458 research outputs found
The Non-Perturbative SO(32) Heterotic String
The SO(32) heterotic string can be obtained from the type IIB string by
gauging a discrete symmetry that acts as on the perturbative
string states and reverses the parity of the D-string. Consistency requires the
presence of 32 NS 9-branes -- the S-duals of D9-branes -- which give SO(32)
Chan-Paton factors to open D-strings. At finite string coupling, there are
SO(32) charges tethered to the heterotic string world-sheet by open D-strings.
At zero-coupling, the D-string tension becomes infinite and the SO(32) charges
are pulled onto the world-sheet, and give the usual SO(32) world-sheet currents
of the heterotic string.Comment: 12 Pages, Tex, Phyzzx Macr
A Space-Time Orbifold: A Toy Model for a Cosmological Singularity
We explore bosonic strings and Type II superstrings in the simplest time
dependent backgrounds, namely orbifolds of Minkowski space by time reversal and
some spatial reflections. We show that there are no negative norm physical
excitations. However, the contributions of negative norm virtual states to
quantum loops do not cancel, showing that a ghost-free gauge cannot be chosen.
The spectrum includes a twisted sector, with strings confined to a ``conical''
singularity which is localized in time. Since these localized strings are not
visible to asymptotic observers, interesting issues arise regarding unitarity
of the S-matrix for scattering of propagating states. The partition function of
our model is modular invariant, and for the superstring, the zero momentum
dilaton tadpole vanishes. Many of the issues we study will be generic to
time-dependent cosmological backgrounds with singularities localized in time,
and we derive some general lessons about quantizing strings on such spaces.Comment: 21 pages, 2 figure
Duality Twists, Orbifolds, and Fluxes
We investigate compactifications with duality twists and their relation to
orbifolds and compactifications with fluxes. Inequivalent compactifications are
classified by conjugacy classes of the U-duality group and result in gauged
supergravities in lower dimensions with nontrivial Scherk-Schwarz potentials on
the moduli space. For certain twists, this mechanism is equivalent to
introducing internal fluxes but is more general and can be used to stabilize
some of the moduli. We show that the potential has stable minima with zero
energy precisely at the fixed points of the twist group. In string theory, when
the twist belongs to the T-duality group, the theory at the minimum has an
exact CFT description as an orbifold. We also discuss more general twists by
nonperturbative U-duality transformations.Comment: 30 pages, harvmac, references and brief comments on gauged
supergravity adde
Gauging and symplectic blowing up in nonlinear sigma-models: I. point singularities
In this paper a two dimensional non-linear sigma model with a general
symplectic manifold with isometry as target space is used to study symplectic
blowing up of a point singularity on the zero level set of the moment map
associated with a quasi-free Hamiltonian action. We discuss in general the
relation between symplectic reduction and gauging of the symplectic isometries
of the sigma model action. In the case of singular reduction, gauging has the
same effect as blowing up the singular point by a small amount. Using the
exponential mapping of the underlying metric, we are able to construct
symplectic diffeomorphisms needed to glue the blow-up to the global reduced
space which is regular, thus providing a transition from one symplectic sigma
model to another one free of singularities.Comment: 32 pages, LaTex, THEP 93/24 (corrected and expanded(about 5 pages)
version
Effects of Amalgam Restorations on the Periodontal Membrane in Monkeys
The response of the periodontal membrane to reimplanted teeth carrying amalgam restorations in the middle third of their roots was studied from seven days to six months after grafting. The study revealed that the amalgam restorations produced an initial localized inflammation in the periodontal tissues that subsided subsequently with the formation of a pseudocapsule.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66490/2/10.1177_00220345770560092001.pd
The Potts Fully Frustrated model: Thermodynamics, percolation and dynamics in 2 dimensions
We consider a Potts model diluted by fully frustrated Ising spins. The model
corresponds to a fully frustrated Potts model with variables having an integer
absolute value and a sign. This model presents precursor phenomena of a glass
transition in the high-temperature region. We show that the onset of these
phenomena can be related to a thermodynamic transition. Furthermore this
transition can be mapped onto a percolation transition. We numerically study
the phase diagram in 2 dimensions (2D) for this model with frustration and {\em
without} disorder and we compare it to the phase diagram of the model with
frustration {\em and} disorder and of the ferromagnetic model.
