Covariant approach to equilibration in effective field theories

The equilibration of two coupled reservoirs is studied using a Green function approach which is suitable for future development with the closed time path method. The problem is solved in two parameterizations, in order to demonstrate the non-trivial issues of parameterization in both the intermediate steps and the interpretation of physical quantities. We use a covariant approach to find self-consistent solutions for the statistical distributions as functions of time. We show that by formally introducing covariant connections, one can rescale a slowly varying non-equilibrium theory so that it appears to be an equilibrium one, for the purposes of calculation. We emphasize the importance of properly tracking variable redefinitions in order to correctly interpret physical quantities.Comment: 11 pages, Late

Prepontine non-giant neurons drive flexible escape behavior in zebrafish

Many species execute ballistic escape reactions to avoid imminent danger. Despite fast reaction times, responses are often highly regulated, reflecting a trade-off between costly motor actions and perceived threat level. However, how sensory cues are integrated within premotor escape circuits remains poorly understood. Here, we show that in zebrafish, less precipitous threats elicit a delayed escape, characterized by flexible trajectories, which are driven by a cluster of 38 prepontine neurons that are completely separate from the fast escape pathway. Whereas neurons that initiate rapid escapes receive direct auditory input and drive motor neurons, input and output pathways for delayed escapes are indirect, facilitating integration of cross-modal sensory information. These results show that rapid decision-making in the escape system is enabled by parallel pathways for ballistic responses and flexible delayed actions and defines a neuronal substrate for hierarchical choice in the vertebrate nervous system

Specialization of the rostral prefrontal cortex for distinct analogy processes

Analogical reasoning is central to learning and abstract thinking. It involves using a more familiar situation (source) to make inferences about a less familiar situation (target). According to the predominant cognitive models, analogical reasoning includes 1) generation of structured mental representations and 2) mapping based on structural similarities between them. This study used functional magnetic resonance imaging to specify the role of rostral prefrontal cortex (PFC) in these distinct processes. An experimental paradigm was designed that enabled differentiation between these processes, by temporal separation of the presentation of the source and the target. Within rostral PFC, a lateral subregion was activated by analogy task both during study of the source (before the source could be compared with a target) and when the target appeared. This may suggest that this subregion supports fundamental analogy processes such as generating structured representations of stimuli but is not specific to one particular processing stage. By contrast, a dorsomedial subregion of rostral PFC showed an interaction between task (analogy vs. control) and period (more activated when the target appeared). We propose that this region is involved in comparison or mapping processes. These results add to the growing evidence for functional differentiation between rostral PFC subregions

The high partial wave phenomenon of spin changing atomic transitions

The collisional transition between two highly excited atomic states with different spin is investigated theoretically. Taking helium-like n1S − n3P as an example, it is found that the transition is driven in the highly ion-ized Fe ion purely by exchange, and the cross section becomes increasingly dominated by partial waves of high orbital angular momentum as the scattering energy increases. Whereas for the near-neutral Li ion the transition is dominated by channel coupling in low partial waves. Analytical bench-marks and numerical methods are developed for the accurate calculation of the exchange integral at high angular momentum. It is shown how the partial wave and energy dependence of the collision strength for high n spin changing transitions in the highly ionized ion is related to the overlap of the extended atomic orbitals.</p

QCD Corrections in two-Higgs-doublet extensions of the Standard Model with Minimal Flavor Violation

We present the QCD corrections to R_b and to the Delta B=1 effective Hamiltonian in models with a second Higgs field that couples to the quarks respecting the criterion of Minimal Flavor Violation, thus belonging either to the (1,2)_1/2 or to the (8,2)_1/2 representation of SU(3)xSU(2)xU(1). After the inclusion of the QCD corrections, the prediction for R_b becomes practically insensitive to the choice of renormalization scheme for the top mass, which for the type-I and type-II models translates in a more robust lower bound on tan(beta). The QCD-corrected determinations of Rb and BR(B->Xs gamma) are used to discuss the constraints on the couplings of a (colored) charged Higgs boson to top and bottom quarks.Comment: 19 pages, 7 figures. v2: version published in Phys. Rev. D, with additional reference and not

