1,054 research outputs found
N=1,2 supersymmetric vacua of IIA supergravity and SU(2) structures
We consider backgrounds of (massive) IIA supergravity of the form of a warped
product , where is a six-dimensional compact
manifold and is or a four-dimensional Minkowski space. We
analyse conditions for and supersymmetry on
manifolds of SU(2) structure. We prove the absence of solutions in certain
cases.Comment: 24 pages; v2: reference adde
Exploring the uncertainty in capacity estimation at roundabouts
Purpose In gap-acceptance theory the critical and the follow up headways have a significant role in determining roundabout entry capacities which in turn depend on circulating flow rates under a specified arrival headway distribution. Calculation considers single mean values of the gap-acceptance parameters,
neglecting the inherent variations in these random variables and providing a single value of entry capacity. The purpose of this paper is to derive the entry capacity distribution which accounts for the variations of the contributing (random) variables and suggest how to consider this issue in the operational analysis of the roundabouts.
Methods We performed a Monte Carlo simulation to get the distribution of entry capacity and found Crystal Ball software effective for performing the random sampling from the probability density functions of each contributing parameter. A steady-state model of capacity was used for performing many runs; in each run, the values of each contributing parameter were randomly drawn from the corresponding distributions.
Results The paper presents the first simulations and the entry capacity distributions at roundabouts, once the probability distributions of the headways were assumed. The results of the analysis were expressed probabilistically, meaning that the probability distributions of capacity rather than the simple point estimates were obtained.
Conclusions Comparing the capacity values based on a meta analytic estimation of critical and follow-up headways and the capacity functions based on the probability distributions of the model parameters, more insights in developing an appropriate approach to capacity estimation at roundabouts can be gained
Towards reduction of type II theories on SU(3) structure manifolds
We revisit the reduction of type II supergravity on SU(3) structure
manifolds, conjectured to lead to gauged N=2 supergravity in 4 dimensions. The
reduction proceeds by expanding the invariant 2- and 3-forms of the SU(3)
structure as well as the gauge potentials of the type II theory in the same set
of forms, the analogues of harmonic forms in the case of Calabi-Yau reductions.
By focussing on the metric sector, we arrive at a list of constraints these
expansion forms should satisfy to yield a base point independent reduction.
Identifying these constraints is a first step towards a first-principles
reduction of type II on SU(3) structure manifolds.Comment: 20 pages; v2: condition (2.13old) on expansion forms weakened,
replaced by (2.13new), (2.14new
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
Canonical differential geometry of string backgrounds
String backgrounds and D-branes do not possess the structure of Lorentzian
manifolds, but that of manifolds with area metric. Area metric geometry is a
true generalization of metric geometry, which in particular may accommodate a
B-field. While an area metric does not determine a connection, we identify the
appropriate differential geometric structure which is of relevance for the
minimal surface equation in such a generalized geometry. In particular the
notion of a derivative action of areas on areas emerges naturally. Area metric
geometry provides new tools in differential geometry, which promise to play a
role in the description of gravitational dynamics on D-branes.Comment: 20 pages, no figures, improved journal versio
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
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
A Decisive Role of [18F] Fdg PET/CT for Diagnosis of Neoplastic Vascular Thrombosis: Report of A Case
The combined use of PET scan with CT and fluorodeoxyglucose ([18F]FDG) can modify, in selected oncologic patients, the clinical management and care. We report here the case of a 63-year old female who showed increase serum level of CEA 3 years following left colectomy for cancer. Ultrasound and CT (computed tomography) of the abdomen showed a single lesion in the segment II of the liver with portal thrombosis. [18F] FDG PET/CT revealed two lesions (the first area was histologically proven to be a colorectal carcinoma metastasis, the second one corresponded to the portal thrombosis). No antithrombotic drugs were administered and patient underwent chemotherapy for 4 courses (eloxatin plus 5-fluorouracil anf leucovorin). Liver resection was performed 2 weeks after the end of chemotherapy, and intraoperative ultrasound revealed a normal portal flow
Heterotic-type IIA duality with fluxes
In this paper we study a possible non-perturbative dual of the heterotic
string compactified on K3 x T^2 in the presence of background fluxes. We show
that type IIA string theory compactified on manifolds with SU(3) structure can
account for a subset of the possible heterotic fluxes. This extends our
previous analysis to a case of a non-perturbative duality with fluxes.Comment: 26 pages, minor corrections; version to appear in JHE
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