Point Form Electrodynamics and the Gupta-Bleuler Formalism

The Gupta-Bleuler formalism for photons is derived from induced representation theory. The representation for the little group for massless particles, the two dimensional Euclidian group, is chosen to be the four dimensional nonunitary representation obtained by restricting elements of the Lorentz group to the Euclidian group. Though the little group representation is nonunitary, it is shown that the representation of the Poincar\'{e} group is unitary. As a consequence of the four dimensional representation, the polarization vector, which connects the four-vector potential with creation and annihilation operators, is given in terms of boosts, coset representatives of the Lorentz group with respect to the Euclidian group. Several polarization vectors (boost choices) are worked out, including a front form polariation vector. The different boost choices are shown to be related by the analogue of Melosh rotations, namely Euclidian group transformations.Comment: 15 page

Eutectic colony formation: A phase field study

Eutectic two-phase cells, also known as eutectic colonies, are commonly observed during the solidification of ternary alloys when the composition is close to a binary eutectic valley. In analogy with the solidification cells formed in dilute binary alloys, colony formation is triggered by a morphological instability of a macroscopically planar eutectic solidification front due to the rejection by both solid phases of a ternary impurity that diffuses in the liquid. Here we develop a phase-field model of a binary eutectic with a dilute ternary impurity and we investigate by dynamical simulations both the initial linear regime of this instability, and the subsequent highly nonlinear evolution of the interface that leads to fully developed two-phase cells with a spacing much larger than the lamellar spacing. We find a good overall agreement with our recent linear stability analysis [M. Plapp and A. Karma, Phys. Rev. E 60, 6865 (1999)], which predicts a destabilization of the front by long-wavelength modes that may be stationary or oscillatory. A fine comparison, however, reveals that the assumption commonly attributed to Cahn that lamella grow perpendicular to the envelope of the solidification front is weakly violated in the phase-field simulations. We show that, even though weak, this violation has an important quantitative effect on the stability properties of the eutectic front. We also investigate the dynamics of fully developed colonies and find that the large-scale envelope of the composite eutectic front does not converge to a steady state, but exhibits cell elimination and tip-splitting events up to the largest times simulated.Comment: 18 pages, 18 EPS figures, RevTeX twocolumn, submitted to Phys. Rev.

SOCS1 Is a Critical Inhibitor of Interferon γ Signaling and Prevents the Potentially Fatal Neonatal Actions of this Cytokine

AbstractMice lacking suppressor of cytokine signaling-1 (SOCS1) develop a complex fatal neonatal disease. In this study, SOCS1−/− mice were shown to exhibit excessive responses typical of those induced by interferon γ (IFNγ), were hyperresponsive to viral infection, and yielded macrophages with an enhanced IFNγ-dependent capacity to kill L. major parasites. The complex disease in SOCS1−/− mice was prevented by administration of anti-IFNγ antibodies and did not occur in SOCS1−/− mice also lacking the IFNγ gene. Although IFNγ is essential for resistance to a variety of infections, the potential toxic action of IFNγ, particularly in neonatal mice, appears to require regulation. Our data indicate that SOCS1 is a key modulator of IFNγ action, allowing the protective effects of this cytokine to occur without the risk of associated pathological responses

Reenacting sensorimotor features of drawing movements from friction sounds

International audienceEven though we generally don't pay attention to the friction sounds produced when we are writing or drawing, these sounds are recordable, and can even evoke the underlying gesture. In this paper, auditory perception of such sounds, and the internal representations they evoke when we listen to them, is considered from the sensorimotor learning point of view. The use of synthesis processes of friction sounds makes it possible to investigate the perceptual influence of each gestures parameter separately. Here, the influence of the velocity profile on the mental representation of the gesture induced by a friction sound was investigated through 3 experiments. The results reveal the perceptual relevance of this parameter, and particularly a specific morphology corresponding to biological movements, the so-called 1/3-power law. The experiments are discussed according to the sensorimotor theory and the invariant taxonomy of the ecological approach

Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)

The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated.Comment: Review on superhard B-C-N-O phase

A hierarchy of heuristic-based models of crowd dynamics

International audienceWe derive a hierarchy of kinetic and macroscopic models from a noisy variant of the heuristic behavioral Individual-Based Model of Moussaid et al, PNAS 2011, where the pedestrians are supposed to have constant speeds. This IBM supposes that the pedestrians seek the best compromise between navigation towards their target and collisions avoidance. We first propose a kinetic model for the probability distribution function of the pedestrians. Then, we derive fluid models and propose three different closure relations. The first two closures assume that the velocity distribution functions are either a Dirac delta or a von Mises-Fisher distribution respectively. The third closure results from a hydrodynamic limit associated to a Local Thermodynamical Equilibrium. We develop an analogy between this equilibrium and Nash equilibia in a game theoretic framework. In each case, we discuss the features of the models and their suitability for practical use

Teaching of Energy Issues: A debate proposal for a GLobal Reorientation

The growing awareness of serious difficulties in the learning of energy issues has produced a great deal of research, most of which is focused on specific conceptual aspects. In our opinion, the difficulties pointed out in the literature are interrelated and connected to other aspects (conceptual as well as procedural and axiological), which are not sufficiently taken into account in previous research. This paper aims to carry out a global analysis in order to avoid the more limited approaches that deal only with individual aspects. From this global analysis we have outlined 24 propositions that are put forward for debate to lay the foundations for a profound reorientation of the teaching of energy topics in upper high school courses, in order to facilitate a better scientific understanding of these topics, avoid many students' misconceptions and enhance awareness of the current situation of planetary emergency

Early carboniferous brachiopod faunas from the Baoshan block, west Yunnan, southwest China

38 brachiopod species in 27 genera and subgenera are described from the Yudong Formation in the Shidian-Baoshan area, west Yunnan, southwest China. New taxa include two new subgenera: Unispirifer (Septimispirifer) and Brachythyrina (Longathyrina), and seven new species: Eomarginifera yunnanensis, Marginatia cylindrica, Unispirifer (Unispirifer) xiangshanensis, Unispirifer (Septimispirifer) wafangjieensis, Brachythyrina (Brachythyrina) transversa, Brachythyrina (Longathyrina) baoshanensis, and Girtyella wafangjieensis. Based on the described material and constraints from associated coral and conodont faunas, the age of the brachiopod fauna from the Yudon Formation is considered late Tournaisian (Early Carboniferous), with a possibility extending into earlyViseacutean.<br /

Competing for What?: Linking Competition to Performance in Social Service Contracting

This article explores the links between competition and contractor performance often assumed by market theory. Using data from Florida social service contracting, the authors test to see if competitively procured vendors outperform their noncompetitive peers regarding adherence to contract terms. It is found that, contrary to market theory, this is not the case. It is also found that district management capacity is positively related to performance and the performance of nonprofit vendors is indistinguishable from for-profits (whereas both appear to be outperformed by other government contractors). Finally, this study finds little evidence that performance or competition is related to the likelihood of maintaining contracts.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline