179 research outputs found
Brane Worlds, the Cosmological Constant and String Theory
We argue that traditional methods of compactification of string theory make
it very difficult to understand how the cosmological constant becomes zero.
String inspired models can give zero cosmological constant after fine tuning
but since string theory has no free parameters it is not clear that this is
allowed. Brane world scenarios on the other hand while they do not answer the
question as to why the cosmological constant is zero do actually allow a choice
of integration constants that permit flat four space solutions. In this paper
we discuss gauged supergravity realizations of such a world. To the extent that
this starting point can be considered a low energy effective action of string
theory (and there is some recent evidence supporting this) our model may be
considered a string theory realization of this scenario.Comment: 18 pages, 5 figures. Shorter version and a few new comments adde
Duality Twists on a Group Manifold
We study duality-twisted dimensional reductions on a group manifold G, where
the twist is in a group \tilde{G} and examine the conditions for consistency.
We find that if the duality twist is introduced through a group element
\tilde{g} in \tilde{G}, then the flat \tilde{G}-connection A =\tilde{g}^{-1}
d\tilde{g} must have constant components M_n with respect to the basis 1-forms
on G, so that the dependence on the internal coordinates cancels out in the
lower dimensional theory. This condition can be satisfied if and only if M_n
forms a representation of the Lie algebra of G, which then ensures that the
lower dimensional gauge algebra closes. We find the form of this gauge algebra
and compare it to that arising from flux compactifications on twisted tori. As
an example of our construction, we find a new five dimensional gauged, massive
supergravity theory by dimensionally reducing the eight dimensional Type II
supergravity on a three dimensional unimodular, non-semi-simple, non-abelian
group manifold with an SL(3,R) twist.Comment: 22 page
Quantum Mechanics of the Doubled Torus
We investigate the quantum mechanics of the doubled torus system, introduced
by Hull [1] to describe T-folds in a more geometric way. Classically, this
system consists of a world-sheet Lagrangian together with some constraints,
which reduce the number of degrees of freedom to the correct physical number.
We consider this system from the point of view of constrained Hamiltonian
dynamics. In this case the constraints are second class, and we can quantize on
the constrained surface using Dirac brackets. We perform the quantization for a
simple T-fold background and compare to results for the conventional
non-doubled torus system. Finally, we formulate a consistent supersymmetric
version of the doubled torus system, including supersymmetric constraints.Comment: 31 pages, 1 figure; v2: references added, minor corrections to final
sectio
Asymmetric Orbifolds, Non-Geometric Fluxes and Non-Commutativity in Closed String Theory
In this paper we consider a class of exactly solvable closed string flux
backgrounds that exhibit non-commutativity in the closed string coordinates.
They are realized in terms of freely-acting asymmetric Z_N-orbifolds, which are
themselves close relatives of twisted torus fibrations with elliptic
Z_N-monodromy (elliptic T-folds). We explicitly construct the modular invariant
partition function of the models and derive the non-commutative algebra in the
string coordinates, which is exact to all orders in {\alpha}'. Finally, we
relate these asymmetric orbifold spaces to inherently stringy Scherk-Schwarz
backgrounds and non-geometric fluxes.Comment: 30 page
Estimands and their estimators for clinical trials Impacted by the COVID-19 pandemic: a report from the NISS Ingram Olkin Forum Series on unplanned clinical trial disruptions
The COVID-19 pandemic continues to affect the conduct of clinical trials globally. Complications may arise from pandemic-related operational challenges such as site closures, travel limitations and interruptions to the supply chain for the investigational product, or from health-related challenges such as COVID-19 infections. Some of these complications lead to unforeseen intercurrent events in the sense that they affect either the interpretation or the existence of the measurements associated with the clinical question of interest. In this article, we demonstrate how the ICH E9(R1) Addendum on estimands and sensitivity analyses provides a rigorous basis to discuss potential pandemic-related trial disruptions and to embed these disruptions in the context of study objectives and design elements. We introduce several hypothetical estimand strategies and review various causal inference and missing data methods, as well as a statistical method that combines unbiased and possibly biased estimators for estimation. To illustrate, we describe the features of a stylized trial, and how it may have been impacted by the pandemic. This stylized trial will then be re-visited by discussing the changes to the estimand and the estimator to account for pandemic disruptions. Finally, we outline considerations for designing future trials in the context of unforeseen disruptions
A Policy Maker’s Guide to Designing Payments for Ecosystem Services
Over the past five years, there has been increasing interest around the globe in payment schemes for the provision of ecosystem services, such as water purification, carbon sequestration, flood control, etc. Written for an Asian Development Bank project in China, this report provides a user-friendly guide to designing payments for the provision of ecosystem services. Part I explains the different types of ecosystem services, different ways of assessing their value, and why they are traditionally under-protected by law and policy. This is followed by an analysis of when payments for services are a preferable approach to other policy instruments. Part II explains the design issues underlying payments for services. These include identification of the service as well as potential buyers and sellers, the level of service needed, payment timing, payment type, and risk allocation. Part II contains a detailed analysis of the different types of payment mechanisms, ranging from general subsidy and certification to mitigation and offset payments. Part III explores the challenges to designing a payment scheme. These include the ability to monitor service provision, secure property rights, perverse incentives, supporting institutions, and poverty alleviation
T-duality in the weakly curved background
We consider the closed string propagating in the weakly curved background
which consists of constant metric and Kalb-Ramond field with infinitesimally
small coordinate dependent part. We propose the procedure for constructing the
T-dual theory, performing T-duality transformations along coordinates on which
the Kalb-Ramond field depends. The obtained theory is defined in the
non-geometric double space, described by the Lagrange multiplier and
its -dual . We apply the proposed T-duality procedure to the
T-dual theory and obtain the initial one. We discuss the standard relations
between T-dual theories that the equations of motion and momenta modes of one
theory are the Bianchi identities and the winding modes of the other
Generalized Flux Vacua
We consider type II string theory compactified on a symmetric T^6/Z_2
orientifold. We study a general class of discrete deformations of the resulting
four-dimensional supergravity theory, including gaugings arising from geometric
and "nongeometric'' fluxes, as well as the usual R-R and NS-NS fluxes. Solving
the equations of motion associated with the resulting N = 1 superpotential, we
find parametrically controllable infinite families of supersymmetric vacua with
all moduli stabilized. We also describe some aspects of the distribution of
generic solutions to the SUSY equations of motion for this model, and note in
particular the existence of an apparently infinite number of solutions in a
finite range of the parameter space of the four-dimensional effective theory.Comment: 30 pages, 4 .eps figures; v2, reference adde
Generalised Geometry for M-Theory
Generalised geometry studies structures on a d-dimensional manifold with a
metric and 2-form gauge field on which there is a natural action of the group
SO(d,d). This is generalised to d-dimensional manifolds with a metric and
3-form gauge field on which there is a natural action of the group .
This provides a framework for the discussion of M-theory solutions with flux. A
different generalisation is to d-dimensional manifolds with a metric, 2-form
gauge field and a set of p-forms for either odd or even on which there is a
natural action of the group . This is useful for type IIA or IIB
string solutions with flux. Further generalisations give extended tangent
bundles and extended spin bundles relevant for non-geometric backgrounds.
Special structures that arise for supersymmetric backgrounds are discussed.Comment: 31 page
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