity) increases obviously in the region of in which the fast mode is weakly damped mode. As a result, the deviation
between the frequency calculated by the linear dispersion relation and that by
the dispersion relation with no damping becomes larger with
increasing. When is not large, such as , the nonlinear IAW can be excited by the driver with the linear frequency
of the modes. However, when is large, such as
, the linear frequency can not be applied to exciting the
nonlinear IAW, while the frequency calculated by the dispersion relation with
no damping can be applied to exciting the nonlinear IAW.Comment: 10 pages, 9 figures, Accepted by POP, Publication in August 1
Upscaling fluxes from towers to regions, continents and global scales using datadriven approaches
Quantifying the current carbon cycle of terrestrial ecosystems requires that we translate spatially sparse measurements into consistent, gridded flux estimates at the regional scale. This is particularly challenging in heterogeneous regions such as the northern forests of the United States. We use a network of 17 eddy covariance flux towers deployed across the Upper Midwest region of northern Wisconsin and Michigan and upscale flux observations from towers to the regional scale. This region is densely instrumented and provides a unique test bed for regional upscaling. We develop a simple Diagnostic Carbon Flux Model (DCFM) and use flux observations and a data assimilation approach to estimate the model parameters. We then use the optimized model to produce gridded flux estimates across the region. We find that model parameters vary not only across plant functional types (PFT) but also within a given PFT. Our results show that the parameter estimates from a single site are not representative of the parameter values of a given PFT; cross-site (or joint) optimization using observations from multiple sites encompassing a range of site and climate conditions considerably improves the representativeness and robustness of parameter estimates. Parameter variability within a PFT can result in substantial variability in regional flux estimates. We also find that land cover representation including land cover heterogeneity and the spatial resolution and accuracy of land cover maps can lead to considerable uncertainty in regional flux estimates. In heterogeneous, complex regions, detailed and accurate land cover maps are essential for accurate estimation of regional fluxes
Upscaling carbon fluxes from towers to the regional scale: Influence of parameter variability and land cover representation on regional flux estimates
Quantifying the current carbon cycle of terrestrial ecosystems requires that we translate spatially sparse measurements into consistent, gridded flux estimates at the regional scale. This is particularly challenging in heterogeneous regions such as the northern forests of the United States. We use a network of 17 eddy covariance flux towers deployed across the Upper Midwest region of northern Wisconsin and Michigan and upscale flux observations from towers to the regional scale. This region is densely instrumented and provides a unique test bed for regional upscaling. We develop a simple Diagnostic Carbon Flux Model (DCFM) and use flux observations and a data assimilation approach to estimate the model parameters. We then use the optimized model to produce gridded flux estimates across the region. We find that model parameters vary not only across plant functional types (PFT) but also within a given PFT. Our results show that the parameter estimates from a single site are not representative of the parameter values of a given PFT; cross-site (or joint) optimization using observations from multiple sites encompassing a range of site and climate conditions considerably improves the representativeness and robustness of parameter estimates. Parameter variability within a PFT can result in substantial variability in regional flux estimates. We also find that land cover representation including land cover heterogeneity and the spatial resolution and accuracy of land cover maps can lead to considerable uncertainty in regional flux estimates. In heterogeneous, complex regions, detailed and accurate land cover maps are essential for accurate estimation of regional fluxes
A system for learning statistical motion patterns
Analysis of motion patterns is an effective approach for anomaly detection and behavior prediction. Current approaches for the analysis of motion patterns depend on known scenes, where objects move in predefined ways. It is highly desirable to automatically construct object motion patterns which reflect the knowledge of the scene. In this paper, we present a system for automatically learning motion patterns for anomaly detection and behavior prediction based on a proposed algorithm for robustly tracking multiple objects. In the tracking algorithm, foreground pixels are clustered using a fast accurate fuzzy k-means algorithm. Growing and prediction of the cluster centroids of foreground pixels ensure that each cluster centroid is associated with a moving object in the scene. In the algorithm for learning motion patterns, trajectories are clustered hierarchically using spatial and temporal information and then each motion pattern is represented with a chain of Gaussian distributions. Based on the learned statistical motion patterns, statistical methods are used to detect anomalies and predict behaviors. Our system is tested using image sequences acquired, respectively, from a crowded real traffic scene and a model traffic scene. Experimental results show the robustness of the tracking algorithm, the efficiency of the algorithm for learning motion patterns, and the encouraging performance of algorithms for anomaly detection and behavior prediction
Charmless decays and new physics effects in the mSUGRA model
By employing the QCD factorization approach, we calculate the new physics
contributions to the branching radios of the two-body charmless and
decays in the framework of the minimal supergravity (mSUGRA) model.
we choose three typical sets of the mSUGRA input parameters in which the Wilson
coefficient can be either SM-like (the case A and C) or has
a flipped-sign (the case B). We found numerically that (a) the SUSY
contributions are always very small for both case A and C; (b) for those
tree-dominated decays, the SUSY contributions in case B are also very small;
(c) for those QCD penguin-dominated decay modes, the SUSY contributions in case
B can be significant, and can provide an enhancement about to
the branching ratios of and decays, but a
reduction about to decays; and (d) the
large SUSY contributions in the case B may be masked by the large theoretical
errors dominated by the uncertainty from our ignorance of calculating the
annihilation contributions in the QCD factorization approach.Comment: 34 pages, 8 PS figures, this is the correct version
Cosmic ray spectral hardening due to dispersion in the source injection spectra
Recent cosmic ray (CR) experiments discovered that the CR spectra experience
a remarkable hardening for rigidity above several hundred GV. We propose that
this is caused by the superposition of the CR energy spectra of many sources
that have a dispersion in the injection spectral indices. Adopting similar
parameters as those of supernova remnants derived from the Fermi -ray
observations, we can reproduce the observational CR spectra of different
species well. This may be interpreted as evidence to support the supernova
remnant origin of CRs below the knee. We further propose that the same
mechanism may explain the "ankle" of the ultra high energy CR spectrum.Comment: 5 pages, 3 figures and 1 table. Updated with the diffusion
propagation model, accepted by Phys. Rev.
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