Symmetries of Doubly Heavy Baryons

For very heavy quarks, the two heavy quarks in a doubly heavy baryon are expected to form a pointlike, heavy diquark in an antitriplet color configuration. In this limit the dynamics of the light degrees of freedom `factorize' from the dynamics of the heavy diquark system, and a superflavor symmetry emerges which relates the properties of doubly heavy baryons to heavy mesons. The charm quark may not be heavy enough for the $cc$ diquark in a $\Xi_{cc}$ to be regarded as pointlike. However, there are indications from the results of a nonrelativistic constituent quark model that many of the consequences of factorization emerge even though the model does not assume a quark-diquark structure and the mean separation of the two heavy quarks in the model is not small. We discuss the consequences of factorization for the spectroscopy of doubly heavy baryons and compare these consequences to results from a quark model. We also discuss the possibility of treating the strange quark as a heavy quark and applying these ideas to the $\Xi$.Comment: To appear in the proceedings for the Eleventh Conference on the Intersections of Particle and Nuclear Physics (CIPANP 2012

Coxeter-Knuth graphs and a signed Little map for type B reduced words

We define an analog of David Little's algorithm for reduced words in type B, and investigate its main properties. In particular, we show that our algorithm preserves the recording tableau of Kra\'{s}kiewicz insertion, and that it provides a bijective realization of the Type B transition equations in Schubert calculus. Many other aspects of type A theory carry over to this new setting. Our primary tool is a shifted version of the dual equivalence graphs defined by Assaf and further developed by Roberts. We provide an axiomatic characterization of shifted dual equivalence graphs, and use them to prove a structure theorem for the graph of Type B Coxeter-Knuth relations.Comment: 41 pages, 10 figures, many improvements from version 1, substantively the same as the version in Electronic Journal of Combinatorics, Vol 21, Issue

Cinema as mnemotechnics: Bernard Stiegler and the "industrialization of memory"

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Robust estimates of changes in poverty and inequality in post-independence Namibia

The authors estimate changes in the distribution of household consumption expenditure in Namibia since Independence in 1990 and the effects on poverty. To produce comparability between two household surveys, they use survey matching techniques and apply the framework of stochastic dominance to test the robustness of the results. The results reveal a significant decrease in the poverty headcount over the period and small but insignificant decreases in the country's extremely high levels of inequality. Decomposition analysis shows that poverty reduction in Namibia is largely driven by growth in mean incomes rather than redistribution. Even so, there have been important changes in inequality, especially between different social groups, as educational attainment has replaced ethnicity as the main determinant of inequality between groups

Scale-dependent angle of alignment between velocity and magnetic field fluctuations in solar wind turbulence

Under certain conditions, freely decaying magnetohydrodynamic (MHD) turbulence evolves in such a way that velocity and magnetic field fluctuations delta v and delta B approach a state of alignment in which delta v proportional to delta B. This process is called dynamic alignment. Boldyrev has suggested that a similar kind of alignment process occurs as energy cascades from large to small scales through the inertial range in strong incompressible MHD turbulence. In this study, plasma and magnetic field data from the Wind spacecraft, data acquired in the ecliptic plane near 1 AU, are employed to investigate the angle theta(tau) between velocity and magnetic field fluctuations in the solar wind as a function of the time scale tau of the fluctuations and to look for the scaling relation similar to tau(1/4) predicted by Boldyrev. We find that the angle appears to scale like a power law at large inertial range scales, but then deviates from power law behavior at medium to small inertial range scales. We also find that small errors in the velocity vector measurements can lead to large errors in the angle measurements at small time scales. As a result, we cannot rule out the possibility that the observed deviations from power law behavior arise from errors in the velocity measurements. When we fit the data from 2 x 10(3) s to 2 x 10(4) s with a power law of the form proportional to tau(p), our best fit values for p are in the range 0.27-0.36

Catalyst warm-up to light-off by pulsating engine exhaust: two-dimensional studies

