Hamiltonian of Tensionless Strings with Tensor Central Charge Coordinates

A new class of twistor-like string models in four-dimensional space-time extended by the addition of six tensorial central charge (TCC) coordinates $z_{mn}$ is studied. The Hamiltonian of tensionless string in the extended space-time is derived and its symmetries are investigated. We establish that the string constraints reduce the number of independent TCC coordinates $z_{mn}$ to one real effective coordinate which composes an effective 5-dimensional target space together with the $x^{m}$ coordinates. We construct the P.B. algebra of the first class constraints and discover that it coincides with the P.B. algebra of tensionless strings. The Lorentz covariant antisymmetric Dirac $\hat{\mathbf C}$-matrix of the P.B. of the second class constraints is constructed and its algebraic structure is further presented.Comment: 18 pages, Latex, no figure

Supersymmetric non-linear sigma-models with boundaries revisited

We study two-dimensional supersymmetric non-linear sigma-models with boundaries. We derive the most general family of boundary conditions in the non-supersymmetric case. Next we show that no further conditions arise when passing to the N=1 model. We present a manifest N=1 off-shell formulation. The analysis is greatly simplified compared to previous studies and there is no need to introduce non-local superspaces nor to go (partially) on-shell. Whether or not torsion is present does not modify the discussion. Subsequently, we determine under which conditions a second supersymmetry exists. As for the case without boundaries, two covariantly constant complex structures are needed. However, because of the presence of the boundary, one gets expressed in terms of the other one and the remainder of the geometric data. Finally we recast some of our results in N=2 superspace and discuss applications.Comment: LaTeX, 23 page

On the covariant quantization of tensionless bosonic strings in AdS spacetime

The covariant quantization of the tensionless free bosonic (open and closed) strings in AdS spaces is obtained. This is done by representing the AdS space as an hyperboloid in a flat auxiliary space and by studying the resulting string constrained hamiltonian system in the tensionless limit. It turns out that the constraint algebra simplifies in the tensionless case in such a way that the closed BRST quantization can be formulated and the theory admits then an explicit covariant quantization scheme. This holds for any value of the dimension of the AdS space.Comment: 1+16 pages; v4 two clarifications adde

D0 and D1 Brane With κ−\kappa_- and κ+\kappa_+ Extended Symmetry

D0 brane (D-particle) and D1 brane actions possess first and second class constraints that result in local $\kappa$ symmetry. The $\kappa$ symmetry of the D-particle and the D1 brane is extended here into a larger symmetry ($\kappa_-$ and $\kappa_+$) in a larger phase space by turning second class constraints into first class. Different gauge fixings of these symmetries result in different presentations of these systems while a "unitary" gauge fixing of the new $\kappa_+$ symmetry retrieves the original action with $\kappa_- = \kappa$ symmetry. For D1 brane our extended phase space makes all constraints into first class in the case of vanishing world sheet electric field (namely $(0, 1)$ string).Comment: A new section on D1 brane with extended kappa symmetry has been added, references have been added. (18 pages, Latex file

New insights into the genetic etiology of Alzheimer's disease and related dementias

Characterization of the genetic landscape of Alzheimer's disease (AD) and related dementias (ADD) provides a unique opportunity for a better understanding of the associated pathophysiological processes. We performed a two-stage genome-wide association study totaling 111,326 clinically diagnosed/'proxy' AD cases and 677,663 controls. We found 75 risk loci, of which 42 were new at the time of analysis. Pathway enrichment analyses confirmed the involvement of amyloid/tau pathways and highlighted microglia implication. Gene prioritization in the new loci identified 31 genes that were suggestive of new genetically associated processes, including the tumor necrosis factor alpha pathway through the linear ubiquitin chain assembly complex. We also built a new genetic risk score associated with the risk of future AD/dementia or progression from mild cognitive impairment to AD/dementia. The improvement in prediction led to a 1.6- to 1.9-fold increase in AD risk from the lowest to the highest decile, in addition to effects of age and the APOE ε4 allele

Biogeography of bacterioplankton in inland waters

Bacteria are among the most abundant groups of organisms. They mediate key ecological processes. Recent molecular advances have provided greater insight into bacterial diversity as well as allowing a more thorough examination of patterns in the spatial and temporal distribution of bacteria. Thus, the study of bacterial biodiversity and biogeographical distribution has stimulated considerable interest and dispute over the last decade. This review summarises the findings obtained from studies on the biogeography of bacterioplankton in inland waters. We examine factors and processes that may determine and maintain bacterial diversity and biogeography, and relate these to the theoretical metacommunity framework. We conclude that the importance of local environmental factors (such as lake character) for local bacterioplankton community compositions (BCC) is much more intensively studied than the importance of regional factors, such as dispersal. Further, few attempts have been made to evaluate simultaneously the relative importance of the two types of factors for BCC. Finally, we summarise gaps in knowledge, delineate challenges and put forward possible future research directions