Loop and Path Spaces and Four-Dimensional BF Theories: Connections, Holonomies and Observables

We study the differential geometry of principal G-bundles whose base space is the space of free paths (loops) on a manifold M. In particular we consider connections defined in terms of pairs (A,B), where A is a connection for a fixed principal bundle P(M,G) and B is a 2-form on M. The relevant curvatures, parallel transports and holonomies are computed and their expressions in local coordinates are exhibited. When the 2-form B is given by the curvature of A, then the so-called non-abelian Stokes formula follows. For a generic 2-form B, we distinguish the cases when the parallel transport depends on the whole path of paths and when it depends only on the spanned surface. In particular we discuss generalizations of the non-abelian Stokes formula. We study also the invariance properties of the (trace of the) holonomy under suitable transformation groups acting on the pairs (A,B). In this way we are able to define observables for both topological and non-topological quantum field theories of the BF type. In the non topological case, the surface terms may be relevant for the understanding of the quark-confinement problem. In the topological case the (perturbative) four-dimensional quantum BF-theory is expected to yield invariants of imbedded (or immersed) surfaces in a 4-manifold M.Comment: TeX, 39 page

Loop observables for BF theories in any dimension and the cohomology of knots

A generalization of Wilson loop observables for BF theories in any dimension is introduced in the Batalin-Vilkovisky framework. The expectation values of these observables are cohomology classes of the space of imbeddings of a circle. One of the resulting theories discussed in the paper has only trivalent interactions and, irrespective of the actual dimension, looks like a 3-dimensional Chern-Simons theory.Comment: 13 page

Determination of oscillator strength of confined excitons in a semiconductor microcavity

We have achieved a significant experimental Rabi-splitting (3.4 meV) for confined polaritons in a planar semiconductor $\lambda$ microcavity for only a single quantum well (SQW) of GaAs (10 nm) placed at the antinode. The Rabi-splitting phenomena are discussed in detail based on the semiclassical theory, where two coupled harmonic oscillators (excitons and photons) are used to describe the system. In this way, we can obtain the dispersion curve of polaritons, the minimum value for the cavity reflectance and the oscillator strength to reach the strong coupling regime. This approach describes an ensemble of excitons confined in a SQW and includes a dissipation component. The results present a weak coupling regime, where an enhanced spontaneous emission takes place, and a strong coupling regime, where Rabi-splitting in the dispersion curve can be observed. The theoretical results are confronted with experimental data for the reflectance behavior in resonant and off-resonant conditions and present a great accuracy. This allows us to determine the oscillator strength of the confined excitons in the SQW with great precision.Comment: 11 pages, 7 figure

Higgs Properties in the Fourth Generation MSSM: Boosted Signals Over the 3G Plan

The generalization of the MSSM to the case of four chiral fermion generations (4GMSSM) can lead to significant changes in the phenomenology of the otherwise familiar Higgs sector. In most of the 3GMSSM parameter space, the lighter CP-even $h$ is $\sim 115-125$ GeV and mostly Standard Model-like while $H,A,H^\pm$ are all relatively heavy. Furthermore, the ratio of Higgs vevs, $\tan \beta$, is relatively unconstrained. In contrast to this, in the 4GMSSM, heavy fourth generation fermion loops drive the masses of $h,H,H^\pm$ to large values while the CP-odd boson, $A$, can remain relatively light and $\tan \beta$ is restricted to the range 1/2 \lsim \tan \beta \lsim 2 due to perturbativity requirements on Yukawa couplings. We explore this scenario in some detail, concentrating on the collider signatures of the light CP-odd Higgs at both the Tevatron and LHC. We find that while $gg \to A$ may lead to a potential signal in the $\tau^+\tau^-$ channel at the LHC, $A$ may first be observed in the $\gamma \gamma$ channel due to a highly loop-enhanced cross section that can be more than an order of magnitude greater than that of a SM Higgs for $A$ masses of $\sim 115-120$ and $\tan\beta<1$. We find that the CP-even states $h,H$ are highly mixed and can have atypical branching fractions. Precision electroweak constraints, particularly for the light $A$ parameter space region, are examined in detail.Comment: 20 pages, 7 figures; typos fixed, refs adde

Localized Charge Transfer Process and Surface Band Bending in Methane Sensing by GaN Nanowires

The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A global resistive response for sensor devices of ensemble nanowires and a localized charge transfer process in single GaN nanowires are studied in situ scanning by Kelvin probe microscopy (SKPM). A localized charge transfer process, involving the VGa-3ON defect complex on nanowire surface is attributed in controlling the global gas sensing behavior of the oxygen rich ensemble GaN nanowires.Comment: 42 pages, 6 figures, Journa

Four-Dimensional Yang-Mills Theory as a Deformation of Topological BF Theory

The classical action for pure Yang--Mills gauge theory can be formulated as a deformation of the topological $BF$ theory where, beside the two-form field $B$, one has to add one extra-field $\eta$ given by a one-form which transforms as the difference of two connections. The ensuing action functional gives a theory that is both classically and quantistically equivalent to the original Yang--Mills theory. In order to prove such an equivalence, it is shown that the dependency on the field $\eta$ can be gauged away completely. This gives rise to a field theory that, for this reason, can be considered as semi-topological or topological in some but not all the fields of the theory. The symmetry group involved in this theory is an affine extension of the tangent gauge group acting on the tangent bundle of the space of connections. A mathematical analysis of this group action and of the relevant BRST complex is discussed in details.Comment: 74 pages, LaTeX, minor corrections; to be published in Commun. Math. Phy

The early steps of drought response mediated by ABA: evolutionary and molecular mechanism of Coffea canephora PYR/PYL/RCAR receptors

