2,734 research outputs found
Many-body theory for positronium-atom interactions
A many-body-theory approach has been developed to study positronium-atom
interactions. As first applications, we calculate the elastic scattering and
momentum-transfer cross sections and the pickoff annihilation rate
for Ps collisions with He and Ne. The cross section for He is
in agreement with previous coupled-state calculations, and the
momentum-transfer cross section for Ne agrees with available experimental data.
is found to be 0.13 and 0.26 for He and Ne, respectively, in
excellent agreement with the measured values.Comment: Accepted by Phys. Rev. Lett. (V2 contains update to text and Figs. 3
and 5. V3 contains further discussion on the calculation of pickoff
annihilation rates.
Potentials for hyper-Kahler metrics with torsion
We prove that locally any hyper-K\"ahler metric with torsion admits an HKT
potential.Comment: 9 page
Holonomy groups of pseudo-quaternionic-K\"ahlerian manifolds of non-zero scalar curvature
The holonomy group of a pseudo-quaternionic-K\"ahlerian manifold of
signature with non-zero scalar curvature is contained in
\Sp(1)\cdot\Sp(r,s) and it contains \Sp(1). It is proved that either is
irreducible, or and preserves an isotropic subspace of dimension
, in the last case, there are only two possibilities for the connected
component of the identity of such . This gives the classification of
possible connected holonomy groups of pseudo-quaternionic-K\"ahlerian manifolds
of non-zero scalar curvature.Comment: 7 pages; Dedicated to Dmitri Vladimirovich Alekseevsky at the
occasion of his 70th birthda
Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms
Predicted twenty years ago, positron binding to neutral atoms has not yet
been observed experimentally. A new scheme is proposed to detect positron-atom
bound states by colliding Rydberg positronium (Ps) with neutral atoms.
Estimates of the charge-transfer reaction cross section are obtained using the
first Born approximation for a selection of neutral atom targets and a wide
range of incident Ps energies and principal quantum numbers. We also estimate
the corresponding Ps ionization cross section. The accuracy of the calculations
is tested by comparison with earlier predictions for Ps charge transfer in
collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps
beam suitable for producing positron-atom bound states and estimate signal
rates based on the calculated cross sections and realistic experimental
parameters. We conclude that the proposed methodology is capable of producing
such states and of testing theoretical predictions of their binding energies.Comment: 19 pages, 11 figures, submitted to Phys. Rev.
Nonlinear Dirac operator and quaternionic analysis
Properties of the Cauchy-Riemann-Fueter equation for maps between
quaternionic manifolds are studied. Spaces of solutions in case of maps from a
K3-surface to the cotangent bundle of a complex projective space are computed.
A relationship between harmonic spinors of a generalized nonlinear Dirac
operator and solutions of the Cauchy-Riemann-Fueter equation are established.Comment: Cosmetic changes onl
Closed forms and multi-moment maps
We extend the notion of multi-moment map to geometries defined by closed
forms of arbitrary degree. We give fundamental existence and uniqueness results
and discuss a number of essential examples, including geometries related to
special holonomy. For forms of degree four, multi-moment maps are guaranteed to
exist and are unique when the symmetry group is (3,4)-trivial, meaning that the
group is connected and the third and fourth Lie algebra Betti numbers vanish.
We give a structural description of some classes of (3,4)-trivial algebras and
provide a number of examples.Comment: 36 page
Para-cresol production by Clostridium difficile affects microbial diversity and membrane integrity of Gram-negative bacteria
Clostridium difficile is a Gram-positive spore-forming anaerobe and a major cause of antibiotic-associated diarrhoea. Disruption of the commensal microbiota, such as through treatment with broad-spectrum antibiotics, is a critical precursor for colonisation by C. difficile and subsequent disease. Furthermore, failure of the gut microbiota to recover colonisation resistance can result in recurrence of infection. An unusual characteristic of C. difficile among gut bacteria is its ability to produce the bacteriostatic compound para-cresol (p-cresol) through fermentation of tyrosine. Here, we demonstrate that the ability of C. difficile to produce p-cresol in vitro provides a competitive advantage over gut bacteria including Escherichia coli, Klebsiella oxytoca and Bacteroides thetaiotaomicron. Metabolic profiling of competitive co-cultures revealed that acetate, alanine, butyrate, isobutyrate, p-cresol and p-hydroxyphenylacetate were the main metabolites responsible for differentiating the parent strain C. difficile (630Δerm) from a defined mutant deficient in p-cresol production. Moreover, we show that the p-cresol mutant displays a fitness defect in a mouse relapse model of C. difficile infection (CDI). Analysis of the microbiome from this mouse model of CDI demonstrates that colonisation by the p-cresol mutant results in a distinctly altered intestinal microbiota, and metabolic profile, with a greater representation of Gammaproteobacteria, including the Pseudomonales and Enterobacteriales. We demonstrate that Gammaproteobacteria are susceptible to exogenous p-cresol in vitro and that there is a clear divide between bacterial Phyla and their susceptibility to p-cresol. In general, Gram-negative species were relatively sensitive to p-cresol, whereas Gram-positive species were more tolerant. This study demonstrates that production of p-cresol by C. difficile has an effect on the viability of intestinal bacteria as well as the major metabolites produced in vitro. These observations are upheld in a mouse model of CDI, in which p-cresol production affects the biodiversity of gut microbiota and faecal metabolite profiles, suggesting that p-cresol production contributes to C. difficile survival and pathogenesis.Peer reviewedFinal Published versio
The isolation and properties of a second glycoprotein (LGP-II) from the articular lubricating fraction from bovine synovial fluid
Hypermultiplets and Topological Strings
The c-map relates classical hypermultiplet moduli spaces in compactifications
of type II strings on a Calabi-Yau threefold to vector multiplet moduli spaces
via a further compactification on a circle. We give an off-shell description of
the c-map in N=2 superspace. The superspace Lagrangian for the hypermultiplets
is a single function directly related to the prepotential of special geometry,
and can therefore be computed using topological string theory. Similarly, a
class of higher derivative terms for hypermultiplets can be computed from the
higher genus topological string amplitudes. Our results provide a framework for
studying quantum corrections to the hypermultiplet moduli space, as well as for
understanding the black hole wave-function as a function of the hypermultiplet
moduli.Comment: 21 pages, references adde
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