This paper presents the results of two-dimensional studies on a range of different catalyst substrates warmed by exhaust from a four-cylinder gasoline engine. Engine speeds investigated were 2000 and 3000 r/min, corresponding to frequencies of 67 and 100 Hz. Of the nine substrates investigated, four were washcoated but non-reactive and four were washcoated and reactive. The ninth substrate was a hybrid sample with its front end inactive and its downstream end active: this is analogous to an aged catalyst. The temperatures at 16 locations within the substrates were measured. All the substrates were warmed by the pulsating exhaust flow from an engine running lean of stoichiometric. Thereactive substratates were warmed through light-off and both substrate temperatures and hydrocarbon conversion were monitored. The latter was measured with very high response time fast flame ionization detectors. Predicted temperatures and conversion were obtained from a two-dimensional computational fluid dynamics (CFD) model. The model was based on the porous medium approach and incorporated a simple chemical scheme for oxidation under lean conditions. Standard values for reaction rate constants were found to be accurate for the lean conditions studied. Comparison was made of measurements of temperature and conversion fraction with predictions, with particular reference to the time taken to achieve light-off. Steady flow CFD predictions were found to be in good overall agreement with engine test data. Pulsing flow CFD predictions were found, however, to improve on the degree of agreement for warm-up of non-reactive samples, but at the expense of increased computation time. The performance of the reactive hybrid sample was accurately predicted by adjustment of the parameter for active area per unit volume, providing the basis for a technique for prediction of performance for aged catalyst

Methodology for modelling a combined DPF and SCR catalyst with the porous medium approach in CFD

In an attempt to reduce particulate and NOx emissions from Diesel exhaust, the combined DPF and SCR filter is now frequently chosen as the preferred catalyst. When this device functions effectively it saves valuable packaging space in a passenger vehicle. As part of its development, modelling of its emissions performance is essential. Single channel modelling would seem to be the obvious choice for an SCRF because of its complex internal geometry. This, however, can be computationally demanding if modelling the full monolith. For a normal flow-through catalyst monolith the porous medium approach is an attractive alternative as it ccounts for nonuniform inlet conditions without the need to model every channel. This paper attempts to model an SCRF by applying the porous medium approach. The model is essentially 1D but as with all porous medium models, can very easily be applied to 3D cases once developed and validated. The model is described in full in this paper and values for all the key parameters are presented. The filter is assumed to collect soot in the inlet channels, but only the outlet channels are coated with SCR washcoat, as in the most recent devices. This aims to avoid back diffusion of NO2 that promotes soot and NOx reactions. But it is necessary to modify the pressure loss expression term to account for the smaller size of the washcoated outlet channel. The SCR model integrated into the CFD coding is simple and based on a scheme available in the literature. This includes the standard and fast SCR reactions and ammonia adsorption and desorption. NO and ammonia oxidation are also included and are important during the high temperature regeneration phase. The detail of the flow at the channel scale is not modelled but the species can be modelled at the channel scale for the monolith by application of source terms in the species transport equation. The source terms are evaluated in user subroutines in commercial CFD software. The species levels of NO, NO2 and NH3 in the flow coming through the filter wall, in the pores in the wall and in the flow in the downstream channel are all modelled as a function of distance along the brick. The simplifying assumptions on which this model is based are stated in this paper. The model produces plausible output when run as a demonstration case for a 1050 s soot storage period at 550 K, followed by a 150 s regeneration period at 900 K, and then for a further soot storage period at 550 K. The simulations are in qualitative agreement with the expected performance of a combined DPF and SCR in a real Diesel exhaust. An attempt has been made to apply the model to a real case based on data available in the literature so that its output can be validated

Automotive catalyst warm-up to light-off by pulsating engine exhaust

This paper presents the results of studies on a range of different catalyst substrates warmed by engineexhaust. Engine speeds were in the range 1200 to 3000 rpm. One substrate was non-washcoated, fourwere washcoated but non-reactive, and four were washcoated and reactive. The temperature at fourlocations within the non-reactive substrates was measured. The reactive substrates were warmed tolight off by the pulsating exhaust flow from an engine running fuel rich of stoichiometric. Bothsubstrate temperatures and hydrocarbon conversion were measured. Predicted temperatures andconversion were obtained from a 1D CFD model. The model was based on the porous mediumapproach and incorporated a simple 3-way chemical scheme. Comparison was made of measurementswith predictions, with particular reference to the time taken to achieve light-off. Pulsing flow CFDpredictions were found to be almost identical to steady flow predictions for the conditions investigated.The CFD predictions were found to be in fair agreement with the engine test results, but using kineticrate constants higher than previously reported values