The static potential in QED3_3 with non-minimal coupling

Here we study the effect of the non-minimal coupling j^{\mu}\eps \partial^{\nu} A^{\alpha} on the static potential in multiflavor QED$_3$. Both cases of four and two components fermions are studied separately at leading order in the $1/N$ expansion. Although a non-local Chern-Simons term appears, in the four components case the photon is still massless leading to a confining logarithmic potential similar to the classical one. In the two components case, as expected, the parity breaking fermion mass term generates a traditional Chern-Simons term which makes the photon massive and we have a screening potential which vanishes at large inter-charge distance. The extra non-minimal couplings have no important influence on the static potential at large inter-charge distances. However, interesting effects show up at finite distances. In particular, for strong enough non-minimal coupling we may have a new massive pole in the photon propagator while in the opposite limit there may be no poles at all in the irreducible case. We also found that, in general, the non-minimal couplings lead to a finite range {\bf repulsive} force between charges of opposite signs.Comment: 19 pages and 7 figure

Component-based model of buckling panels of steel beams at elevated temperatures

Both bottom-flange buckling and beam-web shear buckling have been observed in many full-scale fire tests in the vicinity of beam-to-column connections. These phenomena can influence the load redistribution within the adjacent connections and the global structural behaviour, detrimentally affecting the structural overall fire resistance. However, existing models for bottom-flange buckling overestimate the structural resistance when the beam is slender. In this work, a new analytical model has been created to predict both of these types of buckling behaviour in steel beams in the vicinity of beam-to-column connections at elevated temperatures. The model considers the individual effects of both buckling modes, as well as their interaction. It is capable of predicting the force–deflection relationship of the buckling zone from the initial elastic loading stage to run-away failure. The new analytical model has been compared with the existing Dharma's model and a range of 3D finite element simulations created using the ABAQUS software. Comparisons have shown that the proposed method gives better predictions than Dharma's model. A component-based model of the buckling zone has been created on the basis of this new analysis. The component-based model can provide sufficient accuracy, and will be implemented in the software Vulcan for performance-based global structural fire analysis

A Component-Based Approach to Modelling Beam Bottom Flange Buckling at Elevated Temperatures

In this study, an analytical model of the combination of beam-web shear buckling and bottom-flange buckling at elevated temperatures has been created. This analytical model is able to track the force-deflection path in the post-buckling stage. A range of 3D finite element models has been created using the ABAQUS software. Comparisons have been carried out between the proposed analytical model, finite element modelling and the existing Dharma’s theoretical model. Comparisons indicate that the proposed method is able to provide with accurate predictions for Class 1 and Class 2 beams, and performs better than the existing model, especially for slender beams. A component-based model has been created based on the analytical model, and in due course to be implemented into the software Vulcan for global structural fire analysis

A Parametric Investigation of the Transition Between Beam-Web Shear Buckling and Bottom- Flange Buckling at Elevated Temperatures

In this research, two existing analytical models of beam-web shear buckling and bottom-flange buckling in steel beams at elevated temperatures have been briefly reviewed. Both models are able to track the behaviour from pre-buckling to post-buckling stage, near to the ends of beams. A transition criterion has been proposed to justify which buckling phenomenon occurs in reality, according to the given structural information, such as geometrical dimensions and loading conditions. A number of 3D finite element models have been created using the ABAQUS software. Parametric studies have been carried out to detect the transition from beam-web shear buckling to bottom-flange buckling, as well as an interactive range within which both phenomena occur simultaneously. Comparisons between the analytical and FE models have shown that it is possible to propose criteria to detect the transition between buckling types. The proposed analytical methods provides sufficient accuracy to be developed further, and in due course it will be embodied in global modelling of composite structures in fire as part of a component-based approach to connections and their adjacent zones