Detecting big structural breaks in large factor models

Constant factor loadings is a standard assumption in the analysis of large dimensional factor models. Yet, this assumption may be restrictive unless parameter shifts are mild. In this paper we develop a new testing procedure to detect big breaks in factor loadings at either known or unknown dates. It is based upon testing for structural breaks in a regression of the first of the ¯r factors estimated by PC for the whole sample on the remaining r−1 factors, where r is chosen using Bai and Ng´s (2002) information criteria. We argue that this test is more powerful than other tests available in the literature on this issue.structural break; large factor model

The Rhizobium leguminosarum Norway - Lotus burttii interaction as a model for studying how rhizobia enter plant cells

Nitrogen-fixing rhizobia bacteria engage in a mutualistic symbiosis with legume plants. One of the defining features of this symbiosis is the formation of organs called nodules on the roots of the hosts. Establishment of an efficient interaction requires sophisticated and bidirectional communication between the host and the microsymbiont. The perception of the rhizobial signals by the host leads to the internalisation of rhizobia on the growing root nodule after the epidermal penetration and cortical spreading steps. The perception of rhizobial signal in the early stages has been extensively explored, however, the internalisation mechanism is still under investigation. This is caused by the lack of genetically amenable systems to study. Plant made tubular like structures, called infection threads are formed during rhizobia infection of host cells. A system that can uncouple infection thread formation and host cell infection will be suitable to uncover the mechanism of the internalisation process. To identify a suitable system to study this mechanism, a natural isolate Rhizobium leguminosarum Norway that infects Lotus was explored. Confocal and electron microscopy uncovered that Rl Norway invades the root nodule of Lotus burttii without using infection threads. Strikingly, Rl Norway is directly internalised from the apoplast into the host cell via “peg”-like structures. The expression of symbiotic genes involved in the infection process induced by Rl Norway is delayed and decreased in comparison to the response induced by a strain utilising an infection thread-dependent mode. These results revealed that Rl Norway uses an alternative infection strategy to colonise Lotus cells. Furthermore, a mutant impaired in the biosynthesis of the Lipochitooligosaccharides, known as Nodulation (Nod) factors, failed to induce “peg”-like structures during the internalisation process. This indicates that the formation of “peg”-like structures depends on the Nod factors and reinforces the previous hypothesis that there is signal perception before rhizobia are internalised. In addition to the signalling exchanges with the host, the rhizobia root colonisation is a prerequisite for the establishment of the root nodule symbiosis. However, the root colonisation of the microbe community is a complex process. The interaction between rhizobia, including competition and cooperation, is hypothesised to influence the root colonisation, which is so far not well examined. Rl Norway was co-isolated with Mesorhizobium norvegicum 10.2.2 from the same nodule. Interestingly, the microscopic quantification of root colonisation revealed increasing colonisation of Rl Norway and Mn 10.2.2 in the co-inoculation compared with the single inoculation. To understand the mechanism underlying the increased root colonisation, rhizobia behaviours related to the root colonisation were determined. A swarming assay showed that the motility of Mn 10.2.2 is increased in the presence of Rl Norway. In addition, biofilms were quantified in vitro. Rl Norway formed biofilms alone, while Mn 10.2.2 did not. Interestingly, co-culture of Rl Norway and Mn 10.2.2 enabled Mn 10.2.2 to form mixed biofilms together with Rl Norway. To investigate the role of the surface polysaccharides of Rl Norway during mixed biofilms formation, the biofilms of a mutant, with impaired surface polysaccharides biosynthesis was analysed. The structure of the biofilms formed by this mutant was altered under the single- and co-inoculation condition in comparison with the wild type strain. This indicates that the structure of the biofilms is determined by the surface polysaccharides of Rl Norway. Overall, this thesis concludes that the two strains exhibit synergism, which could possibly contribute to the increased root colonisation

Positivity constraints on the low-energy constants of the chiral pion-nucleon Lagrangian

Positivity constraints on the pion-nucleon scattering amplitude are derived in this article with the help of general S-matrix arguments, such as analyticity, crossing symmetry and unitarity, in the upper part of Mandelstam triangle, R. Scanning inside the region R, the most stringent bounds on the chiral low energy constants of the pion-nucleon Lagrangian are determined. When just considering the central values of the fit results from covariant baryon chiral perturbation theory using extended-on-mass-shell scheme, it is found that these bounds are well respected numerically both at O(p^3) and O(p^4) level. Nevertheless, when taking the errors into account, only the O(p^4) bounds are obeyed in the full error interval, while the bounds on O(p^3) fits are slightly violated. If one disregards loop contributions, the bounds always fail in certain regions of R. Thus, at a given chiral order these terms are not numerically negligible and one needs to consider all possible contributions, i.e., both tree-level and loop diagrams. We have provided the constraints for special points in R where the bounds are nearly optimal in terms of just a few chiral couplings, which can be easily implemented and employed to constrain future analyses. Some issues about calculations with an explicit Delta(1232) resonance are also discussed.Comment: 15 pages, 13 eps figures, 2 table

Ultrastrong coupling phenomena beyond the Dicke model

We study effective light-matter interactions in a circuit QED system consisting of a single $LC$ resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which have a drastic effect on the ground and excited state properties of such circuits in the ultrastrong coupling regime. In contrast to a superradiant phase transition expected from the standard Dicke model, we find an opposite mechanism, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve previous controversies over the existence of superradiant phases in circuit QED, but they more generally show that the physics of two- or multi-atom cavity QED settings can differ significantly from what is commonly assumed.Comment: 11 pages, 8 figure