The abscisic acid (ABA) is a vital phytohormone synthesized in roots and leaves, acting as central regulator involved in plant responses against abiotic stress, such as drought. It mediates stress-responsive gene expression by controlling stomatal closure, root growth modulation and seed dormancy, for example. Water deficit affects coffee plant development and production and consequently the biochemical composition of beans. It is also well known that genetic variability exists in the Coffea species for drought tolerance. Recently, novel intracellular ABA receptors (PYL/RCARs) involved in ABA sensing and signaling were identified. A mechanism of ABA transduction was proposed, involving PYR/PYL/RCARs ABA receptors interacting with PP2Cs phosphatases and SnRK2 protein kinases. The goal of this study was to identify and characterize orthologous genes of PYR/PYL/RCAR family in C. canephora. For this purpose, protein sequences from Arabidopsis, citrus, rice, grape and tomato species were chosen as query to search orthologous genes in coffee genome. This approach allowed the identification and characterization of 9 candidate genes for PYR/PYL/RCAR family in C. canephora genome. The protein domains identified in the predict coffee sequences enabled to characterize these genes as family's members of receptors of ABA response pathway. Phylogenetic analysis allowed classifying coffee polypeptides sequences in three subfamilies expected. These genes were functionally annotated in the Coffee Genome Hub (http://coffee-genome.org/). In silico analyses also revealed differential expression profiles of coffee PYR/PYL/RCAR genes in tissues such as leaves, seeds, and floral organs, although, the highest expression profiles were identified in roots. Regarding drought stress, in silico analyses from RNAseq data obtained from roots of tolerant (Cc14, Cc73, Cc120) and susceptible (Cc22) clones of C. canephora grown with (I) or without (NI) irrigation also confirmed differential expression profiles of PYR/PYL/RCAR genes, suggesting the existence of multiple biological mechanisms for drought tolerance in coffee. Among the 9 coffee candidate genes, only the PYR1, PYL6, PYL8-2, PYL8- 8, PYL9 appeared expressed in roots. By qPCR analyses, we showed that the two copies of PYL8 gene present in C. canephora genome are co-expressed in root. Even members of the same subfamily, an in depth analysis of the coffee genome sequence revealed that these two genes were located in different chromosomes. As previously reported for several specific coffee gene families, such gene expansion could evidence a sequential tandem duplication followed by functional diversification. In order to understand the genetic determinism of drought tolerance in coffee, analyses are currently on going to identify the nucleic polymorphisms in promoter regions and coding sequences of PYR/PYL gene family. (Résumé d'auteur

Enhanced Eshelby twist on thin wurtzite InP nanowires and measurement of local crystal rotation

We have performed a detailed study of the lattice distortions of InP wurtzite nanowires containing an axial screw dislocation. Eshelby predicted that this kind of system should show a crystal rotation due to the dislocation induced torque. We have measured the twisting rate and the dislocation Burgers vector on individual wires, revealing that nanowires with a 10-nm radius have a twist up to 100% larger than estimated from elasticity theory. The strain induced by the deformation has a Mexican-hat-like geometry, which may create a tube-like potential well for carriers

Update on the search of candidate genes for drought-tolerance in coffee

It is well known that drought periods affect coffee plant development, leading to plant death and abortion of developing fruits in case of severe drought. In relation to coffee genetic diversity, several works reported the identification of plants of C. canephora conilon susceptible or tolerant to drought which were analyzed at the physiological level and also used to identify candidate genes underlying stress responses. Even narrow, a genetic diversity for drought tolerance also exist in the species C. arabica. In addition to the identification of undiscovered transcripts, the recent development of low-cost, high throughput next-generation (NGS) sequencing technologies now opens the way to perform expression profiling and to identify gene presenting differential expression patterns by comparing the frequency of reads obtained after sequencing. In order to initiate such kind of approach in coffee, RNAseq approach was performed using (1) roots of C. canephora conilon susceptible (clone 22) or tolerant (clones 14, 73 and 120) to drought grown under greenhouse conditions with (I) or without (NI) irrigation and (2) meristematic tissues from Iapar59 (I59, drought tolerant) and Rubi (R, drought susceptible) cultivars of C. arabica grown under field-grown with (I) or without (NI) irrigation. These data were compared with those of Coffea transcriptome, including the EST sequences from both C. arabica and C. canephora. Electronic northerns produced by these comparisons identified differentially expressed genes between drought-tolerant and -susceptible clones and cultivars. By qPCR experiments, more than 80 candidate genes, that could play a crucial role in the genetic determinism of drought tolerance in coffee plants, were selected. Based on these results, it can be concluded that the abscisic (ABA) signaling pathway (including ABA synthesis and perception) is one of the major molecular determinants that might explain the better efficiency in controlling stomata closure and transpiration displayed by drought-tolerant clones of C. canephora. The high up-regulation of genes encoding for dehydrins, detoxifying enzymes in drought-tolerant clones of C. canephora also suggests a strong induction of antioxidant and osmoprotection systems in these clones. On the other hand, the over-expression in the plagiotropic meristems of drought-tolerant cultivar IAPAR59 of C. arabica grown under NI of genes coding for proteins involved for example in the SAM (S-adenosyl-methionine) pathway and the wax biosynthesis (i.e. lipid transfer proteins) also suggested their involvement in the genetic determinism of drought tolerance in coffee. Interestingly, our work also led to the identification of several "unknown' (orphan) genes highly over-expressed mainly in droughttolerant plants of both C. canephora and C. arabica. All these RNAseq data are now being analyzed with genomic sequences of drought-susceptible (clone 22) or tolerant (clone 14) of C. canephora for example to see if the differential expression profiles that were observed could be explained by the presence of nucleic polyporphisms (SNPs and/or Indels) in promoter regions of corresponding genes. (Texte intégral