355 research outputs found
Fixing All Moduli in a Simple F-Theory Compactification
We discuss a simple example of an F-theory compactification on a Calabi-Yau
fourfold where background fluxes, together with nonperturbative effects from
Euclidean D3 instantons and gauge dynamics on D7 branes, allow us to fix all
closed and open string moduli. We explicitly check that the known higher order
corrections to the potential, which we neglect in our leading approximation,
only shift the results by a small amount. In our exploration of the model, we
encounter interesting new phenomena, including examples of transitions where D7
branes absorb O3 planes, while changing topology to preserve the net D3 charge.Comment: 68 pages, 19 figures; v2: references adde
Conformality and Gauge Coupling Unification
It has been recently proposed to embed the standard model in a conformal
gauge theory to resolve the hierarchy problem, and to avoid assuming either
grand unification or low-energy supersymmetry. By model building based on
string-field duality we show how to maintain the successful prediction of an
electroweak mixing angle with in conformal gauge
theories with three chiral families.Comment: 8 pages LaTe
DBI in the Sky
We analyze the spectrum of density perturbations generated in models of the
recently discovered "D-cceleration" mechanism of inflation. In this scenario,
strong coupling quantum field theoretic effects sum to provide a DBI-like
action for the inflaton. We show that the model has a strict lower bound on the
non-Gaussianity of the CMBR power spectrum at an observable level, and is thus
falsifiable. This in particular observationally distinguishes this mechanism
from traditional slow roll inflation generated by weakly interacting scalar
fields. The model also favors a large observable tensor component to the CMBR
spectrum.Comment: 30 pages latex. v2: references added. v3: correction in three point
function. v4: sign of non-Gaussianity corrected; fNL is negativ
Topological twisted sigma model with H-flux revisited
In this paper we revisit the topological twisted sigma model with H-flux. We
explicitly expand and then twist the worldsheet Lagrangian for bi-Hermitian
geometry. we show that the resulting action consists of a BRST exact term and
pullback terms, which only depend on one of the two generalized complex
structures and the B-field. We then discuss the topological feature of the
model.Comment: 16 pages. Appendix adde
Slow roll in simple non-canonical inflation
We consider inflation using a class of non-canonical Lagrangians for which
the modification to the kinetic term depends on the field, but not its
derivatives. We generalize the standard Hubble slow roll expansion to the
non-canonical case and derive expressions for observables in terms of the
generalized slow roll parameters. We apply the general results to the
illustrative case of ``Slinky'' inflation, which has a simple, exactly
solvable, non-canonical representation. However, when transformed into a
canonical basis, Slinky inflation consists of a field oscillating on a
multi-valued potential. We calculate the power spectrum of curvature
perturbations for Slinky inflation directly in the non-canonical basis, and
show that the spectrum is approximately a power law on large scales, with a
``blue'' power spectrum. On small scales, the power spectrum exhibits strong
oscillatory behavior. This is an example of a model in which the widely used
solution of Garriga and Mukhanov gives the wrong answer for the power spectrum.Comment: 9 pages, LaTeX, four figures. (V2: minor changes to text. Version
submitted to JCAP.
4d-Flat Compactifications With Brane Vorticities
We present solutions in six-dimensional gravity coupled to a sigma model, in
the presence of three-brane sources. The space transverse to the branes is a
compact non-singular manifold. The example of O(3) sigma model in the presence
of two three-branes is worked out in detail. We show that the four-dimensional
flatness is obtained with a single condition involving the brane tensions,
which are in general different and may be both positive, and another
characteristic of the branes, vorticity. We speculate that the adjustment of
the effective four-dimensional cosmological constant may occur through the
exchange of vorticity between the branes. We then give exact instanton type
solutions for sigma models targeted on a general K\"ahler manifold, and
elaborate in this framework on multi-instantons of the O(3) sigma model. The
latter have branes, possibly with vorticities, at the instanton positions, thus
generalizing our two-brane solution.Comment: 8 pages. New references added and minor typos are correcte
NS-NS fluxes in Hitchin's generalized geometry
The standard notion of NS-NS 3-form flux is lifted to Hitchin's generalized
geometry. This generalized flux is given in terms of an integral of a modified
Nijenhuis operator over a generalized 3-cycle. Explicitly evaluating the
generalized flux in a number of familiar examples, we show that it can compute
three-form flux, geometric flux and non-geometric Q-flux. Finally, a
generalized connection that acts on generalized vectors is described and we
show how the flux arises from it.Comment: 21 pages, 1 figure; v3: minor change
From D3-Branes to Lifshitz Space-Times
We present a simple embedding of a z=2 Lifshitz space-time into type IIB
supergravity. This is obtained by considering a stack of D3-branes in type IIB
supergravity and deforming the world-volume by a plane wave. The plane wave is
sourced by the type IIB axion. The superposition of the plane wave and the
D3-branes is 1/4 BPS. The near horizon geometry of this configuration is a
5-dimensional z=0 Schroedinger space-time times a 5-sphere. This geometry is
also 1/4 BPS. Upon compactification along the direction in which the wave is
traveling the 5-dimensional z=0 Schroedinger space-time reduces to a
4-dimensional z=2 Lifshitz space-time. The compactification is such that the
circle is small for weakly coupled type IIB string theory. This reduction
breaks the supersymmetries. Further, we propose a general method to construct
analytic z=2 Lifshitz black brane solutions. The method is based on deforming
5-dimensional AdS black strings by an axion wave and reducing to 4-dimensions.
We illustrate this method with an example.Comment: version 3: version published in Classical and Quantum Gravit
How do supply chain management and information systems practices influence operational performance?:Evidence from emerging country SMEs
This study first provides a comparative analysis of the impact of supply chain management (SCM) and information systems (IS) practices on operational performance (OPER) of small- and medium-sized enterprises (SMEs) operating in two neighbouring emerging country markets of Turkey and Bulgaria. Then, we investigate moderating effects of both SCM–IS-linked enablers and inhibitors on the links between SCM and IS practices and OPER of SMEs. To this end, we first empirically identify the underlying dimensions of SCM and IS practices, and SCM–IS-related enabling and inhibiting factors. Second, a series of regression analyses are undertaken to estimate the impact of the study's constructs on OPER of SMEs. The results are discussed comparatively within the contexts of both Turkish and Bulgarian SMEs and beyond. The study makes a significant contribution to the extant literature through obtaining and analysing cross-national survey data of SCM and IS practices in emerging country markets
Restrictions on dilatonic brane-world models
We consider dilatonic brane-world models with a non-minimal coupling between
a dilaton and usual matter on a brane. We demonstrate that variation of the
fundamental constants on the brane due to such interaction leads to strong
restrictions on parameters of models. In particular, the experimental bounds on
variation of the fine structure constant rule out non-minimal dilatonic models
with a Liouville-type coupling potential f(varphi) = exp (b varphi) where b is
order of 1.Comment: MiKTeX2-LaTeX2e, 10 pages, minor changes, improved references, to
appear in IJMP
- …