Sheath ionization model of beam emissions from large spacecraft

An analytical model of the charging of a spacecraft emitting electron and ion beams has been applied to the case of large spacecraft. In this model, ionization occurs in the sheath due to the return current. Charge neutralization of spherical space charge flow is examined by solving analytical equations numerically. Parametric studies of potential large spacecraft are performed. As in the case of small spacecraft, the ions created in the sheath by the returning current play a large role in determining spacecraft potential

TagF-mediated repression of bacterial type VI secretion systems involves a direct interaction with the cytoplasmic protein Fha

The bacterial type VI secretion system (T6SS) delivers effectors into eukaryotic host cells or toxins into bacterial competitor for survival and fitness. The T6SS is positively regulated by the threonine phosphorylation pathway (TPP) and negatively by the T6SS-accessory protein TagF. Here, we studied the mechanisms underlying TagF-mediated T6SS repression in two distinct bacterial pathogens, Agrobacterium tumefaciens and Pseudomonas aeruginosa. We found that in A. tumefaciens, T6SS toxin secretion and T6SS-dependent antibacterial activity are suppressed by a two-domain chimeric protein consisting of TagF and PppA, a putative phosphatase. Remarkably, this TagF domain is sufficient to post-translationally repress the T6SS, and this inhibition is independent of TPP. This repression requires interaction with a cytoplasmic protein, Fha, critical for activating T6SS assembly. In P. aeruginosa, PppA and TagF are two distinct proteins that repress T6SS in a TPP-dependent and -independent pathways, respectively. P. aeruginosa TagF interacts with Fha1, suggesting that formation of this complex represents a conserved TagF-mediated regulatory mechanism. Using TagF variants with substitutions of conserved amino acid residues at predicted protein-protein interaction interfaces, we uncovered evidence that the TagF-Fha interaction is critical for TagF-mediated T6SS repression in both bacteria. TagF inhibits T6SS without affecting T6SS protein abundance in A. tumefaciens, but TagF overexpression reduces the protein levels of all analyzed T6SS components in P. aeruginosa. Our results indicate that TagF interacts with Fha, which in turn could impact different stages of T6SS assembly in different bacteria, possibly reflecting an evolutionary divergence in T6SS control

Octet baryon magnetic moments from QCD sum rules

A comprehensive study is made for the magnetic moments of octet baryons in the method of QCD sum rules. A complete set of QCD sum rules is derived using the external field method and generalized interpolating fields. For each member, three sum rules are constructed from three independent tensor structures. They are analyzed in conjunction with the corresponding mass sum rules. The performance of each of the sum rules is examined using the criteria of OPE convergence and ground-state dominance, along with the role of the transitions in intermediate states. Individual contributions from the u, d and s quarks are isolated and their implications in the underlying dynamics are explored. Valid sum rules are identified and their predictions are obtained. The results are compared with experiment and previous calculations.Comment: 21 pages, 11 figures, 6 figures; added a reference, minor change in tex

A compressible near-wall turbulence model for boundary layer calculations

A compressible near-wall two-equation model is derived by relaxing the assumption of dynamical field similarity between compressible and incompressible flows. This requires justifications for extending the incompressible models to compressible flows and the formulation of the turbulent kinetic energy equation in a form similar to its incompressible counterpart. As a result, the compressible dissipation function has to be split into a solenoidal part, which is not sensitive to changes of compressibility indicators, and a dilational part, which is directly affected by these changes. This approach isolates terms with explicit dependence on compressibility so that they can be modeled accordingly. An equation that governs the transport of the solenoidal dissipation rate with additional terms that are explicitly dependent on the compressibility effects is derived similarly. A model with an explicit dependence on the turbulent Mach number is proposed for the dilational dissipation rate. Thus formulated, all near-wall incompressible flow models could be expressed in terms of the solenoidal dissipation rate and straight-forwardly extended to compressible flows. Therefore, the incompressible equations are recovered correctly in the limit of constant density. The two-equation model and the assumption of constant turbulent Prandtl number are used to calculate compressible boundary layers on a flat plate with different wall thermal boundary conditions and free-stream Mach numbers. The calculated results, including the near-wall distributions of turbulence statistics and their limiting behavior, are in good agreement with measurements. In particular, the near-wall asymptotic properties are found to be consistent with incompressible behavior; thus suggesting that turbulent flows in the viscous sublayer are not much affected by compressibility effects

New signals of a R-parity violating model of neutrino mass at the Tevatron

In a variety models of neutrino masses and mixings the lighter top squark decays into competing R - parity violating and R - parity conserving channels. Using Pythia we have estimated in a model independent way the minimum value of P $\equiv$ BR($\widetilde t_1 \to c \widetilde \chi_1^0$) $\times$ BR($\widetilde t_1 \to l^+_i b$), where $l_i = e^+$ and $\mu^+$, corresponding to an observable signal involving the final state 1$l$ + jets +\met (carried by the neutrinos from the $\tilde \chi_1^0$ decay) at Tevatron Run II. For the kinematical cuts designed in this paper P depends on $m_{\widetilde t_1}$ only. We then compute P for representative choices of the model parameters constrained by the oscillation data and find that over a significant region of the allowed parameter space P is indeed larger than $P_{min}$. This signal is complementary to the dilepton + dijet signal studied in several earlier experimental and phenomenological analyses and may be observed even if BR($\widetilde t_1 \to l^+_i b$) is an order of magnitude smaller than BR($\widetilde t_1 \to c \widetilde \chi_1^0$). The invariant mass distribution of the hardest lepton and the hardest jet may determine $m_{\widetilde t_1}$ and reveal the lepton number violating nature of the underlying interaction. The invariant mass distribution of the two lowest energy jets may determine $m_{\widetilde \chi_1^0}$.Comment: Some minor changes in language are made at pages 1,2,8 and 18 respectivel