633 research outputs found
Conifolds and Geometric Transitions
Conifold geometries have recieved a lot of attention in string theory and
string-inspired cosmology recently, in particular the Klebanov-Strassler
background that is known as the "warped throat". It is our intention in this
article to give a pedagogical explanation for the singularity resolution in
this geometry and emphasise its connection to geometric transitions. The first
part focuses on the gauge theory dual to the Klebanov-Strassler background,
which we also explain from a T-dual intersecting branes scenario. We then make
the connection to the Gopakumar-Vafa conjecture for open/closed string duality
and summarise a series of papers verifying this model on the supergravity
level.
An appendix provides extensive background material about conifold geometries.
We pay special attention to their complex structures and re-evaluate the
supersymmetry conditions on the background flux in constructions with
fractional D3-branes on the singular (Klebanov-Tseytlin) and resolved (Pando
Zayas-Tseytlin) conifolds. We agree with earlier results that only the singular
solution allows a supersymmetric flux, but point out the importance of using
the correct complex structure to reach this conclusion.Comment: 37 pages, v3: accepted for publication in Reviews of Modern Physic
A Methodological Framework to Assess Road Infrastructure Safety and Performance Efficiency in the Transition toward Cooperative Driving
There is increasing interest in connected and automated vehicles (CAVs), since their implementation will transform the nature of transportation and promote social and economic change. Transition toward cooperative driving still requires the understanding of some key questions to assess the performances of CAVs and human-driven vehicles on roundabouts and to properly balance road safety and traffic efficiency requirements. In this view, this paper proposes a simulation-based methodological framework aiming to assess the presence of increasing proportions of CAVs on roundabouts operating at a high-capacity utilization level. A roundabout was identified in Palermo City, Italy, and built in Aimsun (version 20) to describe the stepwise methodology. The CAV-based curves of capacity by entry mechanism were developed and then used as target capacities. To calibrate the model parameters, the capacity curves were compared with the capacity data simulated by Aimsun. The impact on the safety and performance efficiency of a lane dedicated to CAVs was also examined using surrogate measures of safety. The paper ends with highlighting a general improvement with CAVs on roundabouts, and with providing some insights to assess the advantages of the automated and connected driving technologies in transitioning to smarter mobilit
N=1 effective potential from dual type-IIA D6/O6 orientifolds with general fluxes
We consider N=1 compactifications of the type-IIA theory on the T6/(Z2xZ2)
orbifold and O6 orientifold, in the presence of D6-branes and general NSNS, RR
and Scherk-Schwarz geometrical fluxes. Introducing a suitable dual formulation
of the theory, we derive and solve the Bianchi identities, and show how certain
combinations of fluxes can relax the constraints on D6-brane configurations
coming from the cancellation of RR tadpoles. We then compute, via generalized
dimensional reduction, the N=1, D=4 effective potential for the seven main
moduli, and comment on the relation with truncated N=4 gaugings. As a
byproduct, we obtain a general geometrical expression for the superpotential.
We finally identify a family of fluxes, compatible with all Bianchi identities,
that perturbatively stabilize all seven moduli in supersymmetric AdS4.Comment: 19 pages, no figures, JHEP3 LaTeX. Published versio
Generalized structures of N=1 vacua
We characterize N=1 vacua of type II theories in terms of generalized complex
structure on the internal manifold M. The structure group of T(M) + T*(M) being
SU(3) x SU(3) implies the existence of two pure spinors Phi_1 and Phi_2. The
conditions for preserving N=1 supersymmetry turn out to be simple
generalizations of equations that have appeared in the context of N=2 and
topological strings. They are (d + H wedge) Phi_1=0 and (d + H wedge) Phi_2 =
F_RR. The equation for the first pure spinor implies that the internal space is
a twisted generalized Calabi-Yau manifold of a hybrid complex-symplectic type,
while the RR-fields serve as an integrability defect for the second.Comment: 21 pages. v2, v3: minor changes and correction
Towards Minkowski Vacua in Type II String Compactifications
We study the vacuum structure of compactifications of type II string theories
on orientifolds with SU(3)xSU(3) structure. We argue that generalised geometry
enables us to treat these non-geometric compactifications using a supergravity
analysis in a way very similar to geometric compactifications. We find
supersymmetric Minkowski vacua with all the moduli stabilised at weak string
coupling and all the tadpole conditions satisfied. Generically the value of the
moduli fields in the vacuum is parametrically controlled and can be taken to
arbitrarily large values.Comment: 33 pages; v2 minor corrections, references added, version to appear
in JHE
Radiation-dominated area metric cosmology
We provide further crucial support for a refined, area metric structure of
spacetime. Based on the solution of conceptual issues, such as the consistent
coupling of fermions and the covariant identification of radiation fields on
area metric backgrounds, we show that the radiation-dominated epoch of area
metric cosmology is equivalent to that epoch in standard Einstein cosmology.
This ensures, in particular, successful nucleosynthesis. This surprising result
complements the previously derived prediction of a small late-time acceleration
of an area metric universe.Comment: 23 pages, no figures; references adde
Supersymmetric AdS(4) compactifications of IIA supergravity
We derive necessary and sufficient conditions for N=1 compactifications of
(massive) IIA supergravity to AdS(4) in the language of SU(3) structures. We
find new solutions characterized by constant dilaton and nonzero fluxes for all
form fields. All fluxes are given in terms of the geometrical data of the
internal compact space. The latter is constrained to belong to a special class
of half-flat manifolds.Comment: 24 pages, references adde
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