Introducing a parameter that connects the three models, we generalize the exact
expression of the ferromagnetic Potts transition temperature in 2D to the other
cases. Finally, we estimate the dynamic critical exponents related to the Potts
order parameter and to the energy.Comment: 10 pages, 10 figures, new result
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A multilevel neo-institutional analysis of infection prevention and control in English hospitals: coerced safety culture change?
Despite committed policy, regulative and professional efforts on healthcare safety, little is known about how such macro-interventions permeate organisations and shape culture over time. Informed by neo-institutional theory, we examined how inter-organisational influences shaped safety practices and inter-subjective meanings following efforts for coerced culture change. We traced macro-influences from 2000 to 2015 in infection prevention and control (IPC). Safety perceptions and meanings were inductively analysed from 130 in-depth qualitative interviews with senior- and middle-level managers from 30 English hospitals. A total of 869 institutional interventions were identified; 69% had a regulative component. In this context of forced implementation of safety practices, staff experienced inherent tensions concerning the scope of safety, their ability to be open and prioritisation of external mandates over local need. These tensions stemmed from conflicts among three co-existing institutional logics prevalent in the NHS. In response to requests for change, staff flexibly drew from a repertoire of cognitive, material and symbolic resources within and outside their organisations. They crafted 'strategies of action', guided by a situated assessment of first-hand practice experiences complementing collective evaluations of interventions such as 'pragmatic', 'sensible' and also 'legitimate'. Macro-institutional forces exerted influence either directly on individuals or indirectly by enriching the organisational cultural repertoire
Two-Loop Diagrammatics in a Self-Dual Background
Diagrammatic rules are developed for simplifying two-loop QED diagrams with
propagators in a constant self-dual background field. This diagrammatic
analysis, using dimensional regularization, is used to explain how the fully
renormalized two-loop Euler-Heisenberg effective Lagrangian for QED in a
self-dual background field is naturally expressed in terms of one-loop
diagrams. The connection between the two-loop and one-loop vacuum diagrams in a
background field parallels a corresponding connection for free vacuum diagrams,
without a background field, which can be derived by simple algebraic
manipulations. It also mirrors similar behavior recently found for two-loop
amplitudes in N=4 SUSY Yang-Mills theory.Comment: 16 pp, Latex, Axodra
Atmospheres from very low-mass stars to extrasolar planets
Within the next few years, several instruments aiming at imaging extrasolar
planets will see first light. In parallel, low mass planets are being searched
around red dwarfs which offer more favorable conditions, both for radial
velocity detection and transit studies, than solar-type stars. We review recent
advancements in modeling the stellar to substellar transition. The revised
solar oxygen abundances and cloud models allow to reproduce the photometric and
spectroscopic properties of this transition to a degree never achieved before,
but problems remain in the important M-L transition characteristic of the
effective temperature range of characterizable exoplanets.Comment: submitted to Memorie della Societa Astronomica Italian
Indirect search for dark matter: prospects for GLAST
Possible indirect detection of neutralino, through its gamma-ray annihilation
product, by the forthcoming GLAST satellite from our galactic halo, M31, M87
and the dwarf galaxies Draco and Sagittarius is studied. Gamma-ray fluxes are
evaluated for the two representative energy thresholds, 0.1 GeV and 1.0 GeV, at
which the spatial resolution of GLAST varies considerably. Apart from dwarfs
which are described either by a modified Plummer profile or by a
tidally-truncated King profiles, fluxes are compared for halos with central
cusps and cores. It is demonstrated that substructures, irrespective of their
profiles, enhance the gamma-ray emission only marginally. The expected
gamma-ray intensity above 1 GeV at high galactic latitudes is consistent with
the residual emission derived from EGRET data if the density profile has a
central core and the neutralino mass is less than 50 GeV, whereas for a central
cusp only a substantial enhancement would explain the observations. From M31,
the flux can be detected above 0.1 GeV and 1.0 GeV by GLAST only if the
neutralino mass is below 300 GeV and if the density profile has a central cusp,
case in which a significant boost in the gamma-ray emission is produced by the
central black hole. For Sagittarius, the flux above 0.1 GeV is detectable by
GLAST provided the neutralino mass is below 50 GeV. From M87 and Draco the
fluxes are always below the sensitivity limit of GLAST.Comment: 14 Pages, 7 Figures, 3 Tables, version to appear on Physical